=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.3 $, $Date: 2001/10/03 23:08:02 $)
+perlfaq4 - Data Manipulation
=head1 DESCRIPTION
-The section of the FAQ answers questions related to the manipulation
-of data as numbers, dates, strings, arrays, hashes, and miscellaneous
-data issues.
+This section of the FAQ answers questions related to manipulating
+numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
=head1 Data: Numbers
=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
-The infinite set that a mathematician thinks of as the real numbers can
-only be approximated on a computer, since the computer only has a finite
-number of bits to store an infinite number of, um, numbers.
-
Internally, your computer represents floating-point numbers in binary.
-Floating-point numbers read in from a file or appearing as literals
-in your program are converted from their decimal floating-point
-representation (eg, 19.95) to an internal binary representation.
-
-However, 19.95 can't be precisely represented as a binary
-floating-point number, just like 1/3 can't be exactly represented as a
-decimal floating-point number. The computer's binary representation
-of 19.95, therefore, isn't exactly 19.95.
-
-When a floating-point number gets printed, the binary floating-point
-representation is converted back to decimal. These decimal numbers
-are displayed in either the format you specify with printf(), or the
-current output format for numbers. (See L<perlvar/"$#"> if you use
-print. C<$#> has a different default value in Perl5 than it did in
-Perl4. Changing C<$#> yourself is deprecated.)
-
-This affects B<all> computer languages that represent decimal
-floating-point numbers in binary, not just Perl. Perl provides
-arbitrary-precision decimal numbers with the Math::BigFloat module
-(part of the standard Perl distribution), but mathematical operations
-are consequently slower.
-
-If precision is important, such as when dealing with money, it's good
-to work with integers and then divide at the last possible moment.
-For example, work in pennies (1995) instead of dollars and cents
-(19.95) and divide by 100 at the end.
-
-To get rid of the superfluous digits, just use a format (eg,
-C<printf("%.2f", 19.95)>) to get the required precision.
-See L<perlop/"Floating-point Arithmetic">.
+Digital (as in powers of two) computers cannot store all numbers
+exactly. Some real numbers lose precision in the process. This is a
+problem with how computers store numbers and affects all computer
+languages, not just Perl.
+
+L<perlnumber> shows the gory details of number representations and
+conversions.
+
+To limit the number of decimal places in your numbers, you can use the
+printf or sprintf function. See the L<"Floating Point
+Arithmetic"|perlop> for more details.
+
+ printf "%.2f", 10/3;
+
+ my $number = sprintf "%.2f", 10/3;
+
+=head2 Why is int() broken?
+
+Your C<int()> is most probably working just fine. It's the numbers that
+aren't quite what you think.
+
+First, see the answer to "Why am I getting long decimals
+(eg, 19.9499999999999) instead of the numbers I should be getting
+(eg, 19.95)?".
+
+For example, this
+
+ print int(0.6/0.2-2), "\n";
+
+will in most computers print 0, not 1, because even such simple
+numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
+numbers. What you think in the above as 'three' is really more like
+2.9999999999999995559.
=head2 Why isn't my octal data interpreted correctly?
-Perl only understands octal and hex numbers as such when they occur
-as literals in your program. Octal literals in perl must start with
-a leading "0" and hexadecimal literals must start with a leading "0x".
-If they are read in from somewhere and assigned, no automatic
-conversion takes place. You must explicitly use oct() or hex() if you
-want the values converted to decimal. oct() interprets
-both hex ("0x350") numbers and octal ones ("0350" or even without the
-leading "0", like "377"), while hex() only converts hexadecimal ones,
-with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
-The inverse mapping from decimal to octal can be done with either the
-"%o" or "%O" sprintf() formats. To get from decimal to hex try either
-the "%x" or the "%X" formats to sprintf().
-
-This problem shows up most often when people try using chmod(), mkdir(),
-umask(), or sysopen(), which by widespread tradition typically take
-permissions in octal.
-
- chmod(644, $file); # WRONG
- chmod(0644, $file); # right
-
-Note the mistake in the first line was specifying the decimal literal
-644, rather than the intended octal literal 0644. The problem can
-be seen with:
-
- printf("%#o",644); # prints 01204
-
-Surely you had not intended C<chmod(01204, $file);> - did you? If you
-want to use numeric literals as arguments to chmod() et al. then please
-try to express them as octal constants, that is with a leading zero and
-with the following digits restricted to the set 0..7.
+(contributed by brian d foy)
+
+You're probably trying to convert a string to a number, which Perl only
+converts as a decimal number. When Perl converts a string to a number, it
+ignores leading spaces and zeroes, then assumes the rest of the digits
+are in base 10:
+
+ my $string = '0644';
+
+ print $string + 0; # prints 644
+
+ print $string + 44; # prints 688, certainly not octal!
+
+This problem usually involves one of the Perl built-ins that has the
+same name a unix command that uses octal numbers as arguments on the
+command line. In this example, C<chmod> on the command line knows that
+its first argument is octal because that's what it does:
+
+ %prompt> chmod 644 file
+
+If you want to use the same literal digits (644) in Perl, you have to tell
+Perl to treat them as octal numbers either by prefixing the digits with
+a C<0> or using C<oct>:
+
+ chmod( 0644, $file); # right, has leading zero
+ chmod( oct(644), $file ); # also correct
+
+The problem comes in when you take your numbers from something that Perl
+thinks is a string, such as a command line argument in C<@ARGV>:
+
+ chmod( $ARGV[0], $file); # wrong, even if "0644"
+
+ chmod( oct($ARGV[0]), $file ); # correct, treat string as octal
+
+You can always check the value you're using by printing it in octal
+notation to ensure it matches what you think it should be. Print it
+in octal and decimal format:
+
+ printf "0%o %d", $number, $number;
=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
-Remember that int() merely truncates toward 0. For rounding to a
-certain number of digits, sprintf() or printf() is usually the easiest
-route.
+Remember that C<int()> merely truncates toward 0. For rounding to a
+certain number of digits, C<sprintf()> or C<printf()> is usually the
+easiest route.
- printf("%.3f", 3.1415926535); # prints 3.142
+ printf("%.3f", 3.1415926535); # prints 3.142
-The POSIX module (part of the standard Perl distribution) implements
-ceil(), floor(), and a number of other mathematical and trigonometric
-functions.
+The C<POSIX> module (part of the standard Perl distribution)
+implements C<ceil()>, C<floor()>, and a number of other mathematical
+and trigonometric functions.
- use POSIX;
- $ceil = ceil(3.5); # 4
- $floor = floor(3.5); # 3
+ use POSIX;
+ $ceil = ceil(3.5); # 4
+ $floor = floor(3.5); # 3
-In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
-module. With 5.004, the Math::Trig module (part of the standard Perl
+In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex>
+module. With 5.004, the C<Math::Trig> module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
-uses the Math::Complex module and some functions can break out from
+uses the C<Math::Complex> module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
2.
To see why, notice how you'll still have an issue on half-way-point
alternation:
- for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
+ for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
+
+ 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
+ 0.8 0.8 0.9 0.9 1.0 1.0
+
+Don't blame Perl. It's the same as in C. IEEE says we have to do
+this. Perl numbers whose absolute values are integers under 2**31 (on
+32 bit machines) will work pretty much like mathematical integers.
+Other numbers are not guaranteed.
+
+=head2 How do I convert between numeric representations/bases/radixes?
+
+As always with Perl there is more than one way to do it. Below are a
+few examples of approaches to making common conversions between number
+representations. This is intended to be representational rather than
+exhaustive.
+
+Some of the examples later in L<perlfaq4> use the C<Bit::Vector>
+module from CPAN. The reason you might choose C<Bit::Vector> over the
+perl built in functions is that it works with numbers of ANY size,
+that it is optimized for speed on some operations, and for at least
+some programmers the notation might be familiar.
+
+=over 4
+
+=item How do I convert hexadecimal into decimal
+
+Using perl's built in conversion of C<0x> notation:
+
+ $dec = 0xDEADBEEF;
+
+Using the C<hex> function:
+
+ $dec = hex("DEADBEEF");
+
+Using C<pack>:
+
+ $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
- 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
- 0.8 0.8 0.9 0.9 1.0 1.0
+Using the CPAN module C<Bit::Vector>:
-Don't blame Perl. It's the same as in C. IEEE says we have to do this.
-Perl numbers whose absolute values are integers under 2**31 (on 32 bit
-machines) will work pretty much like mathematical integers. Other numbers
-are not guaranteed.
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
+ $dec = $vec->to_Dec();
-=head2 How do I convert bits into ints?
+=item How do I convert from decimal to hexadecimal
-To turn a string of 1s and 0s like C<10110110> into a scalar containing
-its binary value, use the pack() and unpack() functions (documented in
-L<perlfunc/"pack"> and L<perlfunc/"unpack">):
+Using C<sprintf>:
- $decimal = unpack('c', pack('B8', '10110110'));
+ $hex = sprintf("%X", 3735928559); # upper case A-F
+ $hex = sprintf("%x", 3735928559); # lower case a-f
-This packs the string C<10110110> into an eight bit binary structure.
-This is then unpacked as a character, which returns its ordinal value.
+Using C<unpack>:
-This does the same thing:
+ $hex = unpack("H*", pack("N", 3735928559));
- $decimal = ord(pack('B8', '10110110'));
+Using C<Bit::Vector>:
-Here's an example of going the other way:
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $hex = $vec->to_Hex();
- $binary_string = unpack('B*', "\x29");
+And C<Bit::Vector> supports odd bit counts:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(33, 3735928559);
+ $vec->Resize(32); # suppress leading 0 if unwanted
+ $hex = $vec->to_Hex();
+
+=item How do I convert from octal to decimal
+
+Using Perl's built in conversion of numbers with leading zeros:
+
+ $dec = 033653337357; # note the leading 0!
+
+Using the C<oct> function:
+
+ $dec = oct("33653337357");
+
+Using C<Bit::Vector>:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new(32);
+ $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
+ $dec = $vec->to_Dec();
+
+=item How do I convert from decimal to octal
+
+Using C<sprintf>:
+
+ $oct = sprintf("%o", 3735928559);
+
+Using C<Bit::Vector>:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $oct = reverse join('', $vec->Chunk_List_Read(3));
+
+=item How do I convert from binary to decimal
+
+Perl 5.6 lets you write binary numbers directly with
+the C<0b> notation:
+
+ $number = 0b10110110;
+
+Using C<oct>:
+
+ my $input = "10110110";
+ $decimal = oct( "0b$input" );
+
+Using C<pack> and C<ord>:
+
+ $decimal = ord(pack('B8', '10110110'));
+
+Using C<pack> and C<unpack> for larger strings:
+
+ $int = unpack("N", pack("B32",
+ substr("0" x 32 . "11110101011011011111011101111", -32)));
+ $dec = sprintf("%d", $int);
+
+ # substr() is used to left pad a 32 character string with zeros.
+
+Using C<Bit::Vector>:
+
+ $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
+ $dec = $vec->to_Dec();
+
+=item How do I convert from decimal to binary
+
+Using C<sprintf> (perl 5.6+):
+
+ $bin = sprintf("%b", 3735928559);
+
+Using C<unpack>:
+
+ $bin = unpack("B*", pack("N", 3735928559));
+
+Using C<Bit::Vector>:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $bin = $vec->to_Bin();
+
+The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
+are left as an exercise to the inclined reader.
+
+=back
=head2 Why doesn't & work the way I want it to?
(the number C<3> is treated as the bit pattern C<00000011>).
So, saying C<11 & 3> performs the "and" operation on numbers (yielding
-C<1>). Saying C<"11" & "3"> performs the "and" operation on strings
+C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
(yielding C<"1">).
Most problems with C<&> and C<|> arise because the programmer thinks
they have a number but really it's a string. The rest arise because
the programmer says:
- if ("\020\020" & "\101\101") {
- # ...
- }
+ if ("\020\020" & "\101\101") {
+ # ...
+ }
but a string consisting of two null bytes (the result of C<"\020\020"
& "\101\101">) is not a false value in Perl. You need:
- if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
- # ...
- }
+ if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
+ # ...
+ }
=head2 How do I multiply matrices?
To call a function on each element in an array, and collect the
results, use:
- @results = map { my_func($_) } @array;
+ @results = map { my_func($_) } @array;
For example:
- @triple = map { 3 * $_ } @single;
+ @triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the
results:
- foreach $iterator (@array) {
- some_func($iterator);
- }
+ foreach $iterator (@array) {
+ some_func($iterator);
+ }
To call a function on each integer in a (small) range, you B<can> use:
- @results = map { some_func($_) } (5 .. 25);
+ @results = map { some_func($_) } (5 .. 25);
but you should be aware that the C<..> operator creates an array of
all integers in the range. This can take a lot of memory for large
ranges. Instead use:
- @results = ();
- for ($i=5; $i < 500_005; $i++) {
- push(@results, some_func($i));
- }
+ @results = ();
+ for ($i=5; $i < 500_005; $i++) {
+ push(@results, some_func($i));
+ }
This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
loop will iterate over the range, without creating the entire range.
- for my $i (5 .. 500_005) {
- push(@results, some_func($i));
- }
+ for my $i (5 .. 500_005) {
+ push(@results, some_func($i));
+ }
will not create a list of 500,000 integers.
=head2 How can I output Roman numerals?
-Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
+Get the http://www.cpan.org/modules/by-module/Roman module.
=head2 Why aren't my random numbers random?
If you're using a version of Perl before 5.004, you must call C<srand>
once at the start of your program to seed the random number generator.
+
+ BEGIN { srand() if $] < 5.004 }
+
5.004 and later automatically call C<srand> at the beginning. Don't
-call C<srand> more than once--you make your numbers less random, rather
-than more.
+call C<srand> more than once--you make your numbers less random,
+rather than more.
Computers are good at being predictable and bad at being random
-(despite appearances caused by bugs in your programs :-).
-http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
-Phoenix, talks more about this. John von Neumann said, ``Anyone who
-attempts to generate random numbers by deterministic means is, of
-course, living in a state of sin.''
+(despite appearances caused by bugs in your programs :-). see the
+F<random> article in the "Far More Than You Ever Wanted To Know"
+collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy
+of Tom Phoenix, talks more about this. John von Neumann said, "Anyone
+who attempts to generate random numbers by deterministic means is, of
+course, living in a state of sin."
If you want numbers that are more random than C<rand> with C<srand>
-provides, you should also check out the Math::TrulyRandom module from
+provides, you should also check out the C<Math::TrulyRandom> module from
CPAN. It uses the imperfections in your system's timer to generate
random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
-``Numerical Recipes in C'' at http://www.nr.com/ .
+"Numerical Recipes in C" at http://www.nr.com/ .
+
+=head2 How do I get a random number between X and Y?
+
+To get a random number between two values, you can use the C<rand()>
+built-in to get a random number between 0 and 1. From there, you shift
+that into the range that you want.
+
+C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus
+what you want to have perl figure out is a random number in the range
+from 0 to the difference between your I<X> and I<Y>.
+
+That is, to get a number between 10 and 15, inclusive, you want a
+random number between 0 and 5 that you can then add to 10.
+
+ my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )
+
+Hence you derive the following simple function to abstract
+that. It selects a random integer between the two given
+integers (inclusive), For example: C<random_int_between(50,120)>.
+
+ sub random_int_between {
+ my($min, $max) = @_;
+ # Assumes that the two arguments are integers themselves!
+ return $min if $min == $max;
+ ($min, $max) = ($max, $min) if $min > $max;
+ return $min + int rand(1 + $max - $min);
+ }
=head1 Data: Dates
-=head2 How do I find the week-of-the-year/day-of-the-year?
+=head2 How do I find the day or week of the year?
+
+The localtime function returns the day of the year. Without an
+argument localtime uses the current time.
+
+ $day_of_year = (localtime)[7];
+
+The C<POSIX> module can also format a date as the day of the year or
+week of the year.
-The day of the year is in the array returned by localtime() (see
-L<perlfunc/"localtime">):
+ use POSIX qw/strftime/;
+ my $day_of_year = strftime "%j", localtime;
+ my $week_of_year = strftime "%W", localtime;
- $day_of_year = (localtime(time()))[7];
+To get the day of year for any date, use C<POSIX>'s C<mktime> to get
+a time in epoch seconds for the argument to localtime.
+
+ use POSIX qw/mktime strftime/;
+ my $week_of_year = strftime "%W",
+ localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
+
+The C<Date::Calc> module provides two functions to calculate these.
+
+ use Date::Calc;
+ my $day_of_year = Day_of_Year( 1987, 12, 18 );
+ my $week_of_year = Week_of_Year( 1987, 12, 18 );
=head2 How do I find the current century or millennium?
Use the following simple functions:
- sub get_century {
- return int((((localtime(shift || time))[5] + 1999))/100);
- }
- sub get_millennium {
- return 1+int((((localtime(shift || time))[5] + 1899))/1000);
- }
+ sub get_century {
+ return int((((localtime(shift || time))[5] + 1999))/100);
+ }
+
+ sub get_millennium {
+ return 1+int((((localtime(shift || time))[5] + 1899))/1000);
+ }
-On some systems, you'll find that the POSIX module's strftime() function
-has been extended in a non-standard way to use a C<%C> format, which they
-sometimes claim is the "century". It isn't, because on most such systems,
-this is only the first two digits of the four-digit year, and thus cannot
-be used to reliably determine the current century or millennium.
+On some systems, the C<POSIX> module's C<strftime()> function has been
+extended in a non-standard way to use a C<%C> format, which they
+sometimes claim is the "century". It isn't, because on most such
+systems, this is only the first two digits of the four-digit year, and
+thus cannot be used to reliably determine the current century or
+millennium.
=head2 How can I compare two dates and find the difference?
-If you're storing your dates as epoch seconds then simply subtract one
-from the other. If you've got a structured date (distinct year, day,
-month, hour, minute, seconds values), then for reasons of accessibility,
-simplicity, and efficiency, merely use either timelocal or timegm (from
-the Time::Local module in the standard distribution) to reduce structured
-dates to epoch seconds. However, if you don't know the precise format of
-your dates, then you should probably use either of the Date::Manip and
-Date::Calc modules from CPAN before you go hacking up your own parsing
-routine to handle arbitrary date formats.
+(contributed by brian d foy)
+
+You could just store all your dates as a number and then subtract.
+Life isn't always that simple though. If you want to work with
+formatted dates, the C<Date::Manip>, C<Date::Calc>, or C<DateTime>
+modules can help you.
=head2 How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same format,
you can split it up and pass the parts to C<timelocal> in the standard
-Time::Local module. Otherwise, you should look into the Date::Calc
-and Date::Manip modules from CPAN.
+C<Time::Local> module. Otherwise, you should look into the C<Date::Calc>
+and C<Date::Manip> modules from CPAN.
=head2 How can I find the Julian Day?
-Use the Time::JulianDay module (part of the Time-modules bundle
-available from CPAN.)
-
-Before you immerse yourself too deeply in this, be sure to verify that
-it is the I<Julian> Day you really want. Are you interested in a way
-of getting serial days so that you just can tell how many days they
-are apart or so that you can do also other date arithmetic? If you
-are interested in performing date arithmetic, this can be done using
-modules Date::Manip or Date::Calc.
-
-There is too many details and much confusion on this issue to cover in
-this FAQ, but the term is applied (correctly) to a calendar now
-supplanted by the Gregorian Calendar, with the Julian Calendar failing
-to adjust properly for leap years on centennial years (among other
-annoyances). The term is also used (incorrectly) to mean: [1] days in
-the Gregorian Calendar; and [2] days since a particular starting time
-or `epoch', usually 1970 in the Unix world and 1980 in the
-MS-DOS/Windows world. If you find that it is not the first meaning
-that you really want, then check out the Date::Manip and Date::Calc
-modules. (Thanks to David Cassell for most of this text.)
+(contributed by brian d foy and Dave Cross)
+
+You can use the C<Time::JulianDay> module available on CPAN. Ensure
+that you really want to find a Julian day, though, as many people have
+different ideas about Julian days. See
+http://www.hermetic.ch/cal_stud/jdn.htm for instance.
+
+You can also try the C<DateTime> module, which can convert a date/time
+to a Julian Day.
+
+ $ perl -MDateTime -le'print DateTime->today->jd'
+ 2453401.5
+
+Or the modified Julian Day
+
+ $ perl -MDateTime -le'print DateTime->today->mjd'
+ 53401
+
+Or even the day of the year (which is what some people think of as a
+Julian day)
+
+ $ perl -MDateTime -le'print DateTime->today->doy'
+ 31
=head2 How do I find yesterday's date?
+X<date> X<yesterday> X<DateTime> X<Date::Calc> X<Time::Local>
+X<daylight saving time> X<day> X<Today_and_Now> X<localtime>
+X<timelocal>
-The C<time()> function returns the current time in seconds since the
-epoch. Take twenty-four hours off that:
+(contributed by brian d foy)
- $yesterday = time() - ( 24 * 60 * 60 );
+Use one of the Date modules. The C<DateTime> module makes it simple, and
+give you the same time of day, only the day before.
-Then you can pass this to C<localtime()> and get the individual year,
-month, day, hour, minute, seconds values.
+ use DateTime;
-Note very carefully that the code above assumes that your days are
-twenty-four hours each. For most people, there are two days a year
-when they aren't: the switch to and from summer time throws this off.
-A solution to this issue is offered by Russ Allbery.
+ my $yesterday = DateTime->now->subtract( days => 1 );
- sub yesterday {
- my $now = defined $_[0] ? $_[0] : time;
- my $then = $now - 60 * 60 * 24;
- my $ndst = (localtime $now)[8] > 0;
- my $tdst = (localtime $then)[8] > 0;
- $then - ($tdst - $ndst) * 60 * 60;
- }
- # Should give you "this time yesterday" in seconds since epoch relative to
- # the first argument or the current time if no argument is given and
- # suitable for passing to localtime or whatever else you need to do with
- # it. $ndst is whether we're currently in daylight savings time; $tdst is
- # whether the point 24 hours ago was in daylight savings time. If $tdst
- # and $ndst are the same, a boundary wasn't crossed, and the correction
- # will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
- # from yesterday's time since we gained an extra hour while going off
- # daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
- # negative hour (add an hour) to yesterday's time since we lost an hour.
- #
- # All of this is because during those days when one switches off or onto
- # DST, a "day" isn't 24 hours long; it's either 23 or 25.
- #
- # The explicit settings of $ndst and $tdst are necessary because localtime
- # only says it returns the system tm struct, and the system tm struct at
- # least on Solaris doesn't guarantee any particular positive value (like,
- # say, 1) for isdst, just a positive value. And that value can
- # potentially be negative, if DST information isn't available (this sub
- # just treats those cases like no DST).
- #
- # Note that between 2am and 3am on the day after the time zone switches
- # off daylight savings time, the exact hour of "yesterday" corresponding
- # to the current hour is not clearly defined. Note also that if used
- # between 2am and 3am the day after the change to daylight savings time,
- # the result will be between 3am and 4am of the previous day; it's
- # arguable whether this is correct.
- #
- # This sub does not attempt to deal with leap seconds (most things don't).
- #
- # Copyright relinquished 1999 by Russ Allbery <rra@stanford.edu>
- # This code is in the public domain
-
-=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
+ print "Yesterday was $yesterday\n";
+
+You can also use the C<Date::Calc> module using its C<Today_and_Now>
+function.
+
+ use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
+
+ my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
+
+ print "@date_time\n";
+
+Most people try to use the time rather than the calendar to figure out
+dates, but that assumes that days are twenty-four hours each. For
+most people, there are two days a year when they aren't: the switch to
+and from summer time throws this off. Let the modules do the work.
+
+If you absolutely must do it yourself (or can't use one of the
+modules), here's a solution using C<Time::Local>, which comes with
+Perl:
+
+ # contributed by Gunnar Hjalmarsson
+ use Time::Local;
+ my $today = timelocal 0, 0, 12, ( localtime )[3..5];
+ my ($d, $m, $y) = ( localtime $today-86400 )[3..5];
+ printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;
+
+In this case, you measure the day starting at noon, and subtract 24
+hours. Even if the length of the calendar day is 23 or 25 hours,
+you'll still end up on the previous calendar day, although not at
+noon. Since you don't care about the time, the one hour difference
+doesn't matter and you end up with the previous date.
+
+=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
-Y2K compliant (whatever that means). The programmers you've hired to
+Y2K compliant (whatever that means). The programmers you've hired to
use it, however, probably are not.
Long answer: The question belies a true understanding of the issue.
you can. Is that the pencil's fault? Of course it isn't.
The date and time functions supplied with Perl (gmtime and localtime)
-supply adequate information to determine the year well beyond 2000
-(2038 is when trouble strikes for 32-bit machines). The year returned
-by these functions when used in a list context is the year minus 1900.
-For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
-number. To avoid the year 2000 problem simply do not treat the year as
-a 2-digit number. It isn't.
+supply adequate information to determine the year well beyond 2000 and
+2038. The year returned by these functions when used in a list
+context is the year minus 1900. For years between 1910 and 1999 this
+I<happens> to be a 2-digit decimal number. To avoid the year 2000
+problem simply do not treat the year as a 2-digit number. It isn't.
When gmtime() and localtime() are used in scalar context they return
a timestamp string that contains a fully-expanded year. For example,
That doesn't mean that Perl can't be used to create non-Y2K compliant
programs. It can. But so can your pencil. It's the fault of the user,
-not the language. At the risk of inflaming the NRA: ``Perl doesn't
-break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
+not the language. At the risk of inflaming the NRA: "Perl doesn't
+break Y2K, people do." See http://www.perl.org/about/y2k.html for
a longer exposition.
=head1 Data: Strings
=head2 How do I validate input?
-The answer to this question is usually a regular expression, perhaps
-with auxiliary logic. See the more specific questions (numbers, mail
-addresses, etc.) for details.
+(contributed by brian d foy)
+
+There are many ways to ensure that values are what you expect or
+want to accept. Besides the specific examples that we cover in the
+perlfaq, you can also look at the modules with "Assert" and "Validate"
+in their names, along with other modules such as C<Regexp::Common>.
+
+Some modules have validation for particular types of input, such
+as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>,
+and C<Data::Validate::IP>.
=head2 How do I unescape a string?
-It depends just what you mean by ``escape''. URL escapes are dealt
+It depends just what you mean by "escape". URL escapes are dealt
with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
character are removed with
- s/\\(.)/$1/g;
+ s/\\(.)/$1/g;
This won't expand C<"\n"> or C<"\t"> or any other special escapes.
=head2 How do I remove consecutive pairs of characters?
-To turn C<"abbcccd"> into C<"abccd">:
+(contributed by brian d foy)
+
+You can use the substitution operator to find pairs of characters (or
+runs of characters) and replace them with a single instance. In this
+substitution, we find a character in C<(.)>. The memory parentheses
+store the matched character in the back-reference C<\1> and we use
+that to require that the same thing immediately follow it. We replace
+that part of the string with the character in C<$1>.
- s/(.)\1/$1/g; # add /s to include newlines
+ s/(.)\1/$1/g;
-Here's a solution that turns "abbcccd" to "abcd":
+We can also use the transliteration operator, C<tr///>. In this
+example, the search list side of our C<tr///> contains nothing, but
+the C<c> option complements that so it contains everything. The
+replacement list also contains nothing, so the transliteration is
+almost a no-op since it won't do any replacements (or more exactly,
+replace the character with itself). However, the C<s> option squashes
+duplicated and consecutive characters in the string so a character
+does not show up next to itself
- y///cs; # y == tr, but shorter :-)
+ my $str = 'Haarlem'; # in the Netherlands
+ $str =~ tr///cs; # Now Harlem, like in New York
=head2 How do I expand function calls in a string?
-This is documented in L<perlref>. In general, this is fraught with
-quoting and readability problems, but it is possible. To interpolate
-a subroutine call (in list context) into a string:
+(contributed by brian d foy)
+
+This is documented in L<perlref>, and although it's not the easiest
+thing to read, it does work. In each of these examples, we call the
+function inside the braces used to dereference a reference. If we
+have more than one return value, we can construct and dereference an
+anonymous array. In this case, we call the function in list context.
+
+ print "The time values are @{ [localtime] }.\n";
+
+If we want to call the function in scalar context, we have to do a bit
+more work. We can really have any code we like inside the braces, so
+we simply have to end with the scalar reference, although how you do
+that is up to you, and you can use code inside the braces. Note that
+the use of parens creates a list context, so we need C<scalar> to
+force the scalar context on the function:
- print "My sub returned @{[mysub(1,2,3)]} that time.\n";
+ print "The time is ${\(scalar localtime)}.\n"
-If you prefer scalar context, similar chicanery is also useful for
-arbitrary expressions:
+ print "The time is ${ my $x = localtime; \$x }.\n";
- print "That yields ${\($n + 5)} widgets\n";
+If your function already returns a reference, you don't need to create
+the reference yourself.
-Version 5.004 of Perl had a bug that gave list context to the
-expression in C<${...}>, but this is fixed in version 5.005.
+ sub timestamp { my $t = localtime; \$t }
-See also ``How can I expand variables in text strings?'' in this
-section of the FAQ.
+ print "The time is ${ timestamp() }.\n";
+
+The C<Interpolation> module can also do a lot of magic for you. You can
+specify a variable name, in this case C<E>, to set up a tied hash that
+does the interpolation for you. It has several other methods to do this
+as well.
+
+ use Interpolation E => 'eval';
+ print "The time values are $E{localtime()}.\n";
+
+In most cases, it is probably easier to simply use string concatenation,
+which also forces scalar context.
+
+ print "The time is " . localtime() . ".\n";
=head2 How do I find matching/nesting anything?
matter how complicated. To find something between two single
characters, a pattern like C</x([^x]*)x/> will get the intervening
bits in $1. For multiple ones, then something more like
-C</alpha(.*?)omega/> would be needed. But none of these deals with
-nested patterns, nor can they. For that you'll have to write a
+C</alpha(.*?)omega/> would be needed. But none of these deals with
+nested patterns. For balanced expressions using C<(>, C<{>, C<[> or
+C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
+L<perlre/(??{ code })>. For other cases, you'll have to write a
parser.
If you are serious about writing a parser, there are a number of
modules or oddities that will make your life a lot easier. There are
-the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
-and the byacc program. Starting from perl 5.8 the Text::Balanced
-is part of the standard distribution.
+the CPAN modules C<Parse::RecDescent>, C<Parse::Yapp>, and
+C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8
+the C<Text::Balanced> is part of the standard distribution.
One simple destructive, inside-out approach that you might try is to
pull out the smallest nesting parts one at a time:
- while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
- # do something with $1
- }
+ while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
+ # do something with $1
+ }
A more complicated and sneaky approach is to make Perl's regular
expression engine do it for you. This is courtesy Dean Inada, and
rather has the nature of an Obfuscated Perl Contest entry, but it
really does work:
- # $_ contains the string to parse
- # BEGIN and END are the opening and closing markers for the
- # nested text.
+ # $_ contains the string to parse
+ # BEGIN and END are the opening and closing markers for the
+ # nested text.
- @( = ('(','');
- @) = (')','');
- ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
- @$ = (eval{/$re/},$@!~/unmatched/);
- print join("\n",@$[0..$#$]) if( $$[-1] );
+ @( = ('(','');
+ @) = (')','');
+ ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
+ @$ = (eval{/$re/},$@!~/unmatched/i);
+ print join("\n",@$[0..$#$]) if( $$[-1] );
=head2 How do I reverse a string?
-Use reverse() in scalar context, as documented in
+Use C<reverse()> in scalar context, as documented in
L<perlfunc/reverse>.
- $reversed = reverse $string;
+ $reversed = reverse $string;
=head2 How do I expand tabs in a string?
You can do it yourself:
- 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
+ 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
-Or you can just use the Text::Tabs module (part of the standard Perl
+Or you can just use the C<Text::Tabs> module (part of the standard Perl
distribution).
- use Text::Tabs;
- @expanded_lines = expand(@lines_with_tabs);
+ use Text::Tabs;
+ @expanded_lines = expand(@lines_with_tabs);
=head2 How do I reformat a paragraph?
-Use Text::Wrap (part of the standard Perl distribution):
+Use C<Text::Wrap> (part of the standard Perl distribution):
- use Text::Wrap;
- print wrap("\t", ' ', @paragraphs);
+ use Text::Wrap;
+ print wrap("\t", ' ', @paragraphs);
-The paragraphs you give to Text::Wrap should not contain embedded
-newlines. Text::Wrap doesn't justify the lines (flush-right).
+The paragraphs you give to C<Text::Wrap> should not contain embedded
+newlines. C<Text::Wrap> doesn't justify the lines (flush-right).
-Or use the CPAN module Text::Autoformat. Formatting files can be easily
-done by making a shell alias, like so:
+Or use the CPAN module C<Text::Autoformat>. Formatting files can be
+easily done by making a shell alias, like so:
- alias fmt="perl -i -MText::Autoformat -n0777 \
- -e 'print autoformat $_, {all=>1}' $*"
+ alias fmt="perl -i -MText::Autoformat -n0777 \
+ -e 'print autoformat $_, {all=>1}' $*"
-See the documentation for Text::Autoformat to appreciate its many
+See the documentation for C<Text::Autoformat> to appreciate its many
capabilities.
-=head2 How can I access/change the first N letters of a string?
+=head2 How can I access or change N characters of a string?
+
+You can access the first characters of a string with substr().
+To get the first character, for example, start at position 0
+and grab the string of length 1.
-There are many ways. If you just want to grab a copy, use
-substr():
- $first_byte = substr($a, 0, 1);
+ $string = "Just another Perl Hacker";
+ $first_char = substr( $string, 0, 1 ); # 'J'
-If you want to modify part of a string, the simplest way is often to
-use substr() as an lvalue:
+To change part of a string, you can use the optional fourth
+argument which is the replacement string.
- substr($a, 0, 3) = "Tom";
+ substr( $string, 13, 4, "Perl 5.8.0" );
-Although those with a pattern matching kind of thought process will
-likely prefer
+You can also use substr() as an lvalue.
- $a =~ s/^.../Tom/;
+ substr( $string, 13, 4 ) = "Perl 5.8.0";
=head2 How do I change the Nth occurrence of something?
C<"whosoever"> or C<"whomsoever">, case insensitively. These
all assume that $_ contains the string to be altered.
- $count = 0;
- s{((whom?)ever)}{
- ++$count == 5 # is it the 5th?
- ? "${2}soever" # yes, swap
- : $1 # renege and leave it there
- }ige;
+ $count = 0;
+ s{((whom?)ever)}{
+ ++$count == 5 # is it the 5th?
+ ? "${2}soever" # yes, swap
+ : $1 # renege and leave it there
+ }ige;
In the more general case, you can use the C</g> modifier in a C<while>
loop, keeping count of matches.
- $WANT = 3;
- $count = 0;
- $_ = "One fish two fish red fish blue fish";
- while (/(\w+)\s+fish\b/gi) {
- if (++$count == $WANT) {
- print "The third fish is a $1 one.\n";
- }
- }
+ $WANT = 3;
+ $count = 0;
+ $_ = "One fish two fish red fish blue fish";
+ while (/(\w+)\s+fish\b/gi) {
+ if (++$count == $WANT) {
+ print "The third fish is a $1 one.\n";
+ }
+ }
That prints out: C<"The third fish is a red one."> You can also use a
repetition count and repeated pattern like this:
- /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
+ /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
=head2 How can I count the number of occurrences of a substring within a string?
count of a certain single character (X) within a string, you can use the
C<tr///> function like so:
- $string = "ThisXlineXhasXsomeXx'sXinXit";
- $count = ($string =~ tr/X//);
- print "There are $count X characters in the string";
+ $string = "ThisXlineXhasXsomeXx'sXinXit";
+ $count = ($string =~ tr/X//);
+ print "There are $count X characters in the string";
This is fine if you are just looking for a single character. However,
if you are trying to count multiple character substrings within a
loop around a global pattern match. For example, let's count negative
integers:
- $string = "-9 55 48 -2 23 -76 4 14 -44";
- while ($string =~ /-\d+/g) { $count++ }
- print "There are $count negative numbers in the string";
+ $string = "-9 55 48 -2 23 -76 4 14 -44";
+ while ($string =~ /-\d+/g) { $count++ }
+ print "There are $count negative numbers in the string";
+
+Another version uses a global match in list context, then assigns the
+result to a scalar, producing a count of the number of matches.
+
+ $count = () = $string =~ /-\d+/g;
+
+=head2 Does Perl have a Year 2038 problem?
+
+No, all of Perl's built in date and time functions and modules will
+work to about 2 billion years before and after 1970.
+
+Many systems cannot count time past the year 2038. Older versions of
+Perl were dependent on the system to do date calculation and thus
+shared their 2038 bug.
=head2 How do I capitalize all the words on one line?
+X<Text::Autoformat> X<capitalize> X<case, title> X<case, sentence>
-To make the first letter of each word upper case:
+(contributed by brian d foy)
- $line =~ s/\b(\w)/\U$1/g;
+Damian Conway's L<Text::Autoformat> handles all of the thinking
+for you.
-This has the strange effect of turning "C<don't do it>" into "C<Don'T
-Do It>". Sometimes you might want this. Other times you might need a
-more thorough solution (Suggested by brian d foy):
+ use Text::Autoformat;
+ my $x = "Dr. Strangelove or: How I Learned to Stop ".
+ "Worrying and Love the Bomb";
- $string =~ s/ (
- (^\w) #at the beginning of the line
- | # or
- (\s\w) #preceded by whitespace
- )
- /\U$1/xg;
- $string =~ /([\w']+)/\u\L$1/g;
+ print $x, "\n";
+ for my $style (qw( sentence title highlight )) {
+ print autoformat($x, { case => $style }), "\n";
+ }
-To make the whole line upper case:
+How do you want to capitalize those words?
- $line = uc($line);
+ FRED AND BARNEY'S LODGE # all uppercase
+ Fred And Barney's Lodge # title case
+ Fred and Barney's Lodge # highlight case
-To force each word to be lower case, with the first letter upper case:
+It's not as easy a problem as it looks. How many words do you think
+are in there? Wait for it... wait for it.... If you answered 5
+you're right. Perl words are groups of C<\w+>, but that's not what
+you want to capitalize. How is Perl supposed to know not to capitalize
+that C<s> after the apostrophe? You could try a regular expression:
- $line =~ s/(\w+)/\u\L$1/g;
+ $string =~ s/ (
+ (^\w) #at the beginning of the line
+ | # or
+ (\s\w) #preceded by whitespace
+ )
+ /\U$1/xg;
-You can (and probably should) enable locale awareness of those
-characters by placing a C<use locale> pragma in your program.
-See L<perllocale> for endless details on locales.
+ $string =~ s/([\w']+)/\u\L$1/g;
-This is sometimes referred to as putting something into "title
-case", but that's not quite accurate. Consider the proper
-capitalization of the movie I<Dr. Strangelove or: How I Learned to
-Stop Worrying and Love the Bomb>, for example.
+Now, what if you don't want to capitalize that "and"? Just use
+L<Text::Autoformat> and get on with the next problem. :)
-=head2 How can I split a [character] delimited string except when inside
-[character]? (Comma-separated files)
+=head2 How can I split a [character] delimited string except when inside [character]?
-Take the example case of trying to split a string that is comma-separated
-into its different fields. (We'll pretend you said comma-separated, not
-comma-delimited, which is different and almost never what you mean.) You
-can't use C<split(/,/)> because you shouldn't split if the comma is inside
-quotes. For example, take a data line like this:
+Several modules can handle this sort of parsing--C<Text::Balanced>,
+C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.
- SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
+Take the example case of trying to split a string that is
+comma-separated into its different fields. You can't use C<split(/,/)>
+because you shouldn't split if the comma is inside quotes. For
+example, take a data line like this:
+
+ SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex
-problem. Thankfully, we have Jeffrey Friedl, author of a highly
-recommended book on regular expressions, to handle these for us. He
-suggests (assuming your string is contained in $text):
-
- @new = ();
- push(@new, $+) while $text =~ m{
- "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
- | ([^,]+),?
- | ,
- }gx;
- push(@new, undef) if substr($text,-1,1) eq ',';
+problem. Thankfully, we have Jeffrey Friedl, author of
+I<Mastering Regular Expressions>, to handle these for us. He
+suggests (assuming your string is contained in C<$text>):
+
+ @new = ();
+ push(@new, $+) while $text =~ m{
+ "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
+ | ([^,]+),?
+ | ,
+ }gx;
+ push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a
quotation-mark-delimited field, escape them with backslashes (eg,
-C<"like \"this\"">. Unescaping them is a task addressed earlier in
-this section.
-
-Alternatively, the Text::ParseWords module (part of the standard Perl
-distribution) lets you say:
+C<"like \"this\"">.
- use Text::ParseWords;
- @new = quotewords(",", 0, $text);
+Alternatively, the C<Text::ParseWords> module (part of the standard
+Perl distribution) lets you say:
-There's also a Text::CSV (Comma-Separated Values) module on CPAN.
+ use Text::ParseWords;
+ @new = quotewords(",", 0, $text);
=head2 How do I strip blank space from the beginning/end of a string?
-Although the simplest approach would seem to be
+(contributed by brian d foy)
- $string =~ s/^\s*(.*?)\s*$/$1/;
+A substitution can do this for you. For a single line, you want to
+replace all the leading or trailing whitespace with nothing. You
+can do that with a pair of substitutions.
-not only is this unnecessarily slow and destructive, it also fails with
-embedded newlines. It is much faster to do this operation in two steps:
+ s/^\s+//;
+ s/\s+$//;
- $string =~ s/^\s+//;
- $string =~ s/\s+$//;
+You can also write that as a single substitution, although it turns
+out the combined statement is slower than the separate ones. That
+might not matter to you, though.
-Or more nicely written as:
+ s/^\s+|\s+$//g;
- for ($string) {
- s/^\s+//;
- s/\s+$//;
- }
+In this regular expression, the alternation matches either at the
+beginning or the end of the string since the anchors have a lower
+precedence than the alternation. With the C</g> flag, the substitution
+makes all possible matches, so it gets both. Remember, the trailing
+newline matches the C<\s+>, and the C<$> anchor can match to the
+physical end of the string, so the newline disappears too. Just add
+the newline to the output, which has the added benefit of preserving
+"blank" (consisting entirely of whitespace) lines which the C<^\s+>
+would remove all by itself.
-This idiom takes advantage of the C<foreach> loop's aliasing
-behavior to factor out common code. You can do this
-on several strings at once, or arrays, or even the
-values of a hash if you use a slice:
+ while( <> )
+ {
+ s/^\s+|\s+$//g;
+ print "$_\n";
+ }
- # trim whitespace in the scalar, the array,
- # and all the values in the hash
- foreach ($scalar, @array, @hash{keys %hash}) {
- s/^\s+//;
- s/\s+$//;
- }
+For a multi-line string, you can apply the regular expression
+to each logical line in the string by adding the C</m> flag (for
+"multi-line"). With the C</m> flag, the C<$> matches I<before> an
+embedded newline, so it doesn't remove it. It still removes the
+newline at the end of the string.
-=head2 How do I pad a string with blanks or pad a number with zeroes?
+ $string =~ s/^\s+|\s+$//gm;
+
+Remember that lines consisting entirely of whitespace will disappear,
+since the first part of the alternation can match the entire string
+and replace it with nothing. If need to keep embedded blank lines,
+you have to do a little more work. Instead of matching any whitespace
+(since that includes a newline), just match the other whitespace.
+
+ $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
-(This answer contributed by Uri Guttman, with kibitzing from
-Bart Lateur.)
+=head2 How do I pad a string with blanks or pad a number with zeroes?
In the following examples, C<$pad_len> is the length to which you wish
to pad the string, C<$text> or C<$num> contains the string to be padded,
right with blanks and it will truncate the result to a maximum length of
C<$pad_len>.
- # Left padding a string with blanks (no truncation):
- $padded = sprintf("%${pad_len}s", $text);
+ # Left padding a string with blanks (no truncation):
+ $padded = sprintf("%${pad_len}s", $text);
+ $padded = sprintf("%*s", $pad_len, $text); # same thing
- # Right padding a string with blanks (no truncation):
- $padded = sprintf("%-${pad_len}s", $text);
+ # Right padding a string with blanks (no truncation):
+ $padded = sprintf("%-${pad_len}s", $text);
+ $padded = sprintf("%-*s", $pad_len, $text); # same thing
- # Left padding a number with 0 (no truncation):
- $padded = sprintf("%0${pad_len}d", $num);
+ # Left padding a number with 0 (no truncation):
+ $padded = sprintf("%0${pad_len}d", $num);
+ $padded = sprintf("%0*d", $pad_len, $num); # same thing
- # Right padding a string with blanks using pack (will truncate):
- $padded = pack("A$pad_len",$text);
+ # Right padding a string with blanks using pack (will truncate):
+ $padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you can use
one of the following methods. They all generate a pad string with the
Left and right padding with any character, creating a new string:
- $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
- $padded = $text . $pad_char x ( $pad_len - length( $text ) );
+ $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
+ $padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying C<$text> directly:
- substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
- $text .= $pad_char x ( $pad_len - length( $text ) );
+ substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
+ $text .= $pad_char x ( $pad_len - length( $text ) );
=head2 How do I extract selected columns from a string?
-Use substr() or unpack(), both documented in L<perlfunc>.
-If you prefer thinking in terms of columns instead of widths,
-you can use this kind of thing:
-
- # determine the unpack format needed to split Linux ps output
- # arguments are cut columns
- my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
-
- sub cut2fmt {
- my(@positions) = @_;
- my $template = '';
- my $lastpos = 1;
- for my $place (@positions) {
- $template .= "A" . ($place - $lastpos) . " ";
- $lastpos = $place;
- }
- $template .= "A*";
- return $template;
- }
+(contributed by brian d foy)
+
+If you know where the columns that contain the data, you can
+use C<substr> to extract a single column.
+
+ my $column = substr( $line, $start_column, $length );
+
+You can use C<split> if the columns are separated by whitespace or
+some other delimiter, as long as whitespace or the delimiter cannot
+appear as part of the data.
+
+ my $line = ' fred barney betty ';
+ my @columns = split /\s+/, $line;
+ # ( '', 'fred', 'barney', 'betty' );
+
+ my $line = 'fred||barney||betty';
+ my @columns = split /\|/, $line;
+ # ( 'fred', '', 'barney', '', 'betty' );
+
+If you want to work with comma-separated values, don't do this since
+that format is a bit more complicated. Use one of the modules that
+handle that format, such as C<Text::CSV>, C<Text::CSV_XS>, or
+C<Text::CSV_PP>.
+
+If you want to break apart an entire line of fixed columns, you can use
+C<unpack> with the A (ASCII) format. by using a number after the format
+specifier, you can denote the column width. See the C<pack> and C<unpack>
+entries in L<perlfunc> for more details.
+
+ my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
+
+Note that spaces in the format argument to C<unpack> do not denote literal
+spaces. If you have space separated data, you may want C<split> instead.
=head2 How do I find the soundex value of a string?
-Use the standard Text::Soundex module distributed with Perl.
-Before you do so, you may want to determine whether `soundex' is in
-fact what you think it is. Knuth's soundex algorithm compresses words
-into a small space, and so it does not necessarily distinguish between
-two words which you might want to appear separately. For example, the
-last names `Knuth' and `Kant' are both mapped to the soundex code K530.
-If Text::Soundex does not do what you are looking for, you might want
-to consider the String::Approx module available at CPAN.
+(contributed by brian d foy)
+
+You can use the Text::Soundex module. If you want to do fuzzy or close
+matching, you might also try the C<String::Approx>, and
+C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.
=head2 How can I expand variables in text strings?
-Let's assume that you have a string like:
+(contributed by brian d foy)
+
+If you can avoid it, don't, or if you can use a templating system,
+such as C<Text::Template> or C<Template> Toolkit, do that instead. You
+might even be able to get the job done with C<sprintf> or C<printf>:
- $text = 'this has a $foo in it and a $bar';
+ my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
-If those were both global variables, then this would
-suffice:
+However, for the one-off simple case where I don't want to pull out a
+full templating system, I'll use a string that has two Perl scalar
+variables in it. In this example, I want to expand C<$foo> and C<$bar>
+to their variable's values:
- $text =~ s/\$(\w+)/${$1}/g; # no /e needed
+ my $foo = 'Fred';
+ my $bar = 'Barney';
+ $string = 'Say hello to $foo and $bar';
-But since they are probably lexicals, or at least, they could
-be, you'd have to do this:
+One way I can do this involves the substitution operator and a double
+C</e> flag. The first C</e> evaluates C<$1> on the replacement side and
+turns it into C<$foo>. The second /e starts with C<$foo> and replaces
+it with its value. C<$foo>, then, turns into 'Fred', and that's finally
+what's left in the string:
- $text =~ s/(\$\w+)/$1/eeg;
- die if $@; # needed /ee, not /e
+ $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
-It's probably better in the general case to treat those
-variables as entries in some special hash. For example:
+The C</e> will also silently ignore violations of strict, replacing
+undefined variable names with the empty string. Since I'm using the
+C</e> flag (twice even!), I have all of the same security problems I
+have with C<eval> in its string form. If there's something odd in
+C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then
+I could get myself in trouble.
- %user_defs = (
- foo => 23,
- bar => 19,
- );
- $text =~ s/\$(\w+)/$user_defs{$1}/g;
+To get around the security problem, I could also pull the values from
+a hash instead of evaluating variable names. Using a single C</e>, I
+can check the hash to ensure the value exists, and if it doesn't, I
+can replace the missing value with a marker, in this case C<???> to
+signal that I missed something:
-See also ``How do I expand function calls in a string?'' in this section
-of the FAQ.
+ my $string = 'This has $foo and $bar';
+
+ my %Replacements = (
+ foo => 'Fred',
+ );
+
+ # $string =~ s/\$(\w+)/$Replacements{$1}/g;
+ $string =~ s/\$(\w+)/
+ exists $Replacements{$1} ? $Replacements{$1} : '???'
+ /eg;
+
+ print $string;
=head2 What's wrong with always quoting "$vars"?
-The problem is that those double-quotes force stringification--
-coercing numbers and references into strings--even when you
-don't want them to be strings. Think of it this way: double-quote
-expansion is used to produce new strings. If you already
+The problem is that those double-quotes force
+stringification--coercing numbers and references into strings--even
+when you don't want them to be strings. Think of it this way:
+double-quote expansion is used to produce new strings. If you already
have a string, why do you need more?
If you get used to writing odd things like these:
- print "$var"; # BAD
- $new = "$old"; # BAD
- somefunc("$var"); # BAD
+ print "$var"; # BAD
+ $new = "$old"; # BAD
+ somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases) be
the simpler and more direct:
- print $var;
- $new = $old;
- somefunc($var);
+ print $var;
+ $new = $old;
+ somefunc($var);
Otherwise, besides slowing you down, you're going to break code when
the thing in the scalar is actually neither a string nor a number, but
a reference:
- func(\@array);
- sub func {
- my $aref = shift;
- my $oref = "$aref"; # WRONG
- }
+ func(\@array);
+ sub func {
+ my $aref = shift;
+ my $oref = "$aref"; # WRONG
+ }
You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical C<++> autoincrement operator or the
syscall() function.
-Stringification also destroys arrays.
+Stringification also destroys arrays.
- @lines = `command`;
- print "@lines"; # WRONG - extra blanks
- print @lines; # right
+ @lines = `command`;
+ print "@lines"; # WRONG - extra blanks
+ print @lines; # right
-=head2 Why don't my <<HERE documents work?
+=head2 Why don't my E<lt>E<lt>HERE documents work?
Check for these three things:
=over 4
-=item 1. There must be no space after the << part.
+=item There must be no space after the E<lt>E<lt> part.
-=item 2. There (probably) should be a semicolon at the end.
+=item There (probably) should be a semicolon at the end.
-=item 3. You can't (easily) have any space in front of the tag.
+=item You can't (easily) have any space in front of the tag.
=back
-If you want to indent the text in the here document, you
+If you want to indent the text in the here document, you
can do this:
# all in one
HERE_TARGET
But the HERE_TARGET must still be flush against the margin.
-If you want that indented also, you'll have to quote
+If you want that indented also, you'll have to quote
in the indentation.
($quote = <<' FINIS') =~ s/^\s+//gm;
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
- $quote =~ s/\s*--/\n--/;
+ $quote =~ s/\s+--/\n--/;
A nice general-purpose fixer-upper function for indented here documents
follows. It expects to be called with a here document as its argument.
This works with leading special strings, dynamically determined:
- $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
+ $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
- MAIN_INTERPRETER_LOOP
+ MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:
- $poem = fix<<EVER_ON_AND_ON;
+ $poem = fix<<EVER_ON_AND_ON;
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
- EVER_ON_AND_ON
+ EVER_ON_AND_ON
=head1 Data: Arrays
=head2 What is the difference between a list and an array?
-An array has a changeable length. A list does not. An array is something
-you can push or pop, while a list is a set of values. Some people make
-the distinction that a list is a value while an array is a variable.
-Subroutines are passed and return lists, you put things into list
-context, you initialize arrays with lists, and you foreach() across
-a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
-in scalar context behave like the number of elements in them, subroutines
-access their arguments through the array C<@_>, and push/pop/shift only work
-on arrays.
+An array has a changeable length. A list does not. An array is
+something you can push or pop, while a list is a set of values. Some
+people make the distinction that a list is a value while an array is a
+variable. Subroutines are passed and return lists, you put things into
+list context, you initialize arrays with lists, and you C<foreach()>
+across a list. C<@> variables are arrays, anonymous arrays are
+arrays, arrays in scalar context behave like the number of elements in
+them, subroutines access their arguments through the array C<@_>, and
+C<push>/C<pop>/C<shift> only work on arrays.
As a side note, there's no such thing as a list in scalar context.
When you say
- $scalar = (2, 5, 7, 9);
+ $scalar = (2, 5, 7, 9);
you're using the comma operator in scalar context, so it uses the scalar
-comma operator. There never was a list there at all! This causes the
+comma operator. There never was a list there at all! This causes the
last value to be returned: 9.
=head2 What is the difference between $array[1] and @array[1]?
Sometimes it doesn't make a difference, but sometimes it does.
For example, compare:
- $good[0] = `some program that outputs several lines`;
+ $good[0] = `some program that outputs several lines`;
with
- @bad[0] = `same program that outputs several lines`;
+ @bad[0] = `same program that outputs several lines`;
-The C<use warnings> pragma and the B<-w> flag will warn you about these
+The C<use warnings> pragma and the B<-w> flag will warn you about these
matters.
=head2 How can I remove duplicate elements from a list or array?
-There are several possible ways, depending on whether the array is
-ordered and whether you wish to preserve the ordering.
+(contributed by brian d foy)
-=over 4
+Use a hash. When you think the words "unique" or "duplicated", think
+"hash keys".
-=item a)
+If you don't care about the order of the elements, you could just
+create the hash then extract the keys. It's not important how you
+create that hash: just that you use C<keys> to get the unique
+elements.
-If @in is sorted, and you want @out to be sorted:
-(this assumes all true values in the array)
+ my %hash = map { $_, 1 } @array;
+ # or a hash slice: @hash{ @array } = ();
+ # or a foreach: $hash{$_} = 1 foreach ( @array );
- $prev = "not equal to $in[0]";
- @out = grep($_ ne $prev && ($prev = $_, 1), @in);
+ my @unique = keys %hash;
-This is nice in that it doesn't use much extra memory, simulating
-uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
-guarantees that the expression is true (so that grep picks it up)
-even if the $_ is 0, "", or undef.
+If you want to use a module, try the C<uniq> function from
+C<List::MoreUtils>. In list context it returns the unique elements,
+preserving their order in the list. In scalar context, it returns the
+number of unique elements.
-=item b)
+ use List::MoreUtils qw(uniq);
-If you don't know whether @in is sorted:
+ my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
+ my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
- undef %saw;
- @out = grep(!$saw{$_}++, @in);
+You can also go through each element and skip the ones you've seen
+before. Use a hash to keep track. The first time the loop sees an
+element, that element has no key in C<%Seen>. The C<next> statement
+creates the key and immediately uses its value, which is C<undef>, so
+the loop continues to the C<push> and increments the value for that
+key. The next time the loop sees that same element, its key exists in
+the hash I<and> the value for that key is true (since it's not 0 or
+C<undef>), so the next skips that iteration and the loop goes to the
+next element.
-=item c)
+ my @unique = ();
+ my %seen = ();
-Like (b), but @in contains only small integers:
+ foreach my $elem ( @array )
+ {
+ next if $seen{ $elem }++;
+ push @unique, $elem;
+ }
- @out = grep(!$saw[$_]++, @in);
+You can write this more briefly using a grep, which does the
+same thing.
-=item d)
+ my %seen = ();
+ my @unique = grep { ! $seen{ $_ }++ } @array;
-A way to do (b) without any loops or greps:
-
- undef %saw;
- @saw{@in} = ();
- @out = sort keys %saw; # remove sort if undesired
-
-=item e)
+=head2 How can I tell whether a certain element is contained in a list or array?
-Like (d), but @in contains only small positive integers:
+(portions of this answer contributed by Anno Siegel and brian d foy)
- undef @ary;
- @ary[@in] = @in;
- @out = grep {defined} @ary;
+Hearing the word "in" is an I<in>dication that you probably should have
+used a hash, not a list or array, to store your data. Hashes are
+designed to answer this question quickly and efficiently. Arrays aren't.
-=back
+That being said, there are several ways to approach this. In Perl 5.10
+and later, you can use the smart match operator to check that an item is
+contained in an array or a hash:
-But perhaps you should have been using a hash all along, eh?
+ use 5.010;
-=head2 How can I tell whether a certain element is contained in a list or array?
+ if( $item ~~ @array )
+ {
+ say "The array contains $item"
+ }
-Hearing the word "in" is an I<in>dication that you probably should have
-used a hash, not a list or array, to store your data. Hashes are
-designed to answer this question quickly and efficiently. Arrays aren't.
+ if( $item ~~ %hash )
+ {
+ say "The hash contains $item"
+ }
-That being said, there are several ways to approach this. If you
+With earlier versions of Perl, you have to do a bit more work. If you
are going to make this query many times over arbitrary string values,
-the fastest way is probably to invert the original array and keep an
-associative array lying about whose keys are the first array's values.
+the fastest way is probably to invert the original array and maintain a
+hash whose keys are the first array's values:
- @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
- undef %is_blue;
- for (@blues) { $is_blue{$_} = 1 }
+ @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
+ %is_blue = ();
+ for (@blues) { $is_blue{$_} = 1 }
-Now you can check whether $is_blue{$some_color}. It might have been a
-good idea to keep the blues all in a hash in the first place.
+Now you can check whether C<$is_blue{$some_color}>. It might have
+been a good idea to keep the blues all in a hash in the first place.
If the values are all small integers, you could use a simple indexed
array. This kind of an array will take up less space:
- @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
- undef @is_tiny_prime;
- for (@primes) { $is_tiny_prime[$_] = 1 }
- # or simply @istiny_prime[@primes] = (1) x @primes;
+ @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
+ @is_tiny_prime = ();
+ for (@primes) { $is_tiny_prime[$_] = 1 }
+ # or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings, you can save
quite a lot of space by using bit strings instead:
- @articles = ( 1..10, 150..2000, 2017 );
- undef $read;
- for (@articles) { vec($read,$_,1) = 1 }
+ @articles = ( 1..10, 150..2000, 2017 );
+ undef $read;
+ for (@articles) { vec($read,$_,1) = 1 }
Now check whether C<vec($read,$n,1)> is true for some C<$n>.
-Please do not use
-
- ($is_there) = grep $_ eq $whatever, @array;
+These methods guarantee fast individual tests but require a re-organization
+of the original list or array. They only pay off if you have to test
+multiple values against the same array.
+
+If you are testing only once, the standard module C<List::Util> exports
+the function C<first> for this purpose. It works by stopping once it
+finds the element. It's written in C for speed, and its Perl equivalent
+looks like this subroutine:
+
+ sub first (&@) {
+ my $code = shift;
+ foreach (@_) {
+ return $_ if &{$code}();
+ }
+ undef;
+ }
-or worse yet
+If speed is of little concern, the common idiom uses grep in scalar context
+(which returns the number of items that passed its condition) to traverse the
+entire list. This does have the benefit of telling you how many matches it
+found, though.
- ($is_there) = grep /$whatever/, @array;
+ my $is_there = grep $_ eq $whatever, @array;
-These are slow (checks every element even if the first matches),
-inefficient (same reason), and potentially buggy (what if there are
-regex characters in $whatever?). If you're only testing once, then
-use:
+If you want to actually extract the matching elements, simply use grep in
+list context.
- $is_there = 0;
- foreach $elt (@array) {
- if ($elt eq $elt_to_find) {
- $is_there = 1;
- last;
- }
- }
- if ($is_there) { ... }
+ my @matches = grep $_ eq $whatever, @array;
=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
-Use a hash. Here's code to do both and more. It assumes that
-each element is unique in a given array:
+Use a hash. Here's code to do both and more. It assumes that each
+element is unique in a given array:
- @union = @intersection = @difference = ();
- %count = ();
- foreach $element (@array1, @array2) { $count{$element}++ }
- foreach $element (keys %count) {
- push @union, $element;
- push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
- }
+ @union = @intersection = @difference = ();
+ %count = ();
+ foreach $element (@array1, @array2) { $count{$element}++ }
+ foreach $element (keys %count) {
+ push @union, $element;
+ push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
+ }
-Note that this is the I<symmetric difference>, that is, all elements in
-either A or in B but not in both. Think of it as an xor operation.
+Note that this is the I<symmetric difference>, that is, all elements
+in either A or in B but not in both. Think of it as an xor operation.
=head2 How do I test whether two arrays or hashes are equal?
-The following code works for single-level arrays. It uses a stringwise
-comparison, and does not distinguish defined versus undefined empty
-strings. Modify if you have other needs.
+With Perl 5.10 and later, the smart match operator can give you the answer
+with the least amount of work:
- $are_equal = compare_arrays(\@frogs, \@toads);
+ use 5.010;
- sub compare_arrays {
- my ($first, $second) = @_;
- no warnings; # silence spurious -w undef complaints
- return 0 unless @$first == @$second;
- for (my $i = 0; $i < @$first; $i++) {
- return 0 if $first->[$i] ne $second->[$i];
- }
- return 1;
- }
+ if( @array1 ~~ @array2 )
+ {
+ say "The arrays are the same";
+ }
+
+ if( %hash1 ~~ %hash2 ) # doesn't check values!
+ {
+ say "The hash keys are the same";
+ }
+
+The following code works for single-level arrays. It uses a
+stringwise comparison, and does not distinguish defined versus
+undefined empty strings. Modify if you have other needs.
+
+ $are_equal = compare_arrays(\@frogs, \@toads);
+
+ sub compare_arrays {
+ my ($first, $second) = @_;
+ no warnings; # silence spurious -w undef complaints
+ return 0 unless @$first == @$second;
+ for (my $i = 0; $i < @$first; $i++) {
+ return 0 if $first->[$i] ne $second->[$i];
+ }
+ return 1;
+ }
For multilevel structures, you may wish to use an approach more
-like this one. It uses the CPAN module FreezeThaw:
+like this one. It uses the CPAN module C<FreezeThaw>:
- use FreezeThaw qw(cmpStr);
- @a = @b = ( "this", "that", [ "more", "stuff" ] );
+ use FreezeThaw qw(cmpStr);
+ @a = @b = ( "this", "that", [ "more", "stuff" ] );
- printf "a and b contain %s arrays\n",
- cmpStr(\@a, \@b) == 0
- ? "the same"
- : "different";
+ printf "a and b contain %s arrays\n",
+ cmpStr(\@a, \@b) == 0
+ ? "the same"
+ : "different";
-This approach also works for comparing hashes. Here
-we'll demonstrate two different answers:
+This approach also works for comparing hashes. Here we'll demonstrate
+two different answers:
- use FreezeThaw qw(cmpStr cmpStrHard);
+ use FreezeThaw qw(cmpStr cmpStrHard);
- %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
- $a{EXTRA} = \%b;
- $b{EXTRA} = \%a;
+ %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
+ $a{EXTRA} = \%b;
+ $b{EXTRA} = \%a;
- printf "a and b contain %s hashes\n",
+ printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
- printf "a and b contain %s hashes\n",
+ printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
=head2 How do I find the first array element for which a condition is true?
-You can use this if you care about the index:
+To find the first array element which satisfies a condition, you can
+use the C<first()> function in the C<List::Util> module, which comes
+with Perl 5.8. This example finds the first element that contains
+"Perl".
- for ($i= 0; $i < @array; $i++) {
- if ($array[$i] eq "Waldo") {
- $found_index = $i;
- last;
- }
- }
+ use List::Util qw(first);
+
+ my $element = first { /Perl/ } @array;
+
+If you cannot use C<List::Util>, you can make your own loop to do the
+same thing. Once you find the element, you stop the loop with last.
+
+ my $found;
+ foreach ( @array ) {
+ if( /Perl/ ) { $found = $_; last }
+ }
+
+If you want the array index, you can iterate through the indices
+and check the array element at each index until you find one
+that satisfies the condition.
-Now C<$found_index> has what you want.
+ my( $found, $index ) = ( undef, -1 );
+ for( $i = 0; $i < @array; $i++ ) {
+ if( $array[$i] =~ /Perl/ ) {
+ $found = $array[$i];
+ $index = $i;
+ last;
+ }
+ }
=head2 How do I handle linked lists?
In general, you usually don't need a linked list in Perl, since with
-regular arrays, you can push and pop or shift and unshift at either end,
-or you can use splice to add and/or remove arbitrary number of elements at
-arbitrary points. Both pop and shift are both O(1) operations on Perl's
-dynamic arrays. In the absence of shifts and pops, push in general
-needs to reallocate on the order every log(N) times, and unshift will
-need to copy pointers each time.
+regular arrays, you can push and pop or shift and unshift at either
+end, or you can use splice to add and/or remove arbitrary number of
+elements at arbitrary points. Both pop and shift are O(1)
+operations on Perl's dynamic arrays. In the absence of shifts and
+pops, push in general needs to reallocate on the order every log(N)
+times, and unshift will need to copy pointers each time.
If you really, really wanted, you could use structures as described in
-L<perldsc> or L<perltoot> and do just what the algorithm book tells you
-to do. For example, imagine a list node like this:
+L<perldsc> or L<perltoot> and do just what the algorithm book tells
+you to do. For example, imagine a list node like this:
- $node = {
- VALUE => 42,
- LINK => undef,
- };
+ $node = {
+ VALUE => 42,
+ LINK => undef,
+ };
You could walk the list this way:
- print "List: ";
- for ($node = $head; $node; $node = $node->{LINK}) {
- print $node->{VALUE}, " ";
- }
- print "\n";
+ print "List: ";
+ for ($node = $head; $node; $node = $node->{LINK}) {
+ print $node->{VALUE}, " ";
+ }
+ print "\n";
You could add to the list this way:
- my ($head, $tail);
- $tail = append($head, 1); # grow a new head
- for $value ( 2 .. 10 ) {
- $tail = append($tail, $value);
- }
-
- sub append {
- my($list, $value) = @_;
- my $node = { VALUE => $value };
- if ($list) {
- $node->{LINK} = $list->{LINK};
- $list->{LINK} = $node;
- } else {
- $_[0] = $node; # replace caller's version
- }
- return $node;
- }
+ my ($head, $tail);
+ $tail = append($head, 1); # grow a new head
+ for $value ( 2 .. 10 ) {
+ $tail = append($tail, $value);
+ }
+
+ sub append {
+ my($list, $value) = @_;
+ my $node = { VALUE => $value };
+ if ($list) {
+ $node->{LINK} = $list->{LINK};
+ $list->{LINK} = $node;
+ }
+ else {
+ $_[0] = $node; # replace caller's version
+ }
+ return $node;
+ }
But again, Perl's built-in are virtually always good enough.
=head2 How do I handle circular lists?
+X<circular> X<array> X<Tie::Cycle> X<Array::Iterator::Circular>
+X<cycle> X<modulus>
+
+(contributed by brian d foy)
+
+If you want to cycle through an array endlessy, you can increment the
+index modulo the number of elements in the array:
+
+ my @array = qw( a b c );
+ my $i = 0;
-Circular lists could be handled in the traditional fashion with linked
-lists, or you could just do something like this with an array:
+ while( 1 ) {
+ print $array[ $i++ % @array ], "\n";
+ last if $i > 20;
+ }
- unshift(@array, pop(@array)); # the last shall be first
- push(@array, shift(@array)); # and vice versa
+You can also use C<Tie::Cycle> to use a scalar that always has the
+next element of the circular array:
+
+ use Tie::Cycle;
+
+ tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
+
+ print $cycle; # FFFFFF
+ print $cycle; # 000000
+ print $cycle; # FFFF00
+
+The C<Array::Iterator::Circular> creates an iterator object for
+circular arrays:
+
+ use Array::Iterator::Circular;
+
+ my $color_iterator = Array::Iterator::Circular->new(
+ qw(red green blue orange)
+ );
+
+ foreach ( 1 .. 20 ) {
+ print $color_iterator->next, "\n";
+ }
=head2 How do I shuffle an array randomly?
If you either have Perl 5.8.0 or later installed, or if you have
Scalar-List-Utils 1.03 or later installed, you can say:
- use List::Util 'shuffle';
+ use List::Util 'shuffle';
@shuffled = shuffle(@list);
-If not, you can use this:
+If not, you can use a Fisher-Yates shuffle.
- # fisher_yates_shuffle( \@array ) :
- # generate a random permutation of @array in place
- sub fisher_yates_shuffle {
- my $array = shift;
- my $i = @$array;
- while (--$i) {
- my $j = int rand ($i+1);
- @$array[$i,$j] = @$array[$j,$i];
- }
- }
+ sub fisher_yates_shuffle {
+ my $deck = shift; # $deck is a reference to an array
+ return unless @$deck; # must not be empty!
- fisher_yates_shuffle( \@array ); # permutes @array in place
+ my $i = @$deck;
+ while (--$i) {
+ my $j = int rand ($i+1);
+ @$deck[$i,$j] = @$deck[$j,$i];
+ }
+ }
+
+ # shuffle my mpeg collection
+ #
+ my @mpeg = <audio/*/*.mp3>;
+ fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
+ print @mpeg;
Note that the above implementation shuffles an array in place,
-unlike the List::Util::shuffle() which takes a list and returns
+unlike the C<List::Util::shuffle()> which takes a list and returns
a new shuffled list.
You've probably seen shuffling algorithms that work using splice,
randomly picking another element to swap the current element with
- srand;
- @new = ();
- @old = 1 .. 10; # just a demo
- while (@old) {
- push(@new, splice(@old, rand @old, 1));
- }
+ srand;
+ @new = ();
+ @old = 1 .. 10; # just a demo
+ while (@old) {
+ push(@new, splice(@old, rand @old, 1));
+ }
-This is bad because splice is already O(N), and since you do it N times,
-you just invented a quadratic algorithm; that is, O(N**2). This does
-not scale, although Perl is so efficient that you probably won't notice
-this until you have rather largish arrays.
+This is bad because splice is already O(N), and since you do it N
+times, you just invented a quadratic algorithm; that is, O(N**2).
+This does not scale, although Perl is so efficient that you probably
+won't notice this until you have rather largish arrays.
=head2 How do I process/modify each element of an array?
Use C<for>/C<foreach>:
- for (@lines) {
- s/foo/bar/; # change that word
- y/XZ/ZX/; # swap those letters
- }
+ for (@lines) {
+ s/foo/bar/; # change that word
+ tr/XZ/ZX/; # swap those letters
+ }
Here's another; let's compute spherical volumes:
- for (@volumes = @radii) { # @volumes has changed parts
- $_ **= 3;
- $_ *= (4/3) * 3.14159; # this will be constant folded
- }
+ for (@volumes = @radii) { # @volumes has changed parts
+ $_ **= 3;
+ $_ *= (4/3) * 3.14159; # this will be constant folded
+ }
-If you want to do the same thing to modify the values of the hash,
-you may not use the C<values> function, oddly enough. You need a slice:
+which can also be done with C<map()> which is made to transform
+one list into another:
- for $orbit ( @orbits{keys %orbits} ) {
- ($orbit **= 3) *= (4/3) * 3.14159;
- }
+ @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
+
+If you want to do the same thing to modify the values of the
+hash, you can use the C<values> function. As of Perl 5.6
+the values are not copied, so if you modify $orbit (in this
+case), you modify the value.
+
+ for $orbit ( values %orbits ) {
+ ($orbit **= 3) *= (4/3) * 3.14159;
+ }
+
+Prior to perl 5.6 C<values> returned copies of the values,
+so older perl code often contains constructions such as
+C<@orbits{keys %orbits}> instead of C<values %orbits> where
+the hash is to be modified.
=head2 How do I select a random element from an array?
-Use the
rand() function (see L<perlfunc/rand>):
+Use the
C<rand()> function (see L<perlfunc/rand>):
- # at the top of the program:
- srand; # not needed for 5.004 and later
+ $index = rand @array;
+ $element = $array[$index];
- # then later on
- $index = rand @array;
- $element = $array[$index];
+Or, simply:
-Make sure you I<only call srand once per program, if then>.
-If you are calling it more than once (such as before each
-call to rand), you're almost certainly doing something wrong.
+ my $element = $array[ rand @array ];
=head2 How do I permute N elements of a list?
+X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth>
+X<The Art of Computer Programming> X<Fischer-Krause>
-Here's a little program that generates all permutations
-of all the words on each line of input. The algorithm embodied
-in the permute() function should work on any list:
-
- #!/usr/bin/perl -n
- # tsc-permute: permute each word of input
- permute([split], []);
- sub permute {
- my @items = @{ $_[0] };
- my @perms = @{ $_[1] };
- unless (@items) {
- print "@perms\n";
- } else {
- my(@newitems,@newperms,$i);
- foreach $i (0 .. $#items) {
- @newitems = @items;
- @newperms = @perms;
- unshift(@newperms, splice(@newitems, $i, 1));
- permute([@newitems], [@newperms]);
- }
+Use the C<List::Permutor> module on CPAN. If the list is actually an
+array, try the C<Algorithm::Permute> module (also on CPAN). It's
+written in XS code and is very efficient:
+
+ use Algorithm::Permute;
+
+ my @array = 'a'..'d';
+ my $p_iterator = Algorithm::Permute->new ( \@array );
+
+ while (my @perm = $p_iterator->next) {
+ print "next permutation: (@perm)\n";
+ }
+
+For even faster execution, you could do:
+
+ use Algorithm::Permute;
+
+ my @array = 'a'..'d';
+
+ Algorithm::Permute::permute {
+ print "next permutation: (@array)\n";
+ } @array;
+
+Here's a little program that generates all permutations of all the
+words on each line of input. The algorithm embodied in the
+C<permute()> function is discussed in Volume 4 (still unpublished) of
+Knuth's I<The Art of Computer Programming> and will work on any list:
+
+ #!/usr/bin/perl -n
+ # Fischer-Krause ordered permutation generator
+
+ sub permute (&@) {
+ my $code = shift;
+ my @idx = 0..$#_;
+ while ( $code->(@_[@idx]) ) {
+ my $p = $#idx;
+ --$p while $idx[$p-1] > $idx[$p];
+ my $q = $p or return;
+ push @idx, reverse splice @idx, $p;
+ ++$q while $idx[$p-1] > $idx[$q];
+ @idx[$p-1,$q]=@idx[$q,$p-1];
+ }
}
- }
-Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
-module from CPAN runs at least an order of magnitude faster. If you don't
-have a C compiler (or a binary distribution of Algorithm::Permute), then
-you can use List::Permutor which is written in pure Perl, and is still
-several times faster than the algorithm above.
+ permute { print "@_\n" } split;
+
+The C<Algorithm::Loops> module also provides the C<NextPermute> and
+C<NextPermuteNum> functions which efficiently find all unique permutations
+of an array, even if it contains duplicate values, modifying it in-place:
+if its elements are in reverse-sorted order then the array is reversed,
+making it sorted, and it returns false; otherwise the next
+permutation is returned.
+
+C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so
+you can enumerate all the permutations of C<0..9> like this:
+
+ use Algorithm::Loops qw(NextPermuteNum);
+
+ my @list= 0..9;
+ do { print "@list\n" } while NextPermuteNum @list;
=head2 How do I sort an array by (anything)?
Supply a comparison function to sort() (described in L<perlfunc/sort>):
- @list = sort { $a <=> $b } @list;
+ @list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would
sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
after the first number on each item, and then sort those words
case-insensitively.
- @idx = ();
- for (@data) {
- ($item) = /\d+\s*(\S+)/;
- push @idx, uc($item);
- }
- @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
+ @idx = ();
+ for (@data) {
+ ($item) = /\d+\s*(\S+)/;
+ push @idx, uc($item);
+ }
+ @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
which could also be written this way, using a trick
that's come to be known as the Schwartzian Transform:
- @sorted = map { $_->[0] }
- sort { $a->[1] cmp $b->[1] }
- map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
+ @sorted = map { $_->[0] }
+ sort { $a->[1] cmp $b->[1] }
+ map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is useful.
- @sorted = sort { field1($a) <=> field1($b) ||
- field2($a) cmp field2($b) ||
- field3($a) cmp field3($b)
- } @data;
+ @sorted = sort {
+ field1($a) <=> field1($b) ||
+ field2($a) cmp field2($b) ||
+ field3($a) cmp field3($b)
+ } @data;
This can be conveniently combined with precalculation of keys as given
above.
-See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
-this approach.
+See the F<sort> article in the "Far More Than You Ever Wanted
+To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
+more about this approach.
-See also the question below on sorting hashes.
+See also the question later in L<perlfaq4> on sorting hashes.
=head2 How do I manipulate arrays of bits?
-Use pack() and unpack(), or else vec() and the bitwise operations.
-
-For example, this sets $vec to have bit N set if $ints[N] was set:
-
- $vec = '';
- foreach(@ints) { vec($vec,$_,1) = 1 }
-
-And here's how, given a vector in $vec, you can
-get those bits into your @ints array:
-
- sub bitvec_to_list {
- my $vec = shift;
- my @ints;
- # Find null-byte density then select best algorithm
- if ($vec =~ tr/\0// / length $vec > 0.95) {
- use integer;
- my $i;
- # This method is faster with mostly null-bytes
- while($vec =~ /[^\0]/g ) {
- $i = -9 + 8 * pos $vec;
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- }
- } else {
- # This method is a fast general algorithm
- use integer;
- my $bits = unpack "b*", $vec;
- push @ints, 0 if $bits =~ s/^(\d)// && $1;
- push @ints, pos $bits while($bits =~ /1/g);
- }
- return \@ints;
- }
+Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise
+operations.
+
+For example, you don't have to store individual bits in an array
+(which would mean that you're wasting a lot of space). To convert an
+array of bits to a string, use C<vec()> to set the right bits. This
+sets C<$vec> to have bit N set only if C<$ints[N]> was set:
+
+ @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
+ $vec = '';
+ foreach( 0 .. $#ints ) {
+ vec($vec,$_,1) = 1 if $ints[$_];
+ }
+
+The string C<$vec> only takes up as many bits as it needs. For
+instance, if you had 16 entries in C<@ints>, C<$vec> only needs two
+bytes to store them (not counting the scalar variable overhead).
+
+Here's how, given a vector in C<$vec>, you can get those bits into
+your C<@ints> array:
+
+ sub bitvec_to_list {
+ my $vec = shift;
+ my @ints;
+ # Find null-byte density then select best algorithm
+ if ($vec =~ tr/\0// / length $vec > 0.95) {
+ use integer;
+ my $i;
+
+ # This method is faster with mostly null-bytes
+ while($vec =~ /[^\0]/g ) {
+ $i = -9 + 8 * pos $vec;
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ push @ints, $i if vec($vec, ++$i, 1);
+ }
+ }
+ else {
+ # This method is a fast general algorithm
+ use integer;
+ my $bits = unpack "b*", $vec;
+ push @ints, 0 if $bits =~ s/^(\d)// && $1;
+ push @ints, pos $bits while($bits =~ /1/g);
+ }
+
+ return \@ints;
+ }
This method gets faster the more sparse the bit vector is.
(Courtesy of Tim Bunce and Winfried Koenig.)
-Here's a demo on how to use vec():
+You can make the while loop a lot shorter with this suggestion
+from Benjamin Goldberg:
+
+ while($vec =~ /[^\0]+/g ) {
+ push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
+ }
+
+Or use the CPAN module C<Bit::Vector>:
+
+ $vector = Bit::Vector->new($num_of_bits);
+ $vector->Index_List_Store(@ints);
+ @ints = $vector->Index_List_Read();
- # vec demo
- $vector = "\xff\x0f\xef\xfe";
- print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
+C<Bit::Vector> provides efficient methods for bit vector, sets of
+small integers and "big int" math.
+
+Here's a more extensive illustration using vec():
+
+ # vec demo
+ $vector = "\xff\x0f\xef\xfe";
+ print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", $vector), "\n";
- $is_set = vec($vector, 23, 1);
- print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
- pvec($vector);
-
- set_vec(1,1,1);
- set_vec(3,1,1);
- set_vec(23,1,1);
-
- set_vec(3,1,3);
- set_vec(3,2,3);
- set_vec(3,4,3);
- set_vec(3,4,7);
- set_vec(3,8,3);
- set_vec(3,8,7);
-
- set_vec(0,32,17);
- set_vec(1,32,17);
-
- sub set_vec {
- my ($offset, $width, $value) = @_;
- my $vector = '';
- vec($vector, $offset, $width) = $value;
- print "offset=$offset width=$width value=$value\n";
+ $is_set = vec($vector, 23, 1);
+ print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
pvec($vector);
- }
- sub pvec {
- my $vector = shift;
- my $bits = unpack("b*", $vector);
- my $i = 0;
- my $BASE = 8;
-
- print "vector length in bytes: ", length($vector), "\n";
- @bytes = unpack("A8" x length($vector), $bits);
- print "bits are: @bytes\n\n";
- }
+ set_vec(1,1,1);
+ set_vec(3,1,1);
+ set_vec(23,1,1);
+
+ set_vec(3,1,3);
+ set_vec(3,2,3);
+ set_vec(3,4,3);
+ set_vec(3,4,7);
+ set_vec(3,8,3);
+ set_vec(3,8,7);
+
+ set_vec(0,32,17);
+ set_vec(1,32,17);
+
+ sub set_vec {
+ my ($offset, $width, $value) = @_;
+ my $vector = '';
+ vec($vector, $offset, $width) = $value;
+ print "offset=$offset width=$width value=$value\n";
+ pvec($vector);
+ }
+
+ sub pvec {
+ my $vector = shift;
+ my $bits = unpack("b*", $vector);
+ my $i = 0;
+ my $BASE = 8;
+
+ print "vector length in bytes: ", length($vector), "\n";
+ @bytes = unpack("A8" x length($vector), $bits);
+ print "bits are: @bytes\n\n";
+ }
=head2 Why does defined() return true on empty arrays and hashes?
=head2 How do I process an entire hash?
-Use the each() function (see L<perlfunc/each>) if you don't care
-whether it's sorted:
-
- while ( ($key, $value) = each %hash) {
- print "$key = $value\n";
- }
-
-If you want it sorted, you'll have to use foreach() on the result of
-sorting the keys as shown in an earlier question.
+(contributed by brian d foy)
+
+There are a couple of ways that you can process an entire hash. You
+can get a list of keys, then go through each key, or grab a one
+key-value pair at a time.
+
+To go through all of the keys, use the C<keys> function. This extracts
+all of the keys of the hash and gives them back to you as a list. You
+can then get the value through the particular key you're processing:
+
+ foreach my $key ( keys %hash ) {
+ my $value = $hash{$key}
+ ...
+ }
+
+Once you have the list of keys, you can process that list before you
+process the hash elements. For instance, you can sort the keys so you
+can process them in lexical order:
+
+ foreach my $key ( sort keys %hash ) {
+ my $value = $hash{$key}
+ ...
+ }
+
+Or, you might want to only process some of the items. If you only want
+to deal with the keys that start with C<text:>, you can select just
+those using C<grep>:
+
+ foreach my $key ( grep /^text:/, keys %hash ) {
+ my $value = $hash{$key}
+ ...
+ }
+
+If the hash is very large, you might not want to create a long list of
+keys. To save some memory, you can grab one key-value pair at a time using
+C<each()>, which returns a pair you haven't seen yet:
+
+ while( my( $key, $value ) = each( %hash ) ) {
+ ...
+ }
+
+The C<each> operator returns the pairs in apparently random order, so if
+ordering matters to you, you'll have to stick with the C<keys> method.
+
+The C<each()> operator can be a bit tricky though. You can't add or
+delete keys of the hash while you're using it without possibly
+skipping or re-processing some pairs after Perl internally rehashes
+all of the elements. Additionally, a hash has only one iterator, so if
+you use C<keys>, C<values>, or C<each> on the same hash, you can reset
+the iterator and mess up your processing. See the C<each> entry in
+L<perlfunc> for more details.
+
+=head2 How do I merge two hashes?
+X<hash> X<merge> X<slice, hash>
+
+(contributed by brian d foy)
+
+Before you decide to merge two hashes, you have to decide what to do
+if both hashes contain keys that are the same and if you want to leave
+the original hashes as they were.
+
+If you want to preserve the original hashes, copy one hash (C<%hash1>)
+to a new hash (C<%new_hash>), then add the keys from the other hash
+(C<%hash2> to the new hash. Checking that the key already exists in
+C<%new_hash> gives you a chance to decide what to do with the
+duplicates:
+
+ my %new_hash = %hash1; # make a copy; leave %hash1 alone
+
+ foreach my $key2 ( keys %hash2 )
+ {
+ if( exists $new_hash{$key2} )
+ {
+ warn "Key [$key2] is in both hashes!";
+ # handle the duplicate (perhaps only warning)
+ ...
+ next;
+ }
+ else
+ {
+ $new_hash{$key2} = $hash2{$key2};
+ }
+ }
+
+If you don't want to create a new hash, you can still use this looping
+technique; just change the C<%new_hash> to C<%hash1>.
+
+ foreach my $key2 ( keys %hash2 )
+ {
+ if( exists $hash1{$key2} )
+ {
+ warn "Key [$key2] is in both hashes!";
+ # handle the duplicate (perhaps only warning)
+ ...
+ next;
+ }
+ else
+ {
+ $hash1{$key2} = $hash2{$key2};
+ }
+ }
+
+If you don't care that one hash overwrites keys and values from the other, you
+could just use a hash slice to add one hash to another. In this case, values
+from C<%hash2> replace values from C<%hash1> when they have keys in common:
+
+ @hash1{ keys %hash2 } = values %hash2;
=head2 What happens if I add or remove keys from a hash while iterating over it?
-Don't do that. :-)
+(contributed by brian d foy)
-[lwall] In Perl 4, you were not allowed to modify a hash at all while
-iterating over it. In Perl 5 you can delete from it, but you still
-can't add to it, because that might cause a doubling of the hash table,
-in which half the entries get copied up to the new top half of the
-table, at which point you've totally bamboozled the iterator code.
-Even if the table doesn't double, there's no telling whether your new
-entry will be inserted before or after the current iterator position.
+The easy answer is "Don't do that!"
-Either treasure up your changes and make them after the iterator finishes
-or use keys to fetch all the old keys at once, and iterate over the list
-of keys.
+If you iterate through the hash with each(), you can delete the key
+most recently returned without worrying about it. If you delete or add
+other keys, the iterator may skip or double up on them since perl
+may rearrange the hash table. See the
+entry for C<each()> in L<perlfunc>.
=head2 How do I look up a hash element by value?
Create a reverse hash:
- %by_value = reverse %by_key;
- $key = $by_value{$value};
+ %by_value = reverse %by_key;
+ $key = $by_value{$value};
That's not particularly efficient. It would be more space-efficient
to use:
- while (($key, $value) = each %by_key) {
- $by_value{$value} = $key;
- }
+ while (($key, $value) = each %by_key) {
+ $by_value{$value} = $key;
+ }
If your hash could have repeated values, the methods above will only find
one of the associated keys. This may or may not worry you. If it does
worry you, you can always reverse the hash into a hash of arrays instead:
- while (($key, $value) = each %by_key) {
- push @{$key_list_by_value{$value}}, $key;
- }
+ while (($key, $value) = each %by_key) {
+ push @{$key_list_by_value{$value}}, $key;
+ }
=head2 How can I know how many entries are in a hash?
-If you mean how many keys, then all you have to do is
-take the scalar sense of the keys() function:
+(contributed by brian d foy)
+
+This is very similar to "How do I process an entire hash?", also in
+L<perlfaq4>, but a bit simpler in the common cases.
+
+You can use the C<keys()> built-in function in scalar context to find out
+have many entries you have in a hash:
+
+ my $key_count = keys %hash; # must be scalar context!
+
+If you want to find out how many entries have a defined value, that's
+a bit different. You have to check each value. A C<grep> is handy:
- $num_keys = scalar keys %hash;
+ my $defined_value_count = grep { defined } values %hash;
-The keys() function also resets the iterator, which in void context is
-faster for tied hashes than would be iterating through the whole
-hash, one key-value pair at a time.
+You can use that same structure to count the entries any way that
+you like. If you want the count of the keys with vowels in them,
+you just test for that instead:
+
+ my $vowel_count = grep { /[aeiou]/ } keys %hash;
+
+The C<grep> in scalar context returns the count. If you want the list
+of matching items, just use it in list context instead:
+
+ my @defined_values = grep { defined } values %hash;
+
+The C<keys()> function also resets the iterator, which means that you may
+see strange results if you use this between uses of other hash operators
+such as C<each()>.
=head2 How do I sort a hash (optionally by value instead of key)?
-Internally, hashes are stored in a way that prevents you from imposing
-an order on key-value pairs. Instead, you have to sort a list of the
-keys or values:
+(contributed by brian d foy)
+
+To sort a hash, start with the keys. In this example, we give the list of
+keys to the sort function which then compares them ASCIIbetically (which
+might be affected by your locale settings). The output list has the keys
+in ASCIIbetical order. Once we have the keys, we can go through them to
+create a report which lists the keys in ASCIIbetical order.
+
+ my @keys = sort { $a cmp $b } keys %hash;
+
+ foreach my $key ( @keys )
+ {
+ printf "%-20s %6d\n", $key, $hash{$key};
+ }
+
+We could get more fancy in the C<sort()> block though. Instead of
+comparing the keys, we can compute a value with them and use that
+value as the comparison.
+
+For instance, to make our report order case-insensitive, we use
+the C<\L> sequence in a double-quoted string to make everything
+lowercase. The C<sort()> block then compares the lowercased
+values to determine in which order to put the keys.
+
+ my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
- @keys = sort keys %hash; # sorted by key
- @keys = sort {
- $hash{$a} cmp $hash{$b}
- } keys %hash; # and by value
+Note: if the computation is expensive or the hash has many elements,
+you may want to look at the Schwartzian Transform to cache the
+computation results.
-Here we'll do a reverse numeric sort by value, and if two keys are
-identical, sort by length of key, or if that fails, by straight ASCII
-comparison of the keys (well, possibly modified by your locale--see
-L<perllocale>).
+If we want to sort by the hash value instead, we use the hash key
+to look it up. We still get out a list of keys, but this time they
+are ordered by their value.
- @keys = sort {
- $hash{$b} <=> $hash{$a}
- ||
- length($b) <=> length($a)
- ||
- $a cmp $b
- } keys %hash;
+ my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
+
+From there we can get more complex. If the hash values are the same,
+we can provide a secondary sort on the hash key.
+
+ my @keys = sort {
+ $hash{$a} <=> $hash{$b}
+ or
+ "\L$a" cmp "\L$b"
+ } keys %hash;
=head2 How can I always keep my hash sorted?
+X<hash tie sort DB_File Tie::IxHash>
-You can look into using the DB_File module and tie() using the
-$DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
-The Tie::IxHash module from CPAN might also be instructive.
+You can look into using the C<DB_File> module and C<tie()> using the
+C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory
+Databases">. The C<Tie::IxHash> module from CPAN might also be
+instructive. Although this does keep your hash sorted, you might not
+like the slow down you suffer from the tie interface. Are you sure you
+need to do this? :)
=head2 What's the difference between "delete" and "undef" with hashes?
-Hashes are pairs of scalars: the first is the key, the second is the
-value. The key will be coerced to a string, although the value can be
-any kind of scalar: string, number, or reference. If a key C<$key> is
-present in the array, C<exists($key)> will return true. The value for
-a given key can be C<undef>, in which case C<$array{$key}> will be
-C<undef> while C<$exists{$key}> will return true. This corresponds to
-(C<$key>, C<undef>) being in the hash.
+Hashes contain pairs of scalars: the first is the key, the
+second is the value. The key will be coerced to a string,
+although the value can be any kind of scalar: string,
+number, or reference. If a key C<$key> is present in
+%hash, C<exists($hash{$key})> will return true. The value
+for a given key can be C<undef>, in which case
+C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
+will return true. This corresponds to (C<$key>, C<undef>)
+being in the hash.
-Pictures help... here's the C<%ary> table:
+Pictures help... here's the C<%hash> table:
keys values
+------+------+
And these conditions hold
- $ary{'a'} is true
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is true
- exists $ary{'a'} is true (Perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $hash{'a'} is true
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is true
+ exists $hash{'a'} is true (Perl 5 only)
+ grep ($_ eq 'a', keys %hash) is true
If you now say
- undef $ary{'a'}
+ undef $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is FALSE
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is FALSE
- exists $ary{'a'} is true (Perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $hash{'a'} is FALSE
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is FALSE
+ exists $hash{'a'} is true (Perl 5 only)
+ grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
- delete $ary{'a'}
+ delete $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is false
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is false
- exists $ary{'a'} is FALSE (Perl5 only)
- grep ($_ eq 'a', keys %ary) is FALSE
+ $hash{'a'} is false
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is false
+ exists $hash{'a'} is FALSE (Perl 5 only)
+ grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
=head2 Why don't my tied hashes make the defined/exists distinction?
-They may or may not implement the EXISTS() and DEFINED() methods
-differently. For example, there isn't the concept of undef with hashes
-that are tied to DBM* files. This means the true/false tables above
-will give different results when used on such a hash. It also means
-that exists and defined do the same thing with a DBM* file, and what
-they end up doing is not what they do with ordinary hashes.
+This depends on the tied hash's implementation of EXISTS().
+For example, there isn't the concept of undef with hashes
+that are tied to DBM* files. It also means that exists() and
+defined() do the same thing with a DBM* file, and what they
+end up doing is not what they do with ordinary hashes.
=head2 How do I reset an each() operation part-way through?
-Using C<keys %hash> in scalar context returns the number of keys in
-the hash I<and> resets the iterator associated with the hash. You may
-need to do this if you use C<last> to exit a loop early so that when you
-re-enter it, the hash iterator has been reset.
+(contributed by brian d foy)
+
+You can use the C<keys> or C<values> functions to reset C<each>. To
+simply reset the iterator used by C<each> without doing anything else,
+use one of them in void context:
+
+ keys %hash; # resets iterator, nothing else.
+ values %hash; # resets iterator, nothing else.
+
+See the documentation for C<each> in L<perlfunc>.
=head2 How can I get the unique keys from two hashes?
First you extract the keys from the hashes into lists, then solve
the "removing duplicates" problem described above. For example:
- %seen = ();
- for $element (keys(%foo), keys(%bar)) {
- $seen{$element}++;
- }
- @uniq = keys %seen;
+ %seen = ();
+ for $element (keys(%foo), keys(%bar)) {
+ $seen{$element}++;
+ }
+ @uniq = keys %seen;
Or more succinctly:
- @uniq = keys %{{%foo,%bar}};
+ @uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
- %seen = ();
- while (defined ($key = each %foo)) {
- $seen{$key}++;
- }
- while (defined ($key = each %bar)) {
- $seen{$key}++;
- }
- @uniq = keys %seen;
+ %seen = ();
+ while (defined ($key = each %foo)) {
+ $seen{$key}++;
+ }
+ while (defined ($key = each %bar)) {
+ $seen{$key}++;
+ }
+ @uniq = keys %seen;
=head2 How can I store a multidimensional array in a DBM file?
=head2 How can I make my hash remember the order I put elements into it?
-Use the Tie::IxHash from CPAN.
+Use the C<Tie::IxHash> from CPAN.
- use Tie::IxHash;
- tie(%myhash, Tie::IxHash);
- for ($i=0; $i<20; $i++) {
- $myhash{$i} = 2*$i;
- }
- @keys = keys %myhash;
- # @keys = (0,1,2,3,...)
+ use Tie::IxHash;
+
+ tie my %myhash, 'Tie::IxHash';
+
+ for (my $i=0; $i<20; $i++) {
+ $myhash{$i} = 2*$i;
+ }
+
+ my @keys = keys %myhash;
+ # @keys = (0,1,2,3,...)
=head2 Why does passing a subroutine an undefined element in a hash create it?
-If you say something like:
+(contributed by brian d foy)
+
+Are you using a really old version of Perl?
+
+Normally, accessing a hash key's value for a nonexistent key will
+I<not> create the key.
+
+ my %hash = ();
+ my $value = $hash{ 'foo' };
+ print "This won't print\n" if exists $hash{ 'foo' };
+
+Passing C<$hash{ 'foo' }> to a subroutine used to be a special case, though.
+Since you could assign directly to C<$_[0]>, Perl had to be ready to
+make that assignment so it created the hash key ahead of time:
+
+ my_sub( $hash{ 'foo' } );
+ print "This will print before 5.004\n" if exists $hash{ 'foo' };
- somefunc($hash{"nonesuch key here"});
+ sub my_sub {
+ # $_[0] = 'bar'; # create hash key in case you do this
+ 1;
+ }
-Then that element "autovivifies"; that is, it springs into existence
-whether you store something there or not. That's because functions
-get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
-it has to be ready to write it back into the caller's version.
+Since Perl 5.004, however, this situation is a special case and Perl
+creates the hash key only when you make the assignment:
-This has been fixed as of Perl5.004.
+ my_sub( $hash{ 'foo' } );
+ print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
-Normally, merely accessing a key's value for a nonexistent key does
-I<not> cause that key to be forever there. This is different than
-awk's behavior.
+ sub my_sub {
+ $_[0] = 'bar';
+ }
+
+However, if you want the old behavior (and think carefully about that
+because it's a weird side effect), you can pass a hash slice instead.
+Perl 5.004 didn't make this a special case:
+
+ my_sub( @hash{ qw/foo/ } );
=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Usually a hash ref, perhaps like this:
- $record = {
- NAME => "Jason",
- EMPNO => 132,
- TITLE => "deputy peon",
- AGE => 23,
- SALARY => 37_000,
- PALS => [ "Norbert", "Rhys", "Phineas"],
- };
+ $record = {
+ NAME => "Jason",
+ EMPNO => 132,
+ TITLE => "deputy peon",
+ AGE => 23,
+ SALARY => 37_000,
+ PALS => [ "Norbert", "Rhys", "Phineas"],
+ };
References are documented in L<perlref> and the upcoming L<perlreftut>.
Examples of complex data structures are given in L<perldsc> and
=head2 How can I use a reference as a hash key?
-You can't do this directly, but you could use the standard Tie::Refhash
-module distributed with Perl.
+(contributed by brian d foy and Ben Morrow)
-=head1 Data: Misc
+Hash keys are strings, so you can't really use a reference as the key.
+When you try to do that, perl turns the reference into its stringified
+form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get
+back the reference from the stringified form, at least without doing
+some extra work on your own.
-=head2 How do I handle binary data correctly?
+Remember that the entry in the hash will still be there even if
+the referenced variable goes out of scope, and that it is entirely
+possible for Perl to subsequently allocate a different variable at
+the same address. This will mean a new variable might accidentally
+be associated with the value for an old.
-Perl is binary clean, so this shouldn't be a problem. For example,
-this works fine (assuming the files are found):
+If you have Perl 5.10 or later, and you just want to store a value
+against the reference for lookup later, you can use the core
+Hash::Util::Fieldhash module. This will also handle renaming the
+keys if you use multiple threads (which causes all variables to be
+reallocated at new addresses, changing their stringification), and
+garbage-collecting the entries when the referenced variable goes out
+of scope.
- if (`cat /vmunix` =~ /gzip/) {
- print "Your kernel is GNU-zip enabled!\n";
- }
+If you actually need to be able to get a real reference back from
+each hash entry, you can use the Tie::RefHash module, which does the
+required work for you.
+
+=head1 Data: Misc
+
+=head2 How do I handle binary data correctly?
-On less elegant (read: Byzantine) systems, however, you have
-to play tedious games with "text" versus "binary" files. See
-L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
-systems are curses out of Microsoft, who seem to be committed to putting
-the backward into backward compatibility.
+Perl is binary clean, so it can handle binary data just fine.
+On Windows or DOS, however, you have to use C<binmode> for binary
+files to avoid conversions for line endings. In general, you should
+use C<binmode> any time you want to work with binary data.
-If you're concerned about 8-bit ASCII data, then see L<perllocale>.
+Also see L<perlfunc/"binmode"> or L<perlopentut>.
+If you're concerned about 8-bit textual data then see L<perllocale>.
If you want to deal with multibyte characters, however, there are
some gotchas. See the section on Regular Expressions.
Assuming that you don't care about IEEE notations like "NaN" or
"Infinity", you probably just want to use a regular expression.
- if (/\D/) { print "has nondigits\n" }
- if (/^\d+$/) { print "is a whole number\n" }
- if (/^-?\d+$/) { print "is an integer\n" }
- if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
- if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
- if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
- if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
- { print "a C float" }
-
-If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
-function. Its semantics are somewhat cumbersome, so here's a C<getnum>
-wrapper function for more convenient access. This function takes
-a string and returns the number it found, or C<undef> for input that
-isn't a C float. The C<is_numeric> function is a front end to C<getnum>
-if you just want to say, ``Is this a float?''
-
- sub getnum {
- use POSIX qw(strtod);
- my $str = shift;
- $str =~ s/^\s+//;
- $str =~ s/\s+$//;
- $! = 0;
- my($num, $unparsed) = strtod($str);
- if (($str eq '') || ($unparsed != 0) || $!) {
- return undef;
- } else {
- return $num;
- }
- }
-
- sub is_numeric { defined getnum($_[0]) }
-
-Or you could check out the String::Scanf module on CPAN instead. The
-POSIX module (part of the standard Perl distribution) provides the
-C<strtod> and C<strtol> for converting strings to double and longs,
-respectively.
+ if (/\D/) { print "has nondigits\n" }
+ if (/^\d+$/) { print "is a whole number\n" }
+ if (/^-?\d+$/) { print "is an integer\n" }
+ if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
+ if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
+ if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
+ if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
+ { print "a C float\n" }
+
+There are also some commonly used modules for the task.
+L<Scalar::Util> (distributed with 5.8) provides access to perl's
+internal function C<looks_like_number> for determining whether a
+variable looks like a number. L<Data::Types> exports functions that
+validate data types using both the above and other regular
+expressions. Thirdly, there is C<Regexp::Common> which has regular
+expressions to match various types of numbers. Those three modules are
+available from the CPAN.
+
+If you're on a POSIX system, Perl supports the C<POSIX::strtod>
+function. Its semantics are somewhat cumbersome, so here's a
+C<getnum> wrapper function for more convenient access. This function
+takes a string and returns the number it found, or C<undef> for input
+that isn't a C float. The C<is_numeric> function is a front end to
+C<getnum> if you just want to say, "Is this a float?"
+
+ sub getnum {
+ use POSIX qw(strtod);
+ my $str = shift;
+ $str =~ s/^\s+//;
+ $str =~ s/\s+$//;
+ $! = 0;
+ my($num, $unparsed) = strtod($str);
+ if (($str eq '') || ($unparsed != 0) || $!) {
+ return undef;
+ }
+ else {
+ return $num;
+ }
+ }
+
+ sub is_numeric { defined getnum($_[0]) }
+
+Or you could check out the L<String::Scanf> module on the CPAN
+instead. The C<POSIX> module (part of the standard Perl distribution)
+provides the C<strtod> and C<strtol> for converting strings to double
+and longs, respectively.
=head2 How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM modules.
-See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
-Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
-Storable is part of the standard distribution. Here's one example using
-Storable's C<store> and C<retrieve> functions:
+See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw>
+or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part
+of the standard distribution. Here's one example using C<Storable>'s C<store>
+and C<retrieve> functions:
- use Storable;
- store(\%hash, "filename");
+ use Storable;
+ store(\%hash, "filename");
- # later on...
- $href = retrieve("filename"); # by ref
- %hash = %{ retrieve("filename") }; # direct to hash
+ # later on...
+ $href = retrieve("filename"); # by ref
+ %hash = %{ retrieve("filename") }; # direct to hash
=head2 How do I print out or copy a recursive data structure?
-The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
-for printing out data structures. The Storable module, found on CPAN,
-provides a function called C<dclone> that recursively copies its argument.
+The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great
+for printing out data structures. The C<Storable> module on CPAN (or the
+5.8 release of Perl), provides a function called C<dclone> that recursively
+copies its argument.
- use Storable qw(dclone);
- $r2 = dclone($r1);
+ use Storable qw(dclone);
+ $r2 = dclone($r1);
-Where $r1 can be a reference to any kind of data structure you'd like.
+Where C<$r1> can be a reference to any kind of data structure you'd like.
It will be deeply copied. Because C<dclone> takes and returns references,
you'd have to add extra punctuation if you had a hash of arrays that
you wanted to copy.
- %newhash = %{ dclone(\%oldhash) };
+ %newhash = %{ dclone(\%oldhash) };
=head2 How do I define methods for every class/object?
-Use the UNIVERSAL class (see L<UNIVERSAL>).
+(contributed by Ben Morrow)
+
+You can use the C<UNIVERSAL> class (see L<UNIVERSAL>). However, please
+be very careful to consider the consequences of doing this: adding
+methods to every object is very likely to have unintended
+consequences. If possible, it would be better to have all your object
+inherit from some common base class, or to use an object system like
+Moose that supports roles.
=head2 How do I verify a credit card checksum?
-Get the Business::CreditCard module from CPAN.
+Get the C<Business::CreditCard> module from CPAN.
=head2 How do I pack arrays of doubles or floats for XS code?
-The kgbpack.c code in the PGPLOT module on CPAN does just this.
+The arrays.h/arrays.c code in the C<PGPLOT> module on CPAN does just this.
If you're doing a lot of float or double processing, consider using
-the PDL module from CPAN instead--it makes number-crunching easy.
+the C<PDL> module from CPAN instead--it makes number-crunching easy.
+
+See L<http://search.cpan.org/dist/PGPLOT> for the code.
+
+=head1 REVISION
+
+Revision: $Revision$
+
+Date: $Date$
+
+See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
https://perl5.git.perl.org / perl5.git / blobdiff