=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date: 1997/04/24 22:43:57 $)
+perlfaq4 - Data Manipulation ($Revision: 1.8 $, $Date: 2001/11/09 08:06:04 $)
=head1 DESCRIPTION
-The section of the FAQ answers question related to the manipulation
+The section of the FAQ answers questions related to the manipulation
of data as numbers, dates, strings, arrays, hashes, and miscellaneous
data issues.
=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
+The infinite set that a mathematician thinks of as the real numbers can
+only be approximated on a computer, since the computer only has a finite
+number of bits to store an infinite number of, um, numbers.
+
Internally, your computer represents floating-point numbers in binary.
-Floating-point numbers read in from a file, or appearing as literals
-in your program, are converted from their decimal floating-point
-representation (eg, 19.95) to the internal binary representation.
+Floating-point numbers read in from a file or appearing as literals
+in your program are converted from their decimal floating-point
+representation (eg, 19.95) to an internal binary representation.
However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly represented as a
When a floating-point number gets printed, the binary floating-point
representation is converted back to decimal. These decimal numbers
are displayed in either the format you specify with printf(), or the
-current output format for numbers
(see L<perlvar/"$#"> if you use
+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<$#> yourself is deprecated.
+Perl4. Changing C<$#> yourself is deprecated.)
This affects B<all> computer languages that represent decimal
floating-point numbers in binary, not just Perl. Perl provides
(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">.
=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. 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. oct() interprets
+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 all want permissions in octal.
+umask(), or sysopen(), which by widespread tradition typically take
+permissions in octal.
- chmod(644, $file); # WRONG -- perl -w catches this
+ chmod(644, $file); # WRONG
chmod(0644, $file); # right
-=head2 Does perl have a round function? What about ceil() and floor()?
-Trig functions?
+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.
-For rounding to a certain number of digits, sprintf() or printf() is
-usually the easiest route.
+=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
-The POSIX module (part of the standard perl distribution) implements
+Remember that int() merely truncates toward 0. For rounding to a
+certain number of digits, sprintf() or printf() is usually the easiest
+route.
+
+ 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.
-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
+ use POSIX;
+ $ceil = ceil(3.5); # 4
+ $floor = floor(3.5); # 3
+
+In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
+module. With 5.004, the Math::Trig module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the Math::Complex module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
being used by Perl, but to instead implement the rounding function you
need yourself.
-=head2 How do I convert bits into ints?
+To see why, notice how you'll still have an issue on half-way-point
+alternation:
+
+ for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
+
+ 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:
+
+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 below use the Bit::Vector module from CPAN.
+The reason you might choose 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 B<How do I convert Hexadecimal into decimal:>
+
+Using perl's built in conversion of 0x notation:
+
+ $int = 0xDEADBEEF;
+ $dec = sprintf("%d", $int);
+
+Using the hex function:
+
+ $int = hex("DEADBEEF");
+ $dec = sprintf("%d", $int);
+
+Using pack:
+
+ $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
+ $dec = sprintf("%d", $int);
+
+Using the CPAN module Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
+ $dec = $vec->to_Dec();
+
+=item B<How do I convert from decimal to hexadecimal:>
+
+Using sprint:
+
+ $hex = sprintf("%X", 3735928559);
+
+Using unpack
+
+ $hex = unpack("H*", pack("N", 3735928559));
+
+Using Bit::Vector
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $hex = $vec->to_Hex();
+
+And 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 B<How do I convert from octal to decimal:>
+
+Using Perl's built in conversion of numbers with leading zeros:
+
+ $int = 033653337357; # note the leading 0!
+ $dec = sprintf("%d", $int);
+
+Using the oct function:
+
+ $int = oct("33653337357");
+ $dec = sprintf("%d", $int);
+
+Using Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new(32);
+ $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
+ $dec = $vec->to_Dec();
+
+=item B<How do I convert from decimal to octal:>
+
+Using sprintf:
+
+ $oct = sprintf("%o", 3735928559);
+
+Using Bit::Vector
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $oct = reverse join('', $vec->Chunk_List_Read(3));
+
+=item B<How do I convert from binary to decimal:>
+
+Using pack and ord
+
+ $decimal = ord(pack('B8', '10110110'));
+
+Using pack and unpack for larger strings
+
+ $int = unpack("N", pack("B32",
+ substr("0" x 32 . "11110101011011011111011101111", -32)));
+ $dec = sprintf("%d", $int);
-To turn a string of 1s and 0s like '10110110' into a scalar containing
-its binary value, use the pack() function (documented in
-L<perlfunc/"pack">):
+ # substr() is used to left pad a 32 character string with zeros.
- $decimal = pack('B8', '10110110');
+Using Bit::Vector:
-Here's an example of going the other way:
+ $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
+ $dec = $vec->to_Dec();
- $binary_string = join('', unpack('B*', "\x29"));
+=item B<How do I convert from decimal to binary:>
+
+Using unpack;
+
+ $bin = unpack("B*", pack("N", 3735928559));
+
+Using 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 behavior of binary arithmetic operators depends on whether they're
+used on numbers or strings. The operators treat a string as a series
+of bits and work with that (the string C<"3"> is the bit pattern
+C<00110011>). The operators work with the binary form of a number
+(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
+(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") {
+ # ...
+ }
+
+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]/) {
+ # ...
+ }
=head2 How do I multiply matrices?
results:
foreach $iterator (@array) {
- &my_func($iterator);
+ some_func($iterator);
}
To call a function on each integer in a (small) range, you B<can> use:
- @results = map { &my_func($_) } (5 .. 25);
+ @results = map { some_func($_) } (5 .. 25);
but you should be aware that the C<..> operator creates an array of
all integers in the range. This can take a lot of memory for large
@results = ();
for ($i=5; $i < 500_005; $i++) {
- push(@results, &my_func($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));
}
+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?
-The short explanation is that you're getting pseudorandom numbers, not
-random ones, because that's how these things work. A longer
-explanation is available on
-http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
-Phoenix.
-
-You should also check out the Math::TrulyRandom module from CPAN.
+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.
+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.
+
+Computers are good at being predictable and bad at being random
+(despite appearances caused by bugs in your programs :-).
+http://www.cpan.org/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.''
+
+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
+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/ .
=head1 Data: Dates
$day_of_year = (localtime(time()))[7];
-or more legibly (in 5.004 or higher):
-
- use Time::localtime;
- $day_of_year = localtime(time())->yday;
+=head2 How do I find the current century or millennium?
-You can find the week of the year by dividing this by 7:
+Use the following simple functions:
- $week_of_year = int($day_of_year / 7);
+ sub get_century {
+ return int((((localtime(shift || time))[5] + 1999))/100);
+ }
+ sub get_millennium {
+ return 1+int((((localtime(shift || time))[5] + 1899))/1000);
+ }
-Of course, this believes that weeks start at zero.
+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.
-=head2 How can I compare two date strings?
+=head2 How can I compare two dates and find the difference?
-Use the Date::Manip or Date::DateCalc modules from CPAN.
+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.
=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 timelocal in the standard
-Time::Local module. Otherwise, you should look into one of the
-Date modules from CPAN.
+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.
=head2 How can I find the Julian Day?
-Neither Date::Manip nor Date::DateCalc deal with Julian days.
-Instead, there is an example of Julian date calculation in
-http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz,
-which should help.
-
-=head2 Does Perl have a year 2000 problem?
-
-Not unless you use Perl to create one. 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).
-The year returned by these functions when used in an array 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 a scalar context they return
+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.)
+
+=head2 How do I find yesterday's date?
+
+The C<time()> function returns the current time in seconds since the
+epoch. Take twenty-four hours off that:
+
+ $yesterday = time() - ( 24 * 60 * 60 );
+
+Then you can pass this to C<localtime()> and get the individual year,
+month, day, hour, minute, seconds values.
+
+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.
+
+ 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?
+
+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
+use it, however, probably are not.
+
+Long answer: The question belies a true understanding of the issue.
+Perl is just as Y2K compliant as your pencil--no more, and no less.
+Can you use your pencil to write a non-Y2K-compliant memo? Of course
+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.
+
+When gmtime() and localtime() are used in scalar context they return
a timestamp string that contains a fully-expanded year. For example,
C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
2001". There's no year 2000 problem here.
+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
+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, email
+with auxiliary logic. See the more specific questions (numbers, mail
addresses, etc.) for details.
=head2 How do I unescape a string?
-It depends just what you mean by "escape". URL escapes are dealt with
-in L<perlfaq9>. Shell escapes with the backslash (\)
-character are removed with:
+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;
-Note that this won't expand \n or \t or any other special escapes.
+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 "abbcccd" into "abccd":
+To turn C<"abbcccd"> into C<"abccd">:
+
+ s/(.)\1/$1/g; # add /s to include newlines
- s/(.)\1/$1/g;
+Here's a solution that turns "abbcccd" to "abcd":
+
+ y///cs; # y == tr, but shorter :-)
=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 a list context) into a string:
+a subroutine call (in list context) into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
print "That yields ${\($n + 5)} widgets\n";
-See also "How can I expand variables in text strings?" in this section
-of the FAQ.
+Version 5.004 of Perl had a bug that gave list context to the
+expression in C<${...}>, but this is fixed in version 5.005.
+
+See also ``How can I expand variables in text strings?'' in this
+section of the FAQ.
=head2 How do I find matching/nesting anything?
-This isn't something that can be tackled in one regular expression, no
-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 parser.
+This isn't something that can be done in one regular expression, no
+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
+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.
+
+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
+ }
+
+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:
+
+ # $_ contains the string to parse
+ # BEGIN and END are the opening and closing markers for the
+ # nested text.
+
+ @( = ('(','');
+ @) = (')','');
+ ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
+ @$ = (eval{/$re/},$@!~/unmatched/);
+ print join("\n",@$[0..$#$]) if( $$[-1] );
=head2 How do I reverse a string?
-Use reverse() in a scalar context, as documented in
+Use reverse() in scalar context, as documented in
L<perlfunc/reverse>.
$reversed = reverse $string;
=head2 How do I expand tabs in a string?
-You can do it the old-fashioned way:
+You can do it yourself:
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 Text::Tabs module (part of the standard Perl
distribution).
use Text::Tabs;
=head2 How do I reformat a paragraph?
-Use Text::Wrap (part of the standard perl distribution):
+Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
-The paragraphs you give to Text::Wrap may not contain embedded
+The paragraphs you give to Text::Wrap should not contain embedded
newlines. 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:
+
+ alias fmt="perl -i -MText::Autoformat -n0777 \
+ -e 'print autoformat $_, {all=>1}' $*"
+
+See the documentation for Text::Autoformat to appreciate its many
+capabilities.
+
=head2 How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
-substr:
+substr():
$first_byte = substr($a, 0, 1);
substr($a, 0, 3) = "Tom";
-Although those with a regexp kind of thought process will likely prefer
+Although those with a pattern matching kind of thought process will
+likely prefer
$a =~ s/^.../Tom/;
=head2 How do I change the Nth occurrence of something?
-You have to keep track. For example, let's say you want
-to change the fifth occurrence of "whoever" or "whomever"
-into "whosoever" or "whomsoever", case insensitively.
+You have to keep track of N yourself. For example, let's say you want
+to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
+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
- }igex;
+ }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";
+ }
+ }
+
+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;
=head2 How can I count the number of occurrences of a substring within a string?
-There are a number of ways, with varying efficiency: If you want a
+There are a number of ways, with varying efficiency. If you want a
count of a certain single character (X) within a string, you can use the
C<tr///> function like so:
- $string = "ThisXlineXhasXsomeXx'sXinXit":
+ $string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
- print "There are $count X charcters in the string";
+ 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
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning "C<don't do it>" into "C<Don'T
-Do It>". Sometimes you might want this, instead (Suggested by Brian
-Foy E<lt>comdog@computerdog.comE<gt>):
+Do It>". Sometimes you might want this. Other times you might need a
+more thorough solution (Suggested by brian d foy):
$string =~ s/ (
(^\w) #at the beginning of the line
$line =~ s/(\w+)/\u\L$1/g;
+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.
+
+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.
+
=head2 How can I split a [character] delimited string except when inside
[character]? (Comma-separated files)
C<"like \"this\"">. Unescaping them is a task addressed earlier in
this section.
-Alternatively, the Text::ParseWords module (part of the standard perl
+Alternatively, the Text::ParseWords module (part of the standard Perl
distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
+There's also a Text::CSV (Comma-Separated Values) module on CPAN.
+
=head2 How do I strip blank space from the beginning/end of a string?
-The simplest approach, albeit not the fastest, is probably like this:
+Although the simplest approach would seem to be
$string =~ s/^\s*(.*?)\s*$/$1/;
-It would be faster to do this in two steps:
+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:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
s/\s+$//;
}
+This idiom takes advantage of the C<foreach> loop's aliasing
+behavior to factor out common code. You can do this
+on several strings at once, or arrays, or even the
+values of a hash if you use a slice:
+
+ # trim whitespace in the scalar, the array,
+ # and all the values in the hash
+ foreach ($scalar, @array, @hash{keys %hash}) {
+ s/^\s+//;
+ s/\s+$//;
+ }
+
+=head2 How do I pad a string with blanks or pad a number with zeroes?
+
+(This answer contributed by Uri Guttman, with kibitzing from
+Bart Lateur.)
+
+In the following examples, C<$pad_len> is the length to which you wish
+to pad the string, C<$text> or C<$num> contains the string to be padded,
+and C<$pad_char> contains the padding character. You can use a single
+character string constant instead of the C<$pad_char> variable if you
+know what it is in advance. And in the same way you can use an integer in
+place of C<$pad_len> if you know the pad length in advance.
+
+The simplest method uses the C<sprintf> function. It can pad on the left
+or right with blanks and on the left with zeroes and it will not
+truncate the result. The C<pack> function can only pad strings on the
+right with blanks and it will truncate the result to a maximum length of
+C<$pad_len>.
+
+ # Left padding a string with blanks (no truncation):
+ $padded = sprintf("%${pad_len}s", $text);
+
+ # Right padding a string with blanks (no truncation):
+ $padded = sprintf("%-${pad_len}s", $text);
+
+ # Left padding a number with 0 (no truncation):
+ $padded = sprintf("%0${pad_len}d", $num);
+
+ # Right padding a string with blanks using pack (will truncate):
+ $padded = pack("A$pad_len",$text);
+
+If you need to pad with a character other than blank or zero you can use
+one of the following methods. They all generate a pad string with the
+C<x> operator and combine that with C<$text>. These methods do
+not truncate C<$text>.
+
+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( $text ) );
+
+Left and right padding with any character, modifying C<$text> directly:
+
+ substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
+ $text .= $pad_char x ( $pad_len - length( $text ) );
+
=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;
+ }
=head2 How do I find the soundex value of a string?
-Use the standard Text::Soundex module distributed with perl.
+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.
=head2 How can I expand variables in text strings?
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
- $text =~ s/\$(\w+)/${$1}/g;
-Before version 5 of perl, this had to be done with a double-eval
-substitution:
+If those were both global variables, then this would
+suffice:
+
+ $text =~ s/\$(\w+)/${$1}/g; # no /e needed
+
+But since they are probably lexicals, or at least, they could
+be, you'd have to do this:
$text =~ s/(\$\w+)/$1/eeg;
+ die if $@; # needed /ee, not /e
+
+It's probably better in the general case to treat those
+variables as entries in some special hash. For example:
-Which is bizarre enough that you'll probably actually need an EEG
-afterwards. :-)
+ %user_defs = (
+ foo => 23,
+ bar => 19,
+ );
+ $text =~ s/\$(\w+)/$user_defs{$1}/g;
-See also "How do I expand function calls in a string?" in this section
+See also ``How do I expand function calls in a string?'' in this section
of the FAQ.
=head2 What's wrong with always quoting "$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.
+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:
number, such as the magical C<++> autoincrement operator or the
syscall() function.
+Stringification also destroys arrays.
+
+ @lines = `command`;
+ print "@lines"; # WRONG - extra blanks
+ print @lines; # right
+
=head2 Why don't my <<HERE documents work?
Check for these three things:
=back
+If you want to indent the text in the here document, you
+can do this:
+
+ # all in one
+ ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
+ your text
+ goes here
+ HERE_TARGET
+
+But the HERE_TARGET must still be flush against the margin.
+If you want that indented also, you'll have to quote
+in the indentation.
+
+ ($quote = <<' FINIS') =~ s/^\s+//gm;
+ ...we will have peace, when you and all your works have
+ perished--and the works of your dark master to whom you
+ 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--/;
+
+A nice general-purpose fixer-upper function for indented here documents
+follows. It expects to be called with a here document as its argument.
+It looks to see whether each line begins with a common substring, and
+if so, strips that substring off. Otherwise, it takes the amount of leading
+whitespace found on the first line and removes that much off each
+subsequent line.
+
+ sub fix {
+ local $_ = shift;
+ my ($white, $leader); # common whitespace and common leading string
+ if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
+ ($white, $leader) = ($2, quotemeta($1));
+ } else {
+ ($white, $leader) = (/^(\s+)/, '');
+ }
+ s/^\s*?$leader(?:$white)?//gm;
+ return $_;
+ }
+
+This works with leading special strings, dynamically determined:
+
+ $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
+ @@@ int
+ @@@ runops() {
+ @@@ SAVEI32(runlevel);
+ @@@ runlevel++;
+ @@@ while ( op = (*op->op_ppaddr)() );
+ @@@ TAINT_NOT;
+ @@@ return 0;
+ @@@ }
+ MAIN_INTERPRETER_LOOP
+
+Or with a fixed amount of leading whitespace, with remaining
+indentation correctly preserved:
+
+ $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,
+ Until it joins some larger way
+ Where many paths and errands meet.
+ And whither then? I cannot say.
+ --Bilbo in /usr/src/perl/pp_ctl.c
+ 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.
+
+As a side note, there's no such thing as a list in scalar context.
+When you say
+
+ $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
+last value to be returned: 9.
+
=head2 What is the difference between $array[1] and @array[1]?
-The former is a scalar value, the latter an array slice, which makes
+The former is a scalar value; the latter an array slice, making
it a list with one (scalar) value. You should use $ when you want a
scalar value (most of the time) and @ when you want a list with one
scalar value in it (very, very rarely; nearly never, in fact).
@bad[0] = `same program that outputs several lines`;
-The B<-w> flag will warn you about these matters.
+The C<use warnings> pragma and the B<-w> flag will warn you about these
+matters.
-=head2 How can I extract just the unique elements of an array?
+=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.
=over 4
-=item a) If @in is sorted, and you want @out to be sorted:
+=item a)
+
+If @in is sorted, and you want @out to be sorted:
+(this assumes all true values in the array)
+
+ $prev = "not equal to $in[0]";
+ @out = grep($_ ne $prev && ($prev = $_, 1), @in);
- $prev = 'nonesuch';
- @out = grep($_ ne $prev && ($prev = $_), @in);
+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 $_ is 0, "", or undef.
-This is nice in that it doesn't use much extra memory,
-simulating uniq(1)'s behavior of removing only adjacent
-duplicates.
+=item b)
-=item b) If you don't know whether @in is sorted:
+If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$saw{$_}++, @in);
-=item c) Like (b), but @in contains only small integers:
+=item c)
+
+Like (b), but @in contains only small integers:
@out = grep(!$saw[$_]++, @in);
-=item d) A way to do (b) without any loops or greps:
+=item d)
+
+A way to do (b) without any loops or greps:
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
-=item e) Like (d), but @in contains only small positive integers:
+=item e)
+
+Like (d), but @in contains only small positive integers:
undef @ary;
@ary[@in] = @in;
- @out = @ary;
+ @out = grep {defined} @ary;
=back
-=head2 How can I tell whether an array contains a certain element?
+But perhaps you should have been using a hash all along, eh?
-There are several ways to approach this. If you are going to make
-this query many times and the values are arbitrary strings, the
-fastest way is probably to invert the original array and keep an
+=head2 How can I tell whether a certain element is contained in a list or array?
+
+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.
+
+That being said, there are several ways to approach this. 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.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
undef @is_tiny_prime;
- for (@primes) { $is_tiny_prime[$_] = 1; }
+ for (@primes) { $is_tiny_prime[$_] = 1 }
+ # or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
Please do not use
- $is_there = grep $_ eq $whatever, @array;
+ ($is_there) = grep $_ eq $whatever, @array;
or worse yet
- $is_there = grep /$whatever/, @array;
+ ($is_there) = grep /$whatever/, @array;
These are slow (checks every element even if the first matches),
inefficient (same reason), and potentially buggy (what if there are
-regexp characters in $whatever?).
+regex characters in $whatever?). If you're only testing once, then
+use:
+
+ $is_there = 0;
+ foreach $elt (@array) {
+ if ($elt eq $elt_to_find) {
+ $is_there = 1;
+ last;
+ }
+ }
+ if ($is_there) { ... }
=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
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.
+
+=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.
+
+ $are_equal = compare_arrays(\@frogs, \@toads);
+
+ sub compare_arrays {
+ my ($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:
+
+ 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";
+
+This approach also works for comparing hashes. Here
+we'll demonstrate two different answers:
+
+ use FreezeThaw qw(cmpStr cmpStrHard);
+
+ %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
+ $a{EXTRA} = \%b;
+ $b{EXTRA} = \%a;
+
+ printf "a and b contain %s hashes\n",
+ cmpStr(\%a, \%b) == 0 ? "the same" : "different";
+
+ printf "a and b contain %s hashes\n",
+ cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
+
+
+The first reports that both those the hashes contain the same data,
+while the second reports that they do not. Which you prefer is left as
+an exercise to the reader.
+
=head2 How do I find the first array element for which a condition is true?
You can use this if you care about the index:
- for ($i=0; $i < @array; $i++) {
+ for ($i= 0; $i < @array; $i++) {
if ($array[$i] eq "Waldo") {
$found_index = $i;
last;
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.
+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.
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.
+to do. For example, imagine a list node like this:
+
+ $node = {
+ VALUE => 42,
+ LINK => undef,
+ };
+
+You could walk the list this way:
+
+ print "List: ";
+ for ($node = $head; $node; $node = $node->{LINK}) {
+ print $node->{VALUE}, " ";
+ }
+ print "\n";
+
+You could add to the list this way:
+
+ my ($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;
+ }
+
+But again, Perl's built-in are virtually always good enough.
=head2 How do I handle circular lists?
=head2 How do I shuffle an array randomly?
-Here's a shuffling algorithm which works its way through the list,
-randomly picking another element to swap the current element with:
+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';
+
+ @shuffled = shuffle(@list);
+
+If not, you can use this:
+
+ # fisher_yates_shuffle
+ # generate a random permutation of an array in place
+ # As in shuffling a deck of cards
+ #
+ sub fisher_yates_shuffle {
+ my $deck = shift; # $deck is a reference to an array
+ my $i = @$deck;
+ while (--$i) {
+ my $j = int rand ($i+1);
+ @$deck[$i,$j] = @$deck[$j,$i];
+ }
+ }
+
+And here is an example of using it:
+
+ #
+ # 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
+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 = ();
push(@new, splice(@old, rand @old, 1));
}
-For large arrays, this avoids a lot of the reshuffling:
-
- srand;
- @new = ();
- @old = 1 .. 10000; # just a demo
- for( @old ){
- my $r = rand @new+1;
- push(@new,$new[$r]);
- $new[$r] = $_;
- }
+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/;
- tr[a-z][A-Z];
+ s/foo/bar/; # change that word
+ y/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
- for (@radii) {
+ 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:
+
+ for $orbit ( @orbits{keys %orbits} ) {
+ ($orbit **= 3) *= (4/3) * 3.14159;
+ }
+
=head2 How do I select a random element from an array?
Use the rand() function (see L<perlfunc/rand>):
+ # at the top of the program:
srand; # not needed for 5.004 and later
+
+ # then later on
$index = rand @array;
$element = $array[$index];
+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.
+
=head2 How do I permute N elements of a list?
Here's a little program that generates all permutations
of all the words on each line of input. The algorithm embodied
-in the permut() function should work on any list:
+in the permute() function should work on any list:
#!/usr/bin/perl -n
- # permute - tchrist@perl.com
- permut([split], []);
- sub permut {
- my @head = @{ $_[0] };
- my @tail = @{ $_[1] };
- unless (@head) {
- # stop recursing when there are no elements in the head
- print "@tail\n";
+ # tsc-permute: permute each word of input
+ permute([split], []);
+ sub permute {
+ my @items = @{ $_[0] };
+ my @perms = @{ $_[1] };
+ unless (@items) {
+ print "@perms\n";
} else {
- # for all elements in @head, move one from @head to @tail
- # and call permut() on the new @head and @tail
- my(@newhead,@newtail,$i);
- foreach $i (0 .. $#head) {
- @newhead = @head;
- @newtail = @tail;
- unshift(@newtail, splice(@newhead, $i, 1));
- permut([@newhead], [@newtail]);
+ my(@newitems,@newperms,$i);
+ foreach $i (0 .. $#items) {
+ @newitems = @items;
+ @newperms = @perms;
+ unshift(@newperms, splice(@newitems, $i, 1));
+ permute([@newitems], [@newperms]);
}
}
}
+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.
+
=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;
The default sort function is cmp, string comparison, which would
-sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
+sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
the numerical comparison operator.
If you have a complicated function needed to pull out the part you
}
@sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
-Which could also be written this way, using a trick
+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;
+ map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is useful.
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
+See http://www.cpan.org/doc/FMTEYEWTK/sort.html for more about
this approach.
See also the question below on sorting hashes.
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
-And here's how, given a vector in $vec, you can
+Here's how, given a vector in $vec, you can
get those bits into your @ints array:
sub bitvec_to_list {
This method gets faster the more sparse the bit vector is.
(Courtesy of Tim Bunce and Winfried Koenig.)
+Or use the CPAN module Bit::Vector:
+
+ $vector = Bit::Vector->new($num_of_bits);
+ $vector->Index_List_Store(@ints);
+ @ints = $vector->Index_List_Read();
+
+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";
+ 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), $bits);
+ print "bits are: @bytes\n\n";
+ }
+
=head2 Why does defined() return true on empty arrays and hashes?
-See L<perlfunc/defined> in the 5.004 release or later of Perl.
+The short story is that you should probably only use defined on scalars or
+functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
+in the 5.004 release or later of Perl for more detail.
=head1 Data: Hashes (Associative Arrays)
Use the each() function (see L<perlfunc/each>) if you don't care
whether it's sorted:
- while (($key,$value) = each %hash) {
+ while ( ($key, $value) = each %hash) {
print "$key = $value\n";
}
=head2 What happens if I add or remove keys from a hash while iterating over it?
-Don't do that.
+Don't do that. :-)
+
+[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.
+
+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.
=head2 How do I look up a hash element by value?
$by_value{$value} = $key;
}
-If your hash could have repeated values, the methods above will only
-find one of the associated keys. This may or may not worry you.
+If your hash could have repeated values, the methods above will only find
+one of the associated keys. This may or may not worry you. If it does
+worry you, you can always reverse the hash into a hash of arrays instead:
+
+ while (($key, $value) = each %by_key) {
+ push @{$key_list_by_value{$value}}, $key;
+ }
=head2 How can I know how many entries are in a hash?
$num_keys = scalar keys %hash;
-In void context it just resets the iterator, which is faster
-for tied hashes.
+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.
=head2 How do I sort a hash (optionally by value instead of key)?
} keys %hash; # and by value
Here we'll do a reverse numeric sort by value, and if two keys are
-identical, sort by length of key, and if that fails, by straight ASCII
-comparison of the keys (well, possibly modified by your locale -- see
+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>).
@keys = sort {
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.
=head2 What's the difference between "delete" and "undef" with hashes?
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is true
- exists $ary{'a'} is true (perl5 only)
+ exists $ary{'a'} is true (Perl5 only)
grep ($_ eq 'a', keys %ary) is true
If you now say
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is FALSE
- exists $ary{'a'} is true (perl5 only)
+ exists $ary{'a'} is true (Perl5 only)
grep ($_ eq 'a', keys %ary) is true
Notice the last two: you have an undef value, but a defined key!
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is false
- exists $ary{'a'} is FALSE (perl5 only)
+ exists $ary{'a'} is FALSE (Perl5 only)
grep ($_ eq 'a', keys %ary) is FALSE
See, the whole entry is gone!
=head2 How do I reset an each() operation part-way through?
-Using C<keys %hash> in a scalar context returns the number of keys in
+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.
=head2 How can I get the unique keys from two hashes?
-First you extract the keys from the hashes into arrays, and then solve
-the uniquifying the array problem described above. For example:
+First you extract the keys from the hashes into lists, then solve
+the "removing duplicates" problem described above. For example:
%seen = ();
for $element (keys(%foo), keys(%bar)) {
get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
it has to be ready to write it back into the caller's version.
-This has been fixed as of perl5.004.
+This has been fixed as of Perl5.004.
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
=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
-Use references (documented in L<perlref>). Examples of complex data
-structures are given in L<perldsc> and L<perllol>. Examples of
-structures and object-oriented classes are in L<perltoot>.
+Usually a hash ref, perhaps like this:
+
+ $record = {
+ NAME => "Jason",
+ EMPNO => 132,
+ TITLE => "deputy peon",
+ AGE => 23,
+ SALARY => 37_000,
+ PALS => [ "Norbert", "Rhys", "Phineas"],
+ };
+
+References are documented in L<perlref> and the upcoming L<perlreftut>.
+Examples of complex data structures are given in L<perldsc> and
+L<perllol>. Examples of structures and object-oriented classes are
+in L<perltoot>.
=head2 How can I use a reference as a hash key?
-You can't do this directly, but you could use the standard Tie::Refhash
-module distributed with perl.
+You can't do this directly, but you could use the standard Tie::RefHash
+module distributed with Perl.
=head1 Data: Misc
print "Your kernel is GNU-zip enabled!\n";
}
-On some systems, however, you have to play tedious games with "text"
-versus "binary" files. See L<perlfunc/"binmode">.
+On less elegant (read: Byzantine) systems, however, you have
+to play tedious games with "text" versus "binary" files. See
+L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking
+systems are curses out of Microsoft, who seem to be committed to putting
+the backward into backward compatibility.
If you're concerned about 8-bit ASCII data, then see L<perllocale>.
Assuming that you don't care about IEEE notations like "NaN" or
"Infinity", you probably just want to use a regular expression.
- warn "has nondigits" if /\D/;
- warn "not a whole number" unless /^\d+$/;
- warn "not an integer" unless /^-?\d+$/; # reject +3
- warn "not an integer" unless /^[+-]?\d+$/;
- warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
- warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
- warn "not a C float"
- unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
-
-Or you could check out
-http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
-instead. The POSIX module (part of the standard Perl distribution)
-provides the C<strtol> and C<strtod> for converting strings to double
-and longs, respectively.
+ if (/\D/) { print "has nondigits\n" }
+ if (/^\d+$/) { 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" }
+ 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.
=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.
+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:
+
+ use Storable;
+ store(\%hash, "filename");
+
+ # 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 is nice for printing out
-data structures, and FreezeThaw for copying them. For example:
+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.
+
+ use Storable qw(dclone);
+ $r2 = dclone($r1);
- use FreezeThaw qw(freeze thaw);
- $new = thaw freeze $old;
+Where $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.
-Where $old can be (a reference to) any kind of data structure you'd like.
-It will be deeply copied.
+ %newhash = %{ dclone(\%oldhash) };
=head2 How do I define methods for every class/object?
Get the 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.
+If you're doing a lot of float or double processing, consider using
+the PDL module from CPAN instead--it makes number-crunching easy.
+
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+All rights reserved.
+
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
+Irrespective of its distribution, all code examples in this file
+are hereby placed into the public domain. You are permitted and
+encouraged to use this code in your own programs for fun
+or for profit as you see fit. A simple comment in the code giving
+credit would be courteous but is not required.
https://perl5.git.perl.org / perl5.git / blobdiff