=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date: 1999/01/08 04:26:39 $)
+perlfaq4 - Data Manipulation ($Revision: 1.8 $, $Date: 2001/11/09 08:06:04 $)
=head1 DESCRIPTION
-The section of the FAQ answers question related to the manipulation
+The section of the FAQ answers questions related to the manipulation
of data as numbers, dates, strings, arrays, hashes, and miscellaneous
data issues.
=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
The infinite set that a mathematician thinks of as the real numbers can
-only be approximate on a computer, since the computer only has a finite
+only be approximated on a computer, since the computer only has a finite
number of bits to store an infinite number of, um, numbers.
Internally, your computer represents floating-point numbers in binary.
Floating-point numbers read in from a file or appearing as literals
in your program are converted from their decimal floating-point
-representation (eg, 19.95) to the internal binary representation.
+representation (eg, 19.95) to an internal binary representation.
However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly represented as a
When a floating-point number gets printed, the binary floating-point
representation is converted back to decimal. These decimal numbers
are displayed in either the format you specify with printf(), or the
-current output format for numbers
(see L<perlvar/"$#"> 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
+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.
+
=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
printf("%.3f", 3.1415926535); # prints 3.142
-The POSIX module (part of the standard perl distribution) implements
+The POSIX module (part of the standard Perl distribution) implements
ceil(), floor(), and a number of other mathematical and trigonometric
functions.
$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 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
machines) will work pretty much like mathematical integers. Other numbers
are not guaranteed.
-=head2 How do I convert bits into ints?
+=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 C<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 = $vec->to_Bin();
+
+The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
+are left as an exercise to the inclined reader.
+
+=back
=head2 Why doesn't & work the way I want it to?
push(@results, some_func($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?
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
+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.''
$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. The Date::Calc
-module from CPAN has a lot of date calculation functions, including
-day of the year, week of the year, and so on. Note that not
-all businesses consider ``week 1'' to be the same; for example,
-American businesses often consider the first week with a Monday
-in it to be Work Week #1, despite ISO 8601, which considers
-WW1 to be the first week with a Thursday in it.
+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 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 use one of the Date::Manip
-and Date::Calc modules from CPAN.
+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?
=head2 How can I find the Julian Day?
-Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
-there is an example of Julian date calculation that should help you in
-http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
-.
+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 one day off that:
+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.
-=head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
+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
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)
+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 an array context is the year minus 1900.
+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.
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:
+character are removed with
s/\\(.)/$1/g;
To turn C<"abbcccd"> into C<"abccd">:
- s/(.)\1/$1/g;
+ s/(.)\1/$1/g; # add /s to include newlines
+
+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?
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 is
-the CPAN module Parse::RecDescent, the standard module Text::Balanced,
-the byacc program, the CPAN module Parse::Yapp, and Mark-Jason
-Dominus's excellent I<py> tool at http://www.plover.com/~mjd/perl/py/
-.
+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) {
+ while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
# do something with $1
}
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
-
+
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
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 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}' $*"
+
+See the documentation for Text::Autoformat to appreciate its many
+capabilities.
+
=head2 How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
substr($a, 0, 3) = "Tom";
Although those with a pattern matching kind of thought process will
-likely prefer:
+likely prefer
$a =~ s/^.../Tom/;
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.
+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";
- # Warning: don't `last' out of this loop
}
}
=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";
$count = ($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):
+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
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. Consdier the proper
+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.
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 module on CPAN.
+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?
-Although the simplest approach would seem to be:
+Although the simplest approach would seem to be
$string =~ s/^\s*(.*?)\s*$/$1/;
-This is unnecessarily slow, destructive, and fails with embedded newlines.
-It is much better 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+//;
$string =~ s/\s+$//;
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 slide:
+values of a hash if you use a slice:
# trim whitespace in the scalar, the array,
# and all the values in the hash
=head2 How do I pad a string with blanks or pad a number with zeroes?
-(This answer contributed by Uri Guttman)
+(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<$pad_char> variable
-if you know what it is in advance.
+to pad the string, C<$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 use the C<sprintf> function. It can pad on the
-left or right with blanks and on the left with zeroes.
+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 with blank:
- $padded = sprintf( "%${pad_len}s", $text ) ;
+ # Left padding a string with blanks (no truncation):
+ $padded = sprintf("%${pad_len}s", $text);
- # Right padding with blank:
- $padded = sprintf( "%${pad_len}s", $text ) ;
+ # Right padding a string with blanks (no truncation):
+ $padded = sprintf("%-${pad_len}s", $text);
- # Left padding with 0:
- $padded = sprintf( "%0${pad_len}d", $num ) ;
+ # Left padding a number with 0 (no truncation):
+ $padded = sprintf("%0${pad_len}d", $num);
-If you need to pad with a character other than blank or zero you can use
-one of the following methods.
+ # Right padding a string with blanks using pack (will truncate):
+ $padded = pack("A$pad_len",$text);
-These methods generate a pad string with the C<x> operator and
-concatenate that with the original text.
+If you need to pad with a character other than blank or zero you can use
+one of the following methods. They all generate a pad string with the
+C<x> operator and combine that with C<$text>. These methods do
+not truncate C<$text>.
-Left and right padding with any character:
+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 ) );
-Or you can left or right pad $text directly:
+Left and right padding with any character, modifying C<$text> directly:
- $text .= $pad_char x ( $pad_len - length( $text ) ) ;
- substr( $text, 0, 0 ) = $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( $text ) );
=head2 How do I extract selected columns from a string?
=head2 How do I find the soundex value of a string?
-Use the standard Text::Soundex module distributed with perl.
+Use the standard Text::Soundex module distributed with Perl.
+Before you do so, you may want to determine whether `soundex' is in
+fact what you think it is. Knuth's soundex algorithm compresses words
+into a small space, and so it does not necessarily distinguish between
+two words which you might want to appear separately. For example, the
+last names `Knuth' and `Kant' are both mapped to the soundex code K530.
+If Text::Soundex does not do what you are looking for, you might want
+to consider the String::Approx module available at CPAN.
=head2 How can I expand variables in text strings?
=head2 What's wrong with always quoting "$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. Think of it this way: double-quote
+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?
print "@lines"; # WRONG - extra blanks
print @lines; # right
-=head2 Why don't my E<lt>E<lt>HERE documents work?
+=head2 Why don't my <<HERE documents work?
Check for these three things:
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 off. Otherwise, it takes the amount of leading
-white space found on the first line and removes that much off each
+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, $leader); # common white space and common leading string
+ my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
- @@@ while ( op = (*op->op_ppaddr)() ) ;
+ @@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
-Or with a fixed amount of leading white space, with remaining
+Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:
$poem = fix<<EVER_ON_AND_ON;
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<@_>, push/pop/shift only work
+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.
$scalar = (2, 5, 7, 9);
-you're using the comma operator in scalar context, so it evaluates the
-left hand side, then evaluates and returns the left hand side. This
-causes the last value to be returned: 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)
- $prev = 'nonesuch';
- @out = grep($_ ne $prev && ($prev = $_), @in);
+ $prev = "not equal to $in[0]";
+ @out = grep($_ ne $prev && ($prev = $_, 1), @in);
This is nice in that it doesn't use much extra memory, simulating
-uniq(1)'s behavior of removing only adjacent duplicates. It's less
-nice in that it won't work with false values like undef, 0, or "";
-"0 but true" is ok, though.
+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 $_ is 0, "", or undef.
-=item b) If you don't know whether @in is sorted:
+=item b)
+
+If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$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
But perhaps you should have been using a hash all along, eh?
-=head2 How can I tell whether a list or array contains a certain element?
+=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
@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?). If you're only testing once, then
+regex characters in $whatever?). If you're only testing once, then
use:
$is_there = 0;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
+Note that this is the I<symmetric difference>, that is, all elements in
+either A or in B but not in both. Think of it as an xor operation.
+
=head2 How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a stringwise
sub compare_arrays {
my ($first, $second) = @_;
- local $^W = 0; # silence spurious -w undef complaints
+ no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
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
+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.
}
print "\n";
-You could grow the list this way:
+You could add to the list this way:
my ($head, $tail);
$tail = append($head, 1); # grow a new head
=head2 How do I shuffle an array randomly?
-Use this:
+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( \@array ) :
- # generate a random permutation of @array in place
+ # fisher_yates_shuffle
+ # generate a random permutation of an array in place
+ # As in shuffling a deck of cards
+ #
sub fisher_yates_shuffle {
- my $array = shift;
- my $i;
- for ($i = @$array; --$i; ) {
+ my $deck = shift; # $deck is a reference to an array
+ my $i = @$deck;
+ while (--$i) {
my $j = int rand ($i+1);
- next if $i == $j;
- @$array[$i,$j] = @$array[$j,$i];
+ @$deck[$i,$j] = @$deck[$j,$i];
}
}
- fisher_yates_shuffle( \@array ); # permutes @array in place
+And here is an example of using it:
-You've probably seen shuffling algorithms that works using splice,
-randomly picking another element to swap the current element with:
+ #
+ # 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 = ();
}
}
+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.)
-Here's a demo on how to use vec():
+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";
=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 {
$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 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 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 legacy systems, however, you have to play tedious games with
-"text" versus "binary" files. See L<perlfunc/"binmode">, or the upcoming
-L<perlopentut> manpage.
+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>.
}
}
- sub is_numeric { defined &getnum }
+ sub is_numeric { defined getnum($_[0]) }
-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.
+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. Here's one example using
+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;
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
-When included as part of the Standard Version of Perl, or as part of
-its complete documentation whether printed or otherwise, this work
-may be distributed only under the terms of Perl's Artistic License.
-Any distribution of this file or derivatives thereof I<outside>
-of that package require that special arrangements be made with
-copyright holder.
+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