For the long explanation, see David Goldberg's "What Every ComputerScientist Should Know About Floating-Point Arithmetic"(http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf).

Internally, your computer represents floating-point numbers in binary.Digital (as in powers of two) computers cannot store all numbersexactly. Some real numbers lose precision in the process. This is aproblem with how computers store numbers and affects all computerlanguages, not just Perl.

perlnumber shows the gory details of number representations andconversions.

To limit the number of decimal places in your numbers, you can use theprintf or sprintf function. SeeFloating-point Arithmetic in perlop for more details.

 printf "%.2f", 10/3;
 
  my $number = sprintf "%.2f", 10/3;

Why is int() broken?

Your int() is most probably working just fine. It's the numbers thataren't quite what you think.

First, see the answer to "Why am I getting long decimals(eg, 19.9499999999999) instead of the numbers I should be getting(eg, 19.95)?".

For example, this

 print int(0.6/0.2-2), "\n";

will in most computers print 0, not 1, because even such simplenumbers as 0.6 and 0.2 cannot be presented exactly by floating-pointnumbers. What you think in the above as 'three' is really more like2.9999999999999995559.

Why isn't my octal data interpreted correctly?

(contributed by brian d foy)

You're probably trying to convert a string to a number, which Perl onlyconverts as a decimal number. When Perl converts a string to a number, itignores leading spaces and zeroes, then assumes the rest of the digitsare in base 10:

 my $string = '0644';
 
  print $string + 0;  # prints 644
 
  print $string + 44; # prints 688, certainly not octal!

This problem usually involves one of the Perl built-ins that has thesame name a Unix command that uses octal numbers as arguments on thecommand line. In this example, chmod on the command line knows thatits first argument is octal because that's what it does:

 %prompt> chmod 644 file

If you want to use the same literal digits (644) in Perl, you have to tellPerl to treat them as octal numbers either by prefixing the digits witha 0 or using oct:

 chmod( 0644, $filename );  # right, has leading zero
  chmod( oct(644), $filename );  # also correct

The problem comes in when you take your numbers from something that Perlthinks is a string, such as a command line argument in @ARGV:

 chmod( $ARGV[0],  $filename );  # wrong, even if "0644"
 
  chmod( oct($ARGV[0]), $filename );  # correct, treat string as octal

You can always check the value you're using by printing it in octalnotation to ensure it matches what you think it should be. Print itin octal and decimal format:

 printf "0%o %d", $number, $number;

Does Perl have a round() function? What about ceil() and floor()? Trig functions?

Remember that int() merely truncates toward 0. For rounding to acertain number of digits, sprintf() or printf() is usually theeasiest 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 mathematicaland trigonometric functions.

 use POSIX;
  my $ceil   = ceil(3.5);   # 4
  my $floor  = floor(3.5);  # 3

In 5.000 to 5.003 perls, trigonometry was done in the Math::Complexmodule. With 5.004, the Math::Trig module (part of the standard Perldistribution) implements the trigonometric functions. Internally ituses the Math::Complex module and some functions can break out fromthe real axis into the complex plane, for example the inverse sine of2.

Rounding in financial applications can have serious implications, andthe rounding method used should be specified precisely. In thesecases, it probably pays not to trust whichever system of rounding isbeing used by Perl, but instead to implement the rounding function youneed yourself.

To see why, notice how you'll still have an issue on half-way-pointalternation:

 for (my $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 dothis. Perl numbers whose absolute values are integers under 2**31 (on32-bit machines) will work pretty much like mathematical integers.Other numbers are not guaranteed.

How do I convert between numeric representations/bases/radixes?

As always with Perl there is more than one way to do it. Below are afew examples of approaches to making common conversions between numberrepresentations. This is intended to be representational rather thanexhaustive.

Some of the examples later in perlfaq4 use the Bit::Vectormodule from CPAN. The reason you might choose Bit::Vector over theperl built-in functions is that it works with numbers of ANY size,that it is optimized for speed on some operations, and for at leastsome programmers the notation might be familiar.

How do I convert hexadecimal into decimal

Using perl's built in conversion of 0x notation:

 my $dec = 0xDEADBEEF;

Using the hex function:

 my $dec = hex("DEADBEEF");

Using pack:

 my $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));

Using the CPAN module Bit::Vector:

 use Bit::Vector;
  my $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
  my $dec = $vec->to_Dec();

How do I convert from decimal to hexadecimal

Using sprintf:

 my $hex = sprintf("%X", 3735928559); # upper case A-F
  my $hex = sprintf("%x", 3735928559); # lower case a-f

Using unpack:

 my $hex = unpack("H*", pack("N", 3735928559));

Using Bit::Vector:

 use Bit::Vector;
  my $vec = Bit::Vector->new_Dec(32, -559038737);
  my $hex = $vec->to_Hex();

And Bit::Vector supports odd bit counts:

 use Bit::Vector;
  my $vec = Bit::Vector->new_Dec(33, 3735928559);
  $vec->Resize(32); # suppress leading 0 if unwanted
  my $hex = $vec->to_Hex();

How do I convert from octal to decimal

Using Perl's built in conversion of numbers with leading zeros:

 my $dec = 033653337357; # note the leading 0!

Using the oct function:

 my $dec = oct("33653337357");

Using Bit::Vector:

 use Bit::Vector;
  my $vec = Bit::Vector->new(32);
  $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
  my $dec = $vec->to_Dec();

How do I convert from decimal to octal

Using sprintf:

 my $oct = sprintf("%o", 3735928559);

Using Bit::Vector:

 use Bit::Vector;
  my $vec = Bit::Vector->new_Dec(32, -559038737);
  my $oct = reverse join('', $vec->Chunk_List_Read(3));

How do I convert from binary to decimal

Perl 5.6 lets you write binary numbers directly withthe 0b notation:

 my $number = 0b10110110;

Using oct:

 my $input = "10110110";
  my $decimal = oct( "0b$input" );

Using pack and ord:

 my $decimal = ord(pack('B8', '10110110'));

Using pack and unpack for larger strings:

 my $int = unpack("N", pack("B32",
  substr("0" x 32 . "11110101011011011111011101111", -32)));
  my $dec = sprintf("%d", $int);
 
  # substr() is used to left-pad a 32-character string with zeros.

Using Bit::Vector:

 my $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
  my $dec = $vec->to_Dec();

How do I convert from decimal to binary

Using sprintf (perl 5.6+):

 my $bin = sprintf("%b", 3735928559);

Using unpack:

 my $bin = unpack("B*", pack("N", 3735928559));

Using Bit::Vector:

 use Bit::Vector;
  my $vec = Bit::Vector->new_Dec(32, -559038737);
  my $bin = $vec->to_Bin();

The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)are left as an exercise to the inclined reader.

Why doesn't & work the way I want it to?

The behavior of binary arithmetic operators depends on whether they'reused on numbers or strings. The operators treat a string as a seriesof bits and work with that (the string "3" is the bit pattern00110011). The operators work with the binary form of a number(the number 3 is treated as the bit pattern 00000011).

So, saying 11 & 3 performs the "and" operation on numbers (yielding3). Saying "11" & "3" performs the "and" operation on strings(yielding "1").

Most problems with & and | arise because the programmer thinksthey have a number but really it's a string or vice versa. To avoid this,stringify the arguments explicitly (using "" or qq()) or convert themto numbers explicitly (using 0+$arg). The rest arise becausethe programmer says:

 if ("\020\020" & "\101\101") {
  # ...
  }

but a string consisting of two null bytes (the result of "\020\020"& "\101\101") is not a false value in Perl. You need:

 if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
  # ...
  }

How do I multiply matrices?

Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)or the PDL extension (also available from CPAN).

How do I perform an operation on a series of integers?

To call a function on each element in an array, and collect theresults, use:

 my @results = map { my_func($_) } @array;

For example:

 my @triple = map { 3 * $_ } @single;

To call a function on each element of an array, but ignore theresults:

 foreach my $iterator (@array) {
  some_func($iterator);
  }

To call a function on each integer in a (small) range, you can use:

 my @results = map { some_func($_) } (5 .. 25);

but you should be aware that in this form, the .. operatorcreates a list of all integers in the range, which can take a lot ofmemory for large ranges. However, the problem does not occur whenusing .. within a for loop, because in that case the rangeoperator is optimized to iterate over the range, without creatingthe entire list. So

 my @results = ();
  for my $i (5 .. 500_005) {
  push(@results, some_func($i));
  }

or even

   push(@results, some_func($_)) for 5 .. 500_005;

will not create an intermediate list of 500,000 integers.

How can I output Roman numerals?

Get the http://www.cpan.org/modules/by-module/Roman module.

Why aren't my random numbers random?

If you're using a version of Perl before 5.004, you must call srandonce at the start of your program to seed the random number generator.

 BEGIN { srand() if $] < 5.004 }

5.004 and later automatically call srand at the beginning. Don'tcall 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 :-). Therandom article in the "Far More Than You Ever Wanted To Know"collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz, courtesyof Tom Phoenix, talks more about this. John von Neumann said, "Anyonewho attempts to generate random numbers by deterministic means is, ofcourse, living in a state of sin."

Perl relies on the underlying system for the implementation ofrand and srand; on some systems, the generated numbers arenot random enough (especially on Windows : seehttp://www.perlmonks.org/?node_id=803632).Several CPAN modules in the Math namespace implement betterpseudorandom generators; see for exampleMath::Random::MT ("Mersenne Twister", fast), orMath::TrulyRandom (uses the imperfections in the system'stimer to generate random numbers, which is rather slow).More algorithms for random numbers are described in"Numerical Recipes in C" at http://www.nr.com/

How do I get a random number between X and Y?

To get a random number between two values, you can use the rand()built-in to get a random number between 0 and 1. From there, you shiftthat into the range that you want.

rand($x) returns a number such that 0 <= rand($x) < $x. Thuswhat you want to have perl figure out is a random number in the rangefrom 0 to the difference between your X and Y.

That is, to get a number between 10 and 15, inclusive, you want arandom number between 0 and 5 that you can then add to 10.

 my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )

Hence you derive the following simple function to abstractthat. It selects a random integer between the two givenintegers (inclusive), For example: random_int_between(50,120).

 sub random_int_between {
  my($min, $max) = @_;
  # Assumes that the two arguments are integers themselves!
  return $min if $min == $max;
  ($min, $max) = ($max, $min)  if  $min > $max;
  return $min + int rand(1 + $max - $min);
  }

Data: Dates

How do I find the day or week of the year?

The day of the year is in the list returnedby the localtime function. Without anargument localtime uses the current time.

 my $day_of_year = (localtime)[7];

The POSIX module can also format a date as the day of the year orweek of the year.

 use POSIX qw/strftime/;
  my $day_of_year  = strftime "%j", localtime;
  my $week_of_year = strftime "%W", localtime;

To get the day of year for any date, use POSIX's mktime to geta time in epoch seconds for the argument to localtime.

 use POSIX qw/mktime strftime/;
  my $week_of_year = strftime "%W",
  localtime( mktime( 0, 0, 0, 18, 11, 87 ) );

You can also use Time::Piece, which comes with Perl and provides alocaltime that returns an object:

 use Time::Piece;
  my $day_of_year  = localtime->yday;
  my $week_of_year = localtime->week;

The Date::Calc module provides two functions to calculate these, too:

 use Date::Calc;
  my $day_of_year  = Day_of_Year(  1987, 12, 18 );
  my $week_of_year = Week_of_Year( 1987, 12, 18 );

How do I find the current century or millennium?

Use the following simple functions:

 sub get_century {
  return int((((localtime(shift || time))[5] + 1999))/100);
  }
 
  sub get_millennium {
  return 1+int((((localtime(shift || time))[5] + 1899))/1000);
  }

On some systems, the POSIX module's strftime() function has beenextended in a non-standard way to use a %C format, which theysometimes claim is the "century". It isn't, because on most suchsystems, this is only the first two digits of the four-digit year, andthus cannot be used to determine reliably the current century ormillennium.

How can I compare two dates and find the difference?

(contributed by brian d foy)

You could just store all your dates as a number and then subtract.Life isn't always that simple though.

The Time::Piece module, which comes with Perl, replaces localtimewith a version that returns an object. It also overloads the comparisonoperators so you can compare them directly:

 use Time::Piece;
  my $date1 = localtime( $some_time );
  my $date2 = localtime( $some_other_time );
 
  if( $date1 < $date2 ) {
  print "The date was in the past\n";
  }

You can also get differences with a subtraction, which returns aTime::Seconds object:

 my $diff = $date1 - $date2;
  print "The difference is ", $date_diff->days, " days\n";

If you want to work with formatted dates, the Date::Manip,Date::Calc, or DateTime modules can help you.

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 standardTime::Local module. Otherwise, you should look into the Date::Calc,Date::Parse, and Date::Manip modules from CPAN.

How can I find the Julian Day?

(contributed by brian d foy and Dave Cross)

You can use the Time::Piece module, part of the Standard Library,which can convert a date/time to a Julian Day:

 $ perl -MTime::Piece -le 'print localtime->julian_day'
  2455607.7959375

Or the modified Julian Day:

 $ perl -MTime::Piece -le 'print localtime->mjd'
  55607.2961226851

Or even the day of the year (which is what some people think of as aJulian day):

 $ perl -MTime::Piece -le 'print localtime->yday'
  45

You can also do the same things with the DateTime module:

 $ perl -MDateTime -le'print DateTime->today->jd'
  2453401.5
  $ perl -MDateTime -le'print DateTime->today->mjd'
  53401
  $ perl -MDateTime -le'print DateTime->today->doy'
  31

You can use the Time::JulianDay module available on CPAN. Ensurethat you really want to find a Julian day, though, as many people havedifferent ideas about Julian days (see http://www.hermetic.ch/cal_stud/jdn.htmfor instance):

 $  perl -MTime::JulianDay -le 'print local_julian_day( time )'
  55608

How do I find yesterday's date?

(contributed by brian d foy)

To do it correctly, you can use one of the Date modules since theywork with calendars instead of times. The DateTime module makes itsimple, and give you the same time of day, only the day before,despite daylight saving time changes:

 use DateTime;
 
  my $yesterday = DateTime->now->subtract( days => 1 );
 
  print "Yesterday was $yesterday\n";

You can also use the Date::Calc module using its Today_and_Nowfunction.

 use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
 
  my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
 
  print "@date_time\n";

Most people try to use the time rather than the calendar to figure outdates, but that assumes that days are twenty-four hours each. Formost people, there are two days a year when they aren't: the switch toand from summer time throws this off. For example, the rest of thesuggestions will be wrong sometimes:

Starting with Perl 5.10, Time::Piece and Time::Seconds are partof the standard distribution, so you might think that you could dosomething like this:

 use Time::Piece;
  use Time::Seconds;
 
  my $yesterday = localtime() - ONE_DAY; # WRONG
  print "Yesterday was $yesterday\n";

The Time::Piece module exports a new localtime that returns anobject, and Time::Seconds exports the ONE_DAY constant that is aset number of seconds. This means that it always gives the time 24hours ago, which is not always yesterday. This can cause problemsaround the end of daylight saving time when there's one day that is 25hours long.

You have the same problem with Time::Local, which will give the wronganswer for those same special cases:

 # contributed by Gunnar Hjalmarsson
  use Time::Local;
  my $today = timelocal 0, 0, 12, ( localtime )[3..5];
  my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; # WRONG
  printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;

Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?

(contributed by brian d foy)

Perl itself never had a Y2K problem, although that never stopped peoplefrom creating Y2K problems on their own. See the documentation forlocaltime for its proper use.

Starting with Perl 5.12, localtime and gmtime can handle dates past03:14:08 January 19, 2038, when a 32-bit based time would overflow. Youstill might get a warning on a 32-bit perl:

 % perl5.12 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
  Integer overflow in hexadecimal number at -e line 1.
  Wed Nov  1 19:42:39 5576711

On a 64-bit perl, you can get even larger dates for those really longrunning projects:

 % perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
  Thu Nov  2 00:42:39 5576711

You're still out of luck if you need to keep track of decaying protonsthough.

Data: Strings

How do I validate input?

(contributed by brian d foy)

There are many ways to ensure that values are what you expect orwant to accept. Besides the specific examples that we cover in theperlfaq, you can also look at the modules with "Assert" and "Validate"in their names, along with other modules such as Regexp::Common.

Some modules have validation for particular types of input, suchas Business::ISBN, Business::CreditCard, Email::Valid,and Data::Validate::IP.

How do I unescape a string?

It depends just what you mean by "escape". URL escapes are dealtwith in perlfaq9. Shell escapes with the backslash (\)character are removed with

 s/\(.)/$1/g;

This won't expand "\n" or "\t" or any other special escapes.

How do I remove consecutive pairs of characters?

(contributed by brian d foy)

You can use the substitution operator to find pairs of characters (orruns of characters) and replace them with a single instance. In thissubstitution, we find a character in (.). The memory parenthesesstore the matched character in the back-reference \g1 and we usethat to require that the same thing immediately follow it. We replacethat part of the string with the character in $1.

 s/(.)\g1/$1/g;

We can also use the transliteration operator, tr///. In thisexample, the search list side of our tr/// contains nothing, butthe c option complements that so it contains everything. Thereplacement list also contains nothing, so the transliteration isalmost a no-op since it won't do any replacements (or more exactly,replace the character with itself). However, the s option squashesduplicated and consecutive characters in the string so a characterdoes not show up next to itself

 my $str = 'Haarlem';   # in the Netherlands
  $str =~ tr///cs;   # Now Harlem, like in New York

How do I expand function calls in a string?

(contributed by brian d foy)

This is documented in perlref, and although it's not the easiestthing to read, it does work. In each of these examples, we call thefunction inside the braces used to dereference a reference. If wehave more than one return value, we can construct and dereference ananonymous array. In this case, we call the function in list context.

 print "The time values are @{ [localtime] }.\n";

If we want to call the function in scalar context, we have to do a bitmore work. We can really have any code we like inside the braces, sowe simply have to end with the scalar reference, although how you dothat is up to you, and you can use code inside the braces. Note thatthe use of parens creates a list context, so we need scalar toforce the scalar context on the function:

 print "The time is ${\(scalar localtime)}.\n"
 
  print "The time is ${ my $x = localtime; \$x }.\n";

If your function already returns a reference, you don't need to createthe reference yourself.

 sub timestamp { my $t = localtime; \$t }
 
  print "The time is ${ timestamp() }.\n";

The Interpolation module can also do a lot of magic for you. You canspecify a variable name, in this case E, to set up a tied hash thatdoes the interpolation for you. It has several other methods to do thisas well.

 use Interpolation E => 'eval';
  print "The time values are $E{localtime()}.\n";

In most cases, it is probably easier to simply use string concatenation,which also forces scalar context.

 print "The time is " . localtime() . ".\n";

How do I find matching/nesting anything?

To find something between two singlecharacters, a pattern like /x([^x]*)x/ will get the interveningbits in $1. For multiple ones, then something more like/alpha(.*?)omega/ would be needed. For nested patternsand/or balanced expressions, see the so-called(?PARNO)construct (available since perl 5.10).The CPAN module Regexp::Common can help to build suchregular expressions (see in particularRegexp::Common::balanced and Regexp::Common::delimited).

More complex cases will require to write a parser, probablyusing a parsing module from CPAN, likeRegexp::Grammars, Parse::RecDescent, Parse::Yapp,Text::Balanced, or Marpa::XS.

How do I reverse a string?

Use reverse() in scalar context, as documented inreverse.

 my $reversed = reverse $string;

How do I expand tabs in a string?

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 Perldistribution).

 use Text::Tabs;
  my @expanded_lines = expand(@lines_with_tabs);

How do I reformat a paragraph?

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 embeddednewlines. Text::Wrap doesn't justify the lines (flush-right).

Or use the CPAN module Text::Autoformat. Formatting files can beeasily 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 manycapabilities.

How can I access or change N characters of a string?

You can access the first characters of a string with substr().To get the first character, for example, start at position 0and grab the string of length 1.

 my $string = "Just another Perl Hacker";
  my $first_char = substr( $string, 0, 1 );  #  'J'

To change part of a string, you can use the optional fourthargument which is the replacement string.

 substr( $string, 13, 4, "Perl 5.8.0" );

You can also use substr() as an lvalue.

 substr( $string, 13, 4 ) =  "Perl 5.8.0";

How do I change the Nth occurrence of something?

You have to keep track of N yourself. For example, let's say you wantto change the fifth occurrence of "whoever" or "whomever" into"whosoever" or "whomsoever", case insensitively. Theseall assume that $_ contains the string to be altered.

 $count = 0;
  s{((whom?)ever)}{
   ++$count == 5   # is it the 5th?
   ? "${2}soever"  # yes, swap
   : $1 # renege and leave it there
   }ige;

In the more general case, you can use the /g modifier in a whileloop, 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: "The third fish is a red one." You can also use arepetition count and repeated pattern like this:

 /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;

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 acount of a certain single character (X) within a string, you can use thetr/// function like so:

 my $string = "ThisXlineXhasXsomeXx'sXinXit";
  my $count = ($string =~ tr/X//);
  print "There are $count X characters in the string";

This is fine if you are just looking for a single character. However,if you are trying to count multiple character substrings within alarger string, tr/// won't work. What you can do is wrap a while()loop around a global pattern match. For example, let's count negativeintegers:

 my $string = "-9 55 48 -2 23 -76 4 14 -44";
  my $count = 0;
  while ($string =~ /-\d+/g) { $count++ }
  print "There are $count negative numbers in the string";

Another version uses a global match in list context, then assigns theresult to a scalar, producing a count of the number of matches.

 my $count = () = $string =~ /-\d+/g;

How do I capitalize all the words on one line?

(contributed by brian d foy)

Damian Conway's Text::Autoformat handles all of the thinkingfor you.

 use Text::Autoformat;
  my $x = "Dr. Strangelove or: How I Learned to Stop ".
   "Worrying and Love the Bomb";
 
  print $x, "\n";
  for my $style (qw( sentence title highlight )) {
  print autoformat($x, { case => $style }), "\n";
  }

How do you want to capitalize those words?

 FRED AND BARNEY'S LODGE # all uppercase
  Fred And Barney's Lodge # title case
  Fred and Barney's Lodge # highlight case

It's not as easy a problem as it looks. How many words do you thinkare in there? Wait for it... wait for it.... If you answered 5you're right. Perl words are groups of \w+, but that's not whatyou want to capitalize. How is Perl supposed to know not to capitalizethat s after the apostrophe? You could try a regular expression:

 $string =~ s/ (
   (^\w) #at the beginning of the line
       |  # or
   (\s\w)   #preceded by whitespace
       )
   /\U$1/xg;
 
  $string =~ s/([\w']+)/\u\L$1/g;

Now, what if you don't want to capitalize that "and"? Just useText::Autoformat and get on with the next problem. :)

How can I split a [character]-delimited string except when inside [character]?

Several modules can handle this sort of parsing--Text::Balanced,Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.

Take the example case of trying to split a string that iscomma-separated into its different fields. You can't use split(/,/)because you shouldn't split if the comma is inside quotes. Forexample, take a data line like this:

 SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

Due to the restriction of the quotes, this is a fairly complexproblem. Thankfully, we have Jeffrey Friedl, author ofMastering Regular Expressions, to handle these for us. Hesuggests (assuming your string is contained in $text):

 my @new = ();
  push(@new, $+) while $text =~ m{
   "([^\"\]*(?:\.[^\"\]*)*)",? # groups the phrase inside the quotes
   | ([^,]+),?
   | ,
   }gx;
  push(@new, undef) if substr($text,-1,1) eq ',';

If you want to represent quotation marks inside aquotation-mark-delimited field, escape them with backslashes (eg,"like \"this\"".

Alternatively, the Text::ParseWords module (part of the standardPerl distribution) lets you say:

 use Text::ParseWords;
  @new = quotewords(",", 0, $text);

For parsing or generating CSV, though, using Text::CSV rather thanimplementing it yourself is highly recommended; you'll save yourself odd bugspopping up later by just using code which has already been tried and tested inproduction for years.

How do I strip blank space from the beginning/end of a string?

(contributed by brian d foy)

A substitution can do this for you. For a single line, you want toreplace all the leading or trailing whitespace with nothing. Youcan do that with a pair of substitutions:

 s/^\s+//;
  s/\s+$//;

You can also write that as a single substitution, although it turnsout the combined statement is slower than the separate ones. Thatmight not matter to you, though:

 s/^\s+|\s+$//g;

In this regular expression, the alternation matches either at thebeginning or the end of the string since the anchors have a lowerprecedence than the alternation. With the /g flag, the substitutionmakes all possible matches, so it gets both. Remember, the trailingnewline matches the \s+, and the $ anchor can match to theabsolute end of the string, so the newline disappears too. Just addthe newline to the output, which has the added benefit of preserving"blank" (consisting entirely of whitespace) lines which the ^\s+would remove all by itself:

 while( <> ) {
  s/^\s+|\s+$//g;
  print "$_\n";
  }

For a multi-line string, you can apply the regular expression to eachlogical line in the string by adding the /m flag (for"multi-line"). With the /m flag, the $ matches before anembedded newline, so it doesn't remove it. This pattern still removesthe newline at the end of the string:

 $string =~ s/^\s+|\s+$//gm;

Remember that lines consisting entirely of whitespace will disappear,since the first part of the alternation can match the entire stringand replace it with nothing. If you need to keep embedded blank lines,you have to do a little more work. Instead of matching any whitespace(since that includes a newline), just match the other whitespace:

 $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

How do I pad a string with blanks or pad a number with zeroes?

In the following examples, $pad_len is the length to which you wishto pad the string, $text or $num contains the string to be padded,and $pad_char contains the padding character. You can use a singlecharacter string constant instead of the $pad_char variable if youknow what it is in advance. And in the same way you can use an integer inplace of $pad_len if you know the pad length in advance.

The simplest method uses the sprintf function. It can pad on the leftor right with blanks and on the left with zeroes and it will nottruncate the result. The pack function can only pad strings on theright with blanks and it will truncate the result to a maximum length of$pad_len.

 # Left padding a string with blanks (no truncation):
  my $padded = sprintf("%${pad_len}s", $text);
  my $padded = sprintf("%*s", $pad_len, $text);  # same thing
 
  # Right padding a string with blanks (no truncation):
  my $padded = sprintf("%-${pad_len}s", $text);
  my $padded = sprintf("%-*s", $pad_len, $text); # same thing
 
  # Left padding a number with 0 (no truncation):
  my $padded = sprintf("%0${pad_len}d", $num);
  my $padded = sprintf("%0*d", $pad_len, $num); # same thing
 
  # Right padding a string with blanks using pack (will truncate):
  my $padded = pack("A$pad_len",$text);

If you need to pad with a character other than blank or zero you can useone of the following methods. They all generate a pad string with thex operator and combine that with $text. These methods donot truncate $text.

Left and right padding with any character, creating a new string:

 my $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
  my $padded = $text . $pad_char x ( $pad_len - length( $text ) );

Left and right padding with any character, modifying $text directly:

 substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
  $text .= $pad_char x ( $pad_len - length( $text ) );

How do I extract selected columns from a string?

(contributed by brian d foy)

If you know the columns that contain the data, you canuse substr to extract a single column.

 my $column = substr( $line, $start_column, $length );

You can use split if the columns are separated by whitespace orsome other delimiter, as long as whitespace or the delimiter cannotappear as part of the data.

 my $line = ' fred barney   betty   ';
  my @columns = split /\s+/, $line;
  # ( '', 'fred', 'barney', 'betty' );
 
  my $line = 'fred||barney||betty';
  my @columns = split /\|/, $line;
  # ( 'fred', '', 'barney', '', 'betty' );

If you want to work with comma-separated values, don't do this sincethat format is a bit more complicated. Use one of the modules thathandle that format, such as Text::CSV, Text::CSV_XS, orText::CSV_PP.

If you want to break apart an entire line of fixed columns, you can useunpack with the A (ASCII) format. By using a number after the formatspecifier, you can denote the column width. See the pack and unpackentries in perlfunc for more details.

 my @fields = unpack( $line, "A8 A8 A8 A16 A4" );

Note that spaces in the format argument to unpack do not denote literalspaces. If you have space separated data, you may want split instead.

How do I find the soundex value of a string?

(contributed by brian d foy)

You can use the Text::Soundex module. If you want to do fuzzy or closematching, you might also try the String::Approx, andText::Metaphone, and Text::DoubleMetaphone modules.

How can I expand variables in text strings?

(contributed by brian d foy)

If you can avoid it, don't, or if you can use a templating system,such as Text::Template or Template Toolkit, do that instead. Youmight even be able to get the job done with sprintf or printf:

 my $string = sprintf 'Say hello to %s and %s', $foo, $bar;

However, for the one-off simple case where I don't want to pull out afull templating system, I'll use a string that has two Perl scalarvariables in it. In this example, I want to expand $foo and $barto their variable's values:

 my $foo = 'Fred';
  my $bar = 'Barney';
  $string = 'Say hello to $foo and $bar';

One way I can do this involves the substitution operator and a double/e flag. The first /e evaluates $1 on the replacement side andturns it into $foo. The second /e starts with $foo and replacesit with its value. $foo, then, turns into 'Fred', and that's finallywhat's left in the string:

 $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'

The /e will also silently ignore violations of strict, replacingundefined variable names with the empty string. Since I'm using the/e flag (twice even!), I have all of the same security problems Ihave with eval in its string form. If there's something odd in$foo, perhaps something like @{[ system "rm -rf /" ]}, thenI could get myself in trouble.

To get around the security problem, I could also pull the values froma hash instead of evaluating variable names. Using a single /e, Ican check the hash to ensure the value exists, and if it doesn't, Ican replace the missing value with a marker, in this case ??? tosignal that I missed something:

 my $string = 'This has $foo and $bar';
 
  my %Replacements = (
  foo  => 'Fred',
  );
 
  # $string =~ s/\$(\w+)/$Replacements{$1}/g;
  $string =~ s/\$(\w+)/
   exists $Replacements{$1} ? $Replacements{$1} : '???'
   /eg;
 
  print $string;

What's wrong with always quoting "$vars"?

The problem is that those double-quotes forcestringification--coercing numbers and references into strings--evenwhen you don't want them to be strings. Think of it this way:double-quote expansion is used to produce new strings. If you alreadyhave a string, why do you need more?

If you get used to writing odd things like these:

 print "$var";   # BAD
  my $new = "$old";   # BAD
  somefunc("$var"); # BAD

You'll be in trouble. Those should (in 99.8% of the cases) bethe simpler and more direct:

 print $var;
  my $new = $old;
  somefunc($var);

Otherwise, besides slowing you down, you're going to break code whenthe thing in the scalar is actually neither a string nor a number, buta reference:

 func(\@array);
  sub func {
  my $aref = shift;
  my $oref = "$aref";  # WRONG
  }

You can also get into subtle problems on those few operations in Perlthat actually do care about the difference between a string and anumber, such as the magical ++ autoincrement operator or thesyscall() function.

Stringification also destroys arrays.

 my @lines = `command`;
  print "@lines"; # WRONG - extra blanks
  print @lines;   # right

Why don't my <<HERE documents work?

Here documents are found in perlop. Check for these three things:

There must be no space after the << part.
There (probably) should be a semicolon at the end of the opening token
You can't (easily) have any space in front of the tag.
There needs to be at least a line separator after the end token.

If you want to indent the text in the here document, youcan do this:

 # all in one
  (my $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 quotein the indentation.

 (my $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 documentsfollows. It expects to be called with a here document as its argument.It looks to see whether each line begins with a common substring, andif so, strips that substring off. Otherwise, it takes the amount of leadingwhitespace found on the first line and removes that much off eachsubsequent line.

 sub fix {
  local $_ = shift;
  my ($white, $leader);  # common whitespace and common leading string
  if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) {
  ($white, $leader) = ($2, quotemeta($1));
  } else {
  ($white, $leader) = (/^(\s+)/, '');
  }
  s/^\s*?$leader(?:$white)?//gm;
  return $_;
  }

This works with leading special strings, dynamically determined:

 my $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 remainingindentation correctly preserved:

 my $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

Data: Arrays

What is the difference between a list and an array?

(contributed by brian d foy)

A list is a fixed collection of scalars. An array is a variable thatholds a variable collection of scalars. An array can supply its collectionfor list operations, so list operations also work on arrays:

 # slices
  ( 'dog', 'cat', 'bird' )[2,3];
  @animals[2,3];
 
  # iteration
  foreach ( qw( dog cat bird ) ) { ... }
  foreach ( @animals ) { ... }
 
  my @three = grep { length == 3 } qw( dog cat bird );
  my @three = grep { length == 3 } @animals;
 
  # supply an argument list
  wash_animals( qw( dog cat bird ) );
  wash_animals( @animals );

Array operations, which change the scalars, rearranges them, or addsor subtracts some scalars, only work on arrays. These can't work on alist, which is fixed. Array operations include shift, unshift,push, pop, and splice.

An array can also change its length:

 $#animals = 1;  # truncate to two elements
  $#animals = 10000; # pre-extend to 10,001 elements

You can change an array element, but you can't change a list element:

 $animals[0] = 'Rottweiler';
  qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!
 
  foreach ( @animals ) {
  s/^d/fr/;  # works fine
  }
 
  foreach ( qw( dog cat bird ) ) {
  s/^d/fr/;  # Error! Modification of read only value!
  }

However, if the list element is itself a variable, it appears that youcan change a list element. However, the list element is the variable, notthe data. You're not changing the list element, but something the listelement refers to. The list element itself doesn't change: it's stillthe same variable.

You also have to be careful about context. You can assign an array toa scalar to get the number of elements in the array. This only worksfor arrays, though:

 my $count = @animals;  # only works with arrays

If you try to do the same thing with what you think is a list, youget a quite different result. Although it looks like you have a liston the righthand side, Perl actually sees a bunch of scalars separatedby a comma:

 my $scalar = ( 'dog', 'cat', 'bird' );  # $scalar gets bird

Since you're assigning to a scalar, the righthand side is in scalarcontext. The comma operator (yes, it's an operator!) in scalarcontext evaluates its lefthand side, throws away the result, andevaluates it's righthand side and returns the result. In effect,that list-lookalike assigns to $scalar it's rightmost value. Manypeople mess this up because they choose a list-lookalike whoselast element is also the count they expect:

 my $scalar = ( 1, 2, 3 );  # $scalar gets 3, accidentally

What is the difference between $array[1] and @array[1]?

(contributed by brian d foy)

The difference is the sigil, that special character in front of thearray name. The $ sigil means "exactly one item", while the @sigil means "zero or more items". The $ gets you a single scalar,while the @ gets you a list.

The confusion arises because people incorrectly assume that the sigildenotes the variable type.

The $array[1] is a single-element access to the array. It's goingto return the item in index 1 (or undef if there is no item there).If you intend to get exactly one element from the array, this is theform you should use.

The @array[1] is an array slice, although it has only one index.You can pull out multiple elements simultaneously by specifyingadditional indices as a list, like @array[1,4,3,0].

Using a slice on the lefthand side of the assignment supplies listcontext to the righthand side. This can lead to unexpected results.For instance, if you want to read a single line from a filehandle,assigning to a scalar value is fine:

 $array[1] = <STDIN>;

However, in list context, the line input operator returns all of thelines as a list. The first line goes into @array[1] and the restof the lines mysteriously disappear:

 @array[1] = <STDIN>;  # most likely not what you want

Either the use warnings pragma or the -w flag will warn you whenyou use an array slice with a single index.

How can I remove duplicate elements from a list or array?

(contributed by brian d foy)

Use a hash. When you think the words "unique" or "duplicated", think"hash keys".

If you don't care about the order of the elements, you could justcreate the hash then extract the keys. It's not important how youcreate that hash: just that you use keys to get the uniqueelements.

 my %hash   = map { $_, 1 } @array;
  # or a hash slice: @hash{ @array } = ();
  # or a foreach: $hash{$_} = 1 foreach ( @array );
 
  my @unique = keys %hash;

If you want to use a module, try the uniq function fromList::MoreUtils. In list context it returns the unique elements,preserving their order in the list. In scalar context, it returns thenumber of unique elements.

 use List::MoreUtils qw(uniq);
 
  my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
  my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7

You can also go through each element and skip the ones you've seenbefore. Use a hash to keep track. The first time the loop sees anelement, that element has no key in %Seen. The next statementcreates the key and immediately uses its value, which is undef, sothe loop continues to the push and increments the value for thatkey. The next time the loop sees that same element, its key exists inthe hash and the value for that key is true (since it's not 0 orundef), so the next skips that iteration and the loop goes to thenext element.

 my @unique = ();
  my %seen   = ();
 
  foreach my $elem ( @array ) {
  next if $seen{ $elem }++;
  push @unique, $elem;
  }

You can write this more briefly using a grep, which does thesame thing.

 my %seen = ();
  my @unique = grep { ! $seen{ $_ }++ } @array;

How can I tell whether a certain element is contained in a list or array?

(portions of this answer contributed by Anno Siegel and brian d foy)

Hearing the word "in" is an indication that you probably should haveused a hash, not a list or array, to store your data. Hashes aredesigned to answer this question quickly and efficiently. Arrays aren't.

That being said, there are several ways to approach this. In Perl 5.10and later, you can use the smart match operator to check that an item iscontained in an array or a hash:

 use 5.010;
 
  if( $item ~~ @array ) {
  say "The array contains $item"
  }
 
  if( $item ~~ %hash ) {
  say "The hash contains $item"
  }

With earlier versions of Perl, you have to do a bit more work. If youare going to make this query many times over arbitrary string values,the fastest way is probably to invert the original array and maintain ahash whose keys are the first array's values:

 my @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
  my %is_blue = ();
  for (@blues) { $is_blue{$_} = 1 }

Now you can check whether $is_blue{$some_color}. It might havebeen a good idea to keep the blues all in a hash in the first place.

If the values are all small integers, you could use a simple indexedarray. This kind of an array will take up less space:

 my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
  my @is_tiny_prime = ();
  for (@primes) { $is_tiny_prime[$_] = 1 }
  # or simply  @istiny_prime[@primes] = (1) x @primes;

Now you check whether $is_tiny_prime[$some_number].

If the values in question are integers instead of strings, you can savequite a lot of space by using bit strings instead:

 my @articles = ( 1..10, 150..2000, 2017 );
  undef $read;
  for (@articles) { vec($read,$_,1) = 1 }

Now check whether vec($read,$n,1) is true for some $n.

These methods guarantee fast individual tests but require a re-organizationof the original list or array. They only pay off if you have to testmultiple values against the same array.

If you are testing only once, the standard module List::Util exportsthe function first for this purpose. It works by stopping once itfinds the element. It's written in C for speed, and its Perl equivalentlooks like this subroutine:

 sub first (&@) {
  my $code = shift;
  foreach (@_) {
  return $_ if &{$code}();
  }
  undef;
  }

If speed is of little concern, the common idiom uses grep in scalar context(which returns the number of items that passed its condition) to traverse theentire list. This does have the benefit of telling you how many matches itfound, though.

 my $is_there = grep $_ eq $whatever, @array;

If you want to actually extract the matching elements, simply use grep inlist context.

 my @matches = grep $_ eq $whatever, @array;

How do I compute the difference of two arrays? How do I compute the intersection of two arrays?

Use a hash. Here's code to do both and more. It assumes that eachelement is unique in a given array:

 my (@union, @intersection, @difference);
  my %count = ();
  foreach my $element (@array1, @array2) { $count{$element}++ }
  foreach my $element (keys %count) {
  push @union, $element;
  push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
  }

Note that this is the symmetric difference, that is, all elementsin either A or in B but not in both. Think of it as an xor operation.

How do I test whether two arrays or hashes are equal?

With Perl 5.10 and later, the smart match operator can give you the answerwith the least amount of work:

 use 5.010;
 
  if( @array1 ~~ @array2 ) {
  say "The arrays are the same";
  }
 
  if( %hash1 ~~ %hash2 ) # doesn't check values!  {
  say "The hash keys are the same";
  }

The following code works for single-level arrays. It uses astringwise comparison, and does not distinguish defined versusundefined 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 morelike this one. It uses the CPAN module FreezeThaw:

 use FreezeThaw qw(cmpStr);
  my @a = my @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 demonstratetwo different answers:

 use FreezeThaw qw(cmpStr cmpStrHard);
 
  my %a = my %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 asan exercise to the reader.

How do I find the first array element for which a condition is true?

To find the first array element which satisfies a condition, you canuse the first() function in the List::Util module, which comeswith Perl 5.8. This example finds the first element that contains"Perl".

 use List::Util qw(first);
 
  my $element = first { /Perl/ } @array;

If you cannot use List::Util, you can make your own loop to do thesame thing. Once you find the element, you stop the loop with last.

 my $found;
  foreach ( @array ) {
  if( /Perl/ ) { $found = $_; last }
  }

If you want the array index, use the firstidx() function fromList::MoreUtils:

 use List::MoreUtils qw(firstidx);
  my $index = firstidx { /Perl/ } @array;

Or write it yourself, iterating through the indicesand checking the array element at each index until you find onethat satisfies the condition:

 my( $found, $index ) = ( undef, -1 );
  for( $i = 0; $i < @array; $i++ ) {
  if( $array[$i] =~ /Perl/ ) {
  $found = $array[$i];
  $index = $i;
  last;
  }
  }

How do I handle linked lists?

(contributed by brian d foy)

Perl's arrays do not have a fixed size, so you don't need linked listsif you just want to add or remove items. You can use array operationssuch as push, pop, shift, unshift, or splice to dothat.

Sometimes, however, linked lists can be useful in situations where youwant to "shard" an array so you have have many small arrays instead ofa single big array. You can keep arrays longer than Perl's largestarray index, lock smaller arrays separately in threaded programs,reallocate less memory, or quickly insert elements in the middle ofthe chain.

Steve Lembark goes through the details in his YAPC::NA 2009 talk "PerlyLinked Lists" ( http://www.slideshare.net/lembark/perly-linked-lists ),although you can just use his LinkedList::Single module.

How do I handle circular lists?

(contributed by brian d foy)

If you want to cycle through an array endlessly, you can increment theindex modulo the number of elements in the array:

 my @array = qw( a b c );
  my $i = 0;
 
  while( 1 ) {
  print $array[ $i++ % @array ], "\n";
  last if $i > 20;
  }

You can also use Tie::Cycle to use a scalar that always has thenext element of the circular array:

 use Tie::Cycle;
 
  tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
 
  print $cycle; # FFFFFF
  print $cycle; # 000000
  print $cycle; # FFFF00

The Array::Iterator::Circular creates an iterator object forcircular arrays:

 use Array::Iterator::Circular;
 
  my $color_iterator = Array::Iterator::Circular->new(
  qw(red green blue orange)
  );
 
  foreach ( 1 .. 20 ) {
  print $color_iterator->next, "\n";
  }

How do I shuffle an array randomly?

If you either have Perl 5.8.0 or later installed, or if you haveScalar-List-Utils 1.03 or later installed, you can say:

 use List::Util 'shuffle';
 
  @shuffled = shuffle(@list);

If not, you can use a Fisher-Yates shuffle.

 sub fisher_yates_shuffle {
  my $deck = shift;  # $deck is a reference to an array
  return unless @$deck; # must not be empty!
 
  my $i = @$deck;
  while (--$i) {
  my $j = int rand ($i+1);
  @$deck[$i,$j] = @$deck[$j,$i];
  }
  }
 
  # shuffle my mpeg collection
  #
  my @mpeg = <audio/*/*.mp3>;
  fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
  print @mpeg;

Note that the above implementation shuffles an array in place,unlike the List::Util::shuffle() which takes a list and returnsa new shuffled list.

You've probably seen shuffling algorithms that work using splice,randomly picking another element to swap the current element with

 srand;
  @new = ();
  @old = 1 .. 10;  # just a demo
  while (@old) {
  push(@new, splice(@old, rand @old, 1));
  }

This is bad because splice is already O(N), and since you do it Ntimes, you just invented a quadratic algorithm; that is, O(N**2).This does not scale, although Perl is so efficient that you probablywon't notice this until you have rather largish arrays.

How do I process/modify each element of an array?

Use for/foreach:

 for (@lines) {
  s/foo/bar/; # change that word
  tr/XZ/ZX/; # swap those letters
  }

Here's another; let's compute spherical volumes:

 my @volumes = @radii;
  for (@volumes) {   # @volumes has changed parts
  $_ **= 3;
  $_ *= (4/3) * 3.14159;  # this will be constant folded
  }

which can also be done with map() which is made to transformone list into another:

 my @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;

If you want to do the same thing to modify the values of thehash, you can use the values function. As of Perl 5.6the values are not copied, so if you modify $orbit (in thiscase), you modify the value.

 for my $orbit ( values %orbits ) {
  ($orbit **= 3) *= (4/3) * 3.14159;
  }

Prior to perl 5.6 values returned copies of the values,so older perl code often contains constructions such as@orbits{keys %orbits} instead of values %orbits wherethe hash is to be modified.

How do I select a random element from an array?

Use the rand() function (see rand):

 my $index   = rand @array;
  my $element = $array[$index];

Or, simply:

 my $element = $array[ rand @array ];

How do I permute N elements of a list?

Use the List::Permutor module on CPAN. If the list is actually anarray, try the Algorithm::Permute module (also on CPAN). It'swritten in XS code and is very efficient:

 use Algorithm::Permute;
 
  my @array = 'a'..'d';
  my $p_iterator = Algorithm::Permute->new ( \@array );
 
  while (my @perm = $p_iterator->next) {
    print "next permutation: (@perm)\n";
  }

For even faster execution, you could do:

 use Algorithm::Permute;
 
  my @array = 'a'..'d';
 
  Algorithm::Permute::permute {
  print "next permutation: (@array)\n";
  } @array;

Here's a little program that generates all permutations of all thewords on each line of input. The algorithm embodied in thepermute() function is discussed in Volume 4 (still unpublished) ofKnuth's The Art of Computer Programming and will work on any list:

 #!/usr/bin/perl -n
  # Fischer-Krause ordered permutation generator
 
  sub permute (&@) {
  my $code = shift;
  my @idx = 0..$#_;
  while ( $code->(@_[@idx]) ) {
  my $p = $#idx;
  --$p while $idx[$p-1] > $idx[$p];
  my $q = $p or return;
  push @idx, reverse splice @idx, $p;
  ++$q while $idx[$p-1] > $idx[$q];
  @idx[$p-1,$q]=@idx[$q,$p-1];
  }
  }
 
  permute { print "@_\n" } split;

The Algorithm::Loops module also provides the NextPermute andNextPermuteNum functions which efficiently find all unique permutationsof an array, even if it contains duplicate values, modifying it in-place:if its elements are in reverse-sorted order then the array is reversed,making it sorted, and it returns false; otherwise the nextpermutation is returned.

NextPermute uses string order and NextPermuteNum numeric order, soyou can enumerate all the permutations of 0..9 like this:

 use Algorithm::Loops qw(NextPermuteNum);
 
  my @list= 0..9;
  do { print "@list\n" } while NextPermuteNum @list;

How do I sort an array by (anything)?

Supply a comparison function to sort() (described in sort):

 @list = sort { $a <=> $b } @list;

The default sort function is cmp, string comparison, which wouldsort (1, 2, 10) into (1, 10, 2). <=>, used above, isthe numerical comparison operator.

If you have a complicated function needed to pull out the part youwant to sort on, then don't do it inside the sort function. Pull itout first, because the sort BLOCK can be called many times for thesame element. Here's an example of how to pull out the first wordafter the first number on each item, and then sort those wordscase-insensitively.

 my @idx;
  for (@data) {
  my $item;
  ($item) = /\d+\s*(\S+)/;
  push @idx, uc($item);
  }
  my @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];

which could also be written this way, using a trickthat's come to be known as the Schwartzian Transform:

 my @sorted = map  { $_->[0] }
  sort { $a->[1] cmp $b->[1] }
  map  { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;

If you need to sort on several fields, the following paradigm is useful.

 my @sorted = sort {
  field1($a) <=> field1($b) ||
  field2($a) cmp field2($b) ||
  field3($a) cmp field3($b)
  } @data;

This can be conveniently combined with precalculation of keys as givenabove.

See the sort article in the "Far More Than You Ever WantedTo Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz formore about this approach.

See also the question later in perlfaq4 on sorting hashes.

How do I manipulate arrays of bits?

Use pack() and unpack(), or else vec() and the bitwiseoperations.

For example, you don't have to store individual bits in an array(which would mean that you're wasting a lot of space). To convert anarray of bits to a string, use vec() to set the right bits. Thissets $vec to have bit N set only if $ints[N] was set:

 my @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
  my $vec = '';
  foreach( 0 .. $#ints ) {
  vec($vec,$_,1) = 1 if $ints[$_];
  }

The string $vec only takes up as many bits as it needs. Forinstance, if you had 16 entries in @ints, $vec only needs twobytes to store them (not counting the scalar variable overhead).

Here's how, given a vector in $vec, you can get those bits intoyour @ints array:

 sub bitvec_to_list {
  my $vec = shift;
  my @ints;
  # Find null-byte density then select best algorithm
  if ($vec =~ tr/\0// / length $vec > 0.95) {
  use integer;
  my $i;
 
  # This method is faster with mostly null-bytes
  while($vec =~ /[^\0]/g ) {
  $i = -9 + 8 * pos $vec;
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  push @ints, $i if vec($vec, ++$i, 1);
  }
  }
  else {
  # This method is a fast general algorithm
  use integer;
  my $bits = unpack "b*", $vec;
  push @ints, 0 if $bits =~ s/^(\d)// && $1;
  push @ints, pos $bits while($bits =~ /1/g);
  }
 
  return \@ints;
  }

This method gets faster the more sparse the bit vector is.(Courtesy of Tim Bunce and Winfried Koenig.)

You can make the while loop a lot shorter with this suggestionfrom Benjamin Goldberg:

 while($vec =~ /[^\0]+/g ) {
  push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
  }

Or use the CPAN module Bit::Vector:

 my $vector = Bit::Vector->new($num_of_bits);
  $vector->Index_List_Store(@ints);
  my @ints = $vector->Index_List_Read();

Bit::Vector provides efficient methods for bit vector, sets ofsmall integers and "big int" math.

Here's a more extensive illustration using vec():

 # vec demo
  my $vector = "\xff\x0f\xef\xfe";
  print "Ilya's string \xff\x0f\xef\xfe represents the number ",
  unpack("N", $vector), "\n";
  my $is_set = vec($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";
  }

Why does defined() return true on empty arrays and hashes?

The short story is that you should probably only use defined on scalars orfunctions, not on aggregates (arrays and hashes). See definedin the 5.004 release or later of Perl for more detail.

Data: Hashes (Associative Arrays)

How do I process an entire hash?

(contributed by brian d foy)

There are a couple of ways that you can process an entire hash. Youcan get a list of keys, then go through each key, or grab a onekey-value pair at a time.

To go through all of the keys, use the keys function. This extractsall of the keys of the hash and gives them back to you as a list. Youcan then get the value through the particular key you're processing:

 foreach my $key ( keys %hash ) {
  my $value = $hash{$key}
  ...
  }

Once you have the list of keys, you can process that list before youprocess the hash elements. For instance, you can sort the keys so youcan process them in lexical order:

 foreach my $key ( sort keys %hash ) {
  my $value = $hash{$key}
  ...
  }

Or, you might want to only process some of the items. If you only wantto deal with the keys that start with text:, you can select justthose using grep:

 foreach my $key ( grep /^text:/, keys %hash ) {
  my $value = $hash{$key}
  ...
  }

If the hash is very large, you might not want to create a long list ofkeys. To save some memory, you can grab one key-value pair at a time usingeach(), which returns a pair you haven't seen yet:

 while( my( $key, $value ) = each( %hash ) ) {
  ...
  }

The each operator returns the pairs in apparently random order, so ifordering matters to you, you'll have to stick with the keys method.

The each() operator can be a bit tricky though. You can't add ordelete keys of the hash while you're using it without possiblyskipping or re-processing some pairs after Perl internally rehashesall of the elements. Additionally, a hash has only one iterator, so ifyou mix keys, values, or each on the same hash, you risk resettingthe iterator and messing up your processing. See the each entry inperlfunc for more details.

How do I merge two hashes?

(contributed by brian d foy)

Before you decide to merge two hashes, you have to decide what to doif both hashes contain keys that are the same and if you want to leavethe original hashes as they were.

If you want to preserve the original hashes, copy one hash (%hash1)to a new hash (%new_hash), then add the keys from the other hash(%hash2 to the new hash. Checking that the key already exists in%new_hash gives you a chance to decide what to do with theduplicates:

 my %new_hash = %hash1; # make a copy; leave %hash1 alone
 
  foreach my $key2 ( keys %hash2 ) {
  if( exists $new_hash{$key2} ) {
  warn "Key [$key2] is in both hashes!";
  # handle the duplicate (perhaps only warning)
  ...
  next;
  }
  else {
  $new_hash{$key2} = $hash2{$key2};
  }
  }

If you don't want to create a new hash, you can still use this loopingtechnique; just change the %new_hash to %hash1.

 foreach my $key2 ( keys %hash2 ) {
  if( exists $hash1{$key2} ) {
  warn "Key [$key2] is in both hashes!";
  # handle the duplicate (perhaps only warning)
  ...
  next;
  }
  else {
  $hash1{$key2} = $hash2{$key2};
  }
   }

If you don't care that one hash overwrites keys and values from the other, youcould just use a hash slice to add one hash to another. In this case, valuesfrom %hash2 replace values from %hash1 when they have keys in common:

 @hash1{ keys %hash2 } = values %hash2;

What happens if I add or remove keys from a hash while iterating over it?

(contributed by brian d foy)

The easy answer is "Don't do that!"

If you iterate through the hash with each(), you can delete the keymost recently returned without worrying about it. If you delete or addother keys, the iterator may skip or double up on them since perlmay rearrange the hash table. See theentry for each() in perlfunc.

How do I look up a hash element by value?

Create a reverse hash:

 my %by_value = reverse %by_key;
  my $key = $by_value{$value};

That's not particularly efficient. It would be more space-efficientto use:

 while (my ($key, $value) = each %by_key) {
  $by_value{$value} = $key;
  }

If your hash could have repeated values, the methods above will only findone of the associated keys. This may or may not worry you. If it doesworry you, you can always reverse the hash into a hash of arrays instead:

 while (my ($key, $value) = each %by_key) {
  push @{$key_list_by_value{$value}}, $key;
  }

How can I know how many entries are in a hash?

(contributed by brian d foy)

This is very similar to "How do I process an entire hash?", also inperlfaq4, but a bit simpler in the common cases.

You can use the keys() built-in function in scalar context to find outhave many entries you have in a hash:

 my $key_count = keys %hash; # must be scalar context!

If you want to find out how many entries have a defined value, that'sa bit different. You have to check each value. A grep is handy:

 my $defined_value_count = grep { defined } values %hash;

You can use that same structure to count the entries any way thatyou like. If you want the count of the keys with vowels in them,you just test for that instead:

 my $vowel_count = grep { /[aeiou]/ } keys %hash;

The grep in scalar context returns the count. If you want the listof matching items, just use it in list context instead:

 my @defined_values = grep { defined } values %hash;

The keys() function also resets the iterator, which means that you maysee strange results if you use this between uses of other hash operatorssuch as each().

How do I sort a hash (optionally by value instead of key)?

(contributed by brian d foy)

To sort a hash, start with the keys. In this example, we give the list ofkeys to the sort function which then compares them ASCIIbetically (whichmight be affected by your locale settings). The output list has the keysin ASCIIbetical order. Once we have the keys, we can go through them tocreate a report which lists the keys in ASCIIbetical order.

 my @keys = sort { $a cmp $b } keys %hash;
 
  foreach my $key ( @keys ) {
  printf "%-20s %6d\n", $key, $hash{$key};
  }

We could get more fancy in the sort() block though. Instead ofcomparing the keys, we can compute a value with them and use thatvalue as the comparison.

For instance, to make our report order case-insensitive, we uselc to lowercase the keys before comparing them:

 my @keys = sort { lc $a cmp lc $b } keys %hash;

Note: if the computation is expensive or the hash has many elements,you may want to look at the Schwartzian Transform to cache thecomputation results.

If we want to sort by the hash value instead, we use the hash keyto look it up. We still get out a list of keys, but this time theyare ordered by their value.

 my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;

From there we can get more complex. If the hash values are the same,we can provide a secondary sort on the hash key.

 my @keys = sort {
  $hash{$a} <=> $hash{$b}
  or
  "\L$a" cmp "\L$b"
  } keys %hash;

How can I always keep my hash sorted?

You can look into using the DB_File module and tie() using the$DB_BTREE hash bindings as documented in In Memory Databases in DB_File. The Tie::IxHash module from CPAN might also beinstructive. Although this does keep your hash sorted, you might notlike the slowdown you suffer from the tie interface. Are you sure youneed to do this? :)

What's the difference between "delete" and "undef" with hashes?

Hashes contain pairs of scalars: the first is the key, thesecond is the value. The key will be coerced to a string,although the value can be any kind of scalar: string,number, or reference. If a key $key is present in%hash, exists($hash{$key}) will return true. The valuefor a given key can be undef, in which case$hash{$key} will be undef while exists $hash{$key}will return true. This corresponds to ($key, undef)being in the hash.

Pictures help... Here's the %hash table:

  keys  values
  +------+------+
  |  a   |  3   |
  |  x   |  7   |
  |  d   |  0   |
  |  e   |  2   |
  +------+------+

And these conditions hold

 $hash{'a'}   is true
  $hash{'d'}   is false
  defined $hash{'d'}   is true
  defined $hash{'a'}   is true
  exists $hash{'a'} is true (Perl 5 only)
  grep ($_ eq 'a', keys %hash) is true

If you now say

 undef $hash{'a'}

your table now reads:

  keys  values
  +------+------+
  |  a   | undef|
  |  x   |  7   |
  |  d   |  0   |
  |  e   |  2   |
  +------+------+

and these conditions now hold; changes in caps:

 $hash{'a'}   is FALSE
  $hash{'d'}   is false
  defined $hash{'d'}   is true
  defined $hash{'a'}   is FALSE
  exists $hash{'a'} is true (Perl 5 only)
  grep ($_ eq 'a', keys %hash) is true

Notice the last two: you have an undef value, but a defined key!

Now, consider this:

 delete $hash{'a'}

your table now reads:

  keys  values
  +------+------+
  |  x   |  7   |
  |  d   |  0   |
  |  e   |  2   |
  +------+------+

and these conditions now hold; changes in caps:

 $hash{'a'}   is false
  $hash{'d'}   is false
  defined $hash{'d'}   is true
  defined $hash{'a'}   is false
  exists $hash{'a'} is FALSE (Perl 5 only)
  grep ($_ eq 'a', keys %hash) is FALSE

See, the whole entry is gone!

Why don't my tied hashes make the defined/exists distinction?

This depends on the tied hash's implementation of EXISTS().For example, there isn't the concept of undef with hashesthat are tied to DBM* files. It also means that exists() anddefined() do the same thing with a DBM* file, and what theyend up doing is not what they do with ordinary hashes.

How do I reset an each() operation part-way through?

(contributed by brian d foy)

You can use the keys or values functions to reset each. Tosimply reset the iterator used by each without doing anything else,use one of them in void context:

 keys %hash; # resets iterator, nothing else.
  values %hash; # resets iterator, nothing else.

See the documentation for each in perlfunc.

How can I get the unique keys from two hashes?

First you extract the keys from the hashes into lists, then solvethe "removing duplicates" problem described above. For example:

 my %seen = ();
  for my $element (keys(%foo), keys(%bar)) {
  $seen{$element}++;
  }
  my @uniq = keys %seen;

Or more succinctly:

 my @uniq = keys %{{%foo,%bar}};

Or if you really want to save space:

 my %seen = ();
  while (defined ($key = each %foo)) {
  $seen{$key}++;
  }
  while (defined ($key = each %bar)) {
  $seen{$key}++;
  }
  my @uniq = keys %seen;

How can I store a multidimensional array in a DBM file?

Either stringify the structure yourself (no fun), or elseget the MLDBM (which uses Data::Dumper) module from CPAN and layerit on top of either DB_File or GDBM_File. You might also try DBM::Deep, butit can be a bit slow.

How can I make my hash remember the order I put elements into it?

Use the Tie::IxHash from CPAN.

 use Tie::IxHash;
 
  tie my %myhash, 'Tie::IxHash';
 
  for (my $i=0; $i<20; $i++) {
  $myhash{$i} = 2*$i;
  }
 
  my @keys = keys %myhash;
  # @keys = (0,1,2,3,...)

Why does passing a subroutine an undefined element in a hash create it?

(contributed by brian d foy)

Are you using a really old version of Perl?

Normally, accessing a hash key's value for a nonexistent key willnot create the key.

 my %hash  = ();
  my $value = $hash{ 'foo' };
  print "This won't print\n" if exists $hash{ 'foo' };

Passing $hash{ 'foo' } to a subroutine used to be a special case, though.Since you could assign directly to $_[0], Perl had to be ready tomake that assignment so it created the hash key ahead of time:

 my_sub( $hash{ 'foo' } );
  print "This will print before 5.004\n" if exists $hash{ 'foo' };
 
  sub my_sub {
  # $_[0] = 'bar' # create hash key in case you do this
  1;
  }

Since Perl 5.004, however, this situation is a special case and Perlcreates the hash key only when you make the assignment:

 my_sub( $hash{ 'foo' } );
  print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
 
  sub my_sub {
  $_[0] = 'bar';
  }

However, if you want the old behavior (and think carefully about thatbecause it's a weird side effect), you can pass a hash slice instead.Perl 5.004 didn't make this a special case:

 my_sub( @hash{ qw/foo/ } );

How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?

Usually a hash ref, perhaps like this:

 $record = {
  NAME   => "Jason",
  EMPNO  => 132,
  TITLE  => "deputy peon",
  AGE => 23,
  SALARY => 37_000,
  PALS   => [ "Norbert", "Rhys", "Phineas"],
  };

References are documented in perlref and perlreftut.Examples of complex data structures are given in perldsc andperllol. Examples of structures and object-oriented classes arein perltoot.

How can I use a reference as a hash key?

(contributed by brian d foy and Ben Morrow)

Hash keys are strings, so you can't really use a reference as the key.When you try to do that, perl turns the reference into its stringifiedform (for instance, HASH(0xDEADBEEF)). From there you can't getback the reference from the stringified form, at least without doingsome extra work on your own.

Remember that the entry in the hash will still be there even ifthe referenced variable goes out of scope, and that it is entirelypossible for Perl to subsequently allocate a different variable atthe same address. This will mean a new variable might accidentallybe associated with the value for an old.

If you have Perl 5.10 or later, and you just want to store a valueagainst the reference for lookup later, you can use the coreHash::Util::Fieldhash module. This will also handle renaming thekeys if you use multiple threads (which causes all variables to bereallocated at new addresses, changing their stringification), andgarbage-collecting the entries when the referenced variable goes outof scope.

If you actually need to be able to get a real reference back fromeach hash entry, you can use the Tie::RefHash module, which does therequired work for you.

How can I check if a key exists in a multilevel hash?

(contributed by brian d foy)

The trick to this problem is avoiding accidental autovivification. Ifyou want to check three keys deep, you might naïvely try this:

 my %hash;
  if( exists $hash{key1}{key2}{key3} ) {
  ...;
  }

Even though you started with a completely empty hash, after that call toexists you've created the structure you needed to check for key3:

 %hash = (
   'key1' => {
   'key2' => {}
  }
  );

That's autovivification. You can get around this in a few ways. Theeasiest way is to just turn it off. The lexical autovivificationpragma is available on CPAN. Now you don't add to the hash:

 {
  no autovivification;
  my %hash;
  if( exists $hash{key1}{key2}{key3} ) {
  ...;
  }
  }

The Data::Diver module on CPAN can do it for you too. Its Divesubroutine can tell you not only if the keys exist but also get thevalue:

 use Data::Diver qw(Dive);
 
  my @exists = Dive( \%hash, qw(key1 key2 key3) );
  if(  ! @exists  ) {
  ...; # keys do not exist
  }
  elsif(  ! defined $exists[0]  ) {
  ...; # keys exist but value is undef
  }

You can easily do this yourself too by checking each level of the hashbefore you move onto the next level. This is essentially whatData::Diver does for you:

 if( check_hash( \%hash, qw(key1 key2 key3) ) ) {
  ...;
  }
 
  sub check_hash {
    my( $hash, @keys ) = @_;
 
    return unless @keys;
 
    foreach my $key ( @keys ) {
    return unless eval { exists $hash->{$key} };
    $hash = $hash->{$key};
  }
 
    return 1;
  }

How can I prevent addition of unwanted keys into a hash?

Since version 5.8.0, hashes can be restricted to a fixed numberof given keys. Methods for creating and dealing with restricted hashesare exported by the Hash::Util module.

Data: Misc

How do I handle binary data correctly?

Perl is binary-clean, so it can handle binary data just fine.On Windows or DOS, however, you have to use binmode for binaryfiles to avoid conversions for line endings. In general, you shoulduse binmode any time you want to work with binary data.

Also see binmode or perlopentut.

If you're concerned about 8-bit textual data then see perllocale.If you want to deal with multibyte characters, however, there aresome gotchas. See the section on Regular Expressions.

How do I determine whether a scalar is a number/whole/integer/float?

Assuming that you don't care about IEEE notations like "NaN" or"Infinity", you probably just want to use a regular expression:

 use 5.010;
 
  given( $number ) {
  when( /\D/ )
  { say "\thas nondigits"; continue }
  when( /^\d+\z/ )
  { say "\tis a whole number"; continue }
  when( /^-?\d+\z/ )
  { say "\tis an integer"; continue }
  when( /^[+-]?\d+\z/ )
  { say "\tis a +/- integer"; continue }
  when( /^-?(?:\d+\.?|\.\d)\d*\z/ )
  { say "\tis a real number"; continue }
  when( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i)
  { say "\tis a C float" }
  }

There are also some commonly used modules for the task.Scalar::Util (distributed with 5.8) provides access to perl'sinternal function looks_like_number for determining whether avariable looks like a number. Data::Types exports functions thatvalidate data types using both the above and other regularexpressions. Thirdly, there is Regexp::Common which has regularexpressions to match various types of numbers. Those three modules areavailable from the CPAN.

If you're on a POSIX system, Perl supports the POSIX::strtodfunction for converting strings to doubles (and also POSIX::strtolfor longs). Its semantics are somewhat cumbersome, so here's agetnum wrapper function for more convenient access. This functiontakes a string and returns the number it found, or undef for inputthat isn't a C float. The is_numeric function is a front end togetnum 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 the CPANinstead.

How do I keep persistent data across program calls?

For some specific applications, you can use one of the DBM modules.See AnyDBM_File. More generically, you should consult the FreezeThawor Storable modules from CPAN. Starting from Perl 5.8, Storable is partof the standard distribution. Here's one example using Storable's storeand retrieve functions:

 use Storable;
  store(\%hash, "filename");
 
  # later on...
  $href = retrieve("filename"); # by ref
  %hash = %{ retrieve("filename") };   # direct to hash

How do I print out or copy a recursive data structure?

The Data::Dumper module on CPAN (or the 5.005 release of Perl) is greatfor printing out data structures. The Storable module on CPAN (or the5.8 release of Perl), provides a function called dclone that recursivelycopies its argument.

 use Storable qw(dclone);
  $r2 = dclone($r1);

Where $r1 can be a reference to any kind of data structure you'd like.It will be deeply copied. Because dclone takes and returns references,you'd have to add extra punctuation if you had a hash of arrays thatyou wanted to copy.

 %newhash = %{ dclone(\%oldhash) };

How do I define methods for every class/object?

(contributed by Ben Morrow)

You can use the UNIVERSAL class (see UNIVERSAL). However, pleasebe very careful to consider the consequences of doing this: addingmethods to every object is very likely to have unintendedconsequences. If possible, it would be better to have all your objectinherit from some common base class, or to use an object system likeMoose that supports roles.

How do I verify a credit card checksum?

Get the Business::CreditCard module from CPAN.

How do I pack arrays of doubles or floats for XS code?

The arrays.h/arrays.c code in the PGPLOT module on CPAN does just this.If you're doing a lot of float or double processing, consider usingthe PDL module from CPAN instead--it makes number-crunching easy.

See http://search.cpan.org/dist/PGPLOT for the code.

AUTHOR AND COPYRIGHT

This documentation is free; you can redistribute it and/or modify itunder the same terms as Perl itself.

Irrespective of its distribution, all code examples in this fileare hereby placed into the public domain. You are permitted andencouraged to use this code in your own programs for funor for profit as you see fit. A simple comment in the code givingcredit would be courteous but is not required.

Page index

NAME
DESCRIPTION
Data: Numbers
Data: Dates
Data: Strings
Data: Arrays
Data: Hashes (Associative Arrays)
Data: Misc
AUTHOR AND COPYRIGHT

Source : perldoc.perl.org - Official documentation for the Perl programming language
Site maintained by Jon Allen (JJ) See the project page for more details
Documentation maintained by the Perl 5 Porters

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