perlfaq4 - Data Manipulation ($Revision: 1.61 $, $Date: 2005/03/11 16:27:53 $)

This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.

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

perlnumber show the gory details of number representations and conversions.

To limit the number of decimal places in your numbers, you can use the printf or sprintf function. See the Floating Point Arithmetic for more details.

        printf "%.2f", 10/3;

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

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

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

For example, this

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

will in most computers print 0, not 1, because even such simple numbers as 0.6 and 0.2 cannot be presented exactly by floating-point numbers. What you think in the above as 'three' is really more like 2.9999999999999995559.

Perl only understands octal and hex numbers as such when they occur as literals in your program. Octal literals in perl must start with a leading 0 and hexadecimal literals must start with a leading 0x. If they are read in from somewhere and assigned, no automatic conversion takes place. You must explicitly use oct() or hex() if you want the values converted to decimal. oct() interprets hex (0x350), octal (0350 or even without the leading 0, like 377) and binary (0b1010) numbers, 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.

This problem shows up most often when people try using chmod(), mkdir(), umask(), or sysopen(), which by widespread tradition typically take permissions in octal.

    chmod(644,  $file); # WRONG
    chmod(0644, $file); # right

Note the mistake in the first line was specifying the decimal literal 644, rather than the intended octal literal 0644. The problem can be seen with:

    printf("%#o",644); # prints 01204

Surely you had not intended CWchmod(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.

Remember that int() merely truncates toward 0. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route.

    printf("%.3f", 3.1415926535);       # prints 3.142

The POSIX module (part of the standard Perl distribution) implements ceil(), floor(), and a number of other mathematical and trigonometric functions.

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

In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2.

Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.

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

    for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

    0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
    0.8 0.8 0.9 0.9 1.0 1.0

Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32 bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed.

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.

How do I convert hexadecimal into decimal

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

    $dec = 0xDEADBEEF;

Using the hex function:

    $dec = hex("DEADBEEF");

Using pack:

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

Using the CPAN module Bit::Vector:

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

How do I convert from decimal to hexadecimal

Using sprintf:

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

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(3); # suppress leading 0 if unwanted
    $hex = $vec->to_Hex();

How do I convert from octal to decimal

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

    $dec = 033653337357; # note the leading 0!

Using the oct function:

    $dec = oct("33653337357");

Using Bit::Vector:

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

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));

How do I convert from binary to decimal

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

    $number = 0b10110110;

Using oct:

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

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);

    # substr() is used to left pad a 32 character string with zeros.

Using Bit::Vector:

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

How do I convert from decimal to binary

Using sprintf (perl 5.6+):

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

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.

The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string CW"3" is the bit pattern CW00110011). The operators work with the binary form of a number (the number CW3 is treated as the bit pattern CW00000011).

So, saying CW11 & 3 performs the and operation on numbers (yielding CW3). Saying CW"11" & "3" performs the and operation on strings (yielding CW"1").

Most problems with CW& and CW| arise because the programmer thinks they have a number but really it's a string. The rest arise because the programmer says:

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

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

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

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

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

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

For example:

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

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

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

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

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

but you should be aware that the CW.. operator creates an array of all integers in the range. This can take a lot of memory for large ranges. Instead use:

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

This situation has been fixed in Perl5.005. Use of CW.. in a CWfor 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.

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

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

         BEGIN { srand() if $] < 5.004 }

5.004 and later automatically call CWsrand at the beginning. Don't call CWsrand 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 :-). see the random article in the Far More Than You Ever Wanted To Know collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of Tom Phoenix, talks more about this. John von Neumann said, ``Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.''

If you want numbers that are more random than CWrand with CWsrand provides, you should also check out the Math::TrulyRandom module from CPAN. It uses the imperfections in your system's timer to generate random numbers, but this takes quite a while. If you want a better pseudorandom generator than comes with your operating system, look at ``Numerical Recipes in C'' at http://www.nr.com/ .

CWrand($x) returns a number such that CW0 <= rand($x) < $x. Thus what you want to have perl figure out is a random number in the range from 0 to the difference between your X and Y.

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

    my $number = 10 + int rand( 15-10+1 );

Hence you derive the following simple function to abstract that. It selects a random integer between the two given integers (inclusive), For example: CWrandom_int_in(50,120).

   sub random_int_in ($$) {
     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);
   }

The localtime function returns the day of the year. Without an argument localtime uses the current time.

    $day_of_year = (localtime)[7];

The POSIX module can also format a date as the day of the year or week 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 the Time::Local module to get a time in epoch seconds for the argument to localtime.

        use POSIX qw/strftime/;
        use Time::Local;
        my $week_of_year = strftime "%W", 
                localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );

The Date::Calc module provides two functions for to calculate these.

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

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 been extended in a non-standard way to use a CW%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.

If you're storing your dates as epoch seconds then simply subtract one from the other. If you've got a structured date (distinct year, day, month, hour, minute, seconds values), then for reasons of accessibility, simplicity, and efficiency, merely use either timelocal or timegm (from the Time::Local module in the standard distribution) to reduce structured dates to epoch seconds. However, if you don't know the precise format of your dates, then you should probably use either of the Date::Manip and Date::Calc modules from CPAN before you go hacking up your own parsing routine to handle arbitrary date formats.

If it's a regular enough string that it always has the same format, you can split it up and pass the parts to CWtimelocal in the standard Time::Local module. Otherwise, you should look into the Date::Calc and Date::Manip modules from CPAN.

(contributed by brian d foy and Dave Cross)

You can use the Time::JulianDay module available on CPAN. Ensure that you really want to find a Julian day, though, as many people have different ideas about Julian days. See http://www.hermetic.ch/cal_stud/jdn.htm for instance.

You can also try the DateTime module, which can convert a date/time to a Julian Day.

  $ perl -MDateTime -le'print DateTime->today->jd'
  2453401.5

Or the modified Julian Day

  $ perl -MDateTime -le'print DateTime->today->mjd'
  53401

Or even the day of the year (which is what some people think of as a Julian day)

  $ perl -MDateTime -le'print DateTime->today->doy'
  31

If you only need to find the date (and not the same time), you can use the Date::Calc module.

        use Date::Calc qw(Today Add_Delta_Days);

        my @date = Add_Delta_Days( Today(), -1 );

        print "@date\n";

Most people try to use the time rather than the calendar to figure out dates, but that 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. Russ Allbery offers this solution.

    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. CW$ndst is whether we're currently in daylight savings time; CW$tdst is whether the point 24 hours ago was in daylight savings time. If CW$tdst and CW$ndst are the same, a boundary wasn't crossed, and the correction will subtract 0. If CW$tdst is 1 and CW$ndst is 0, subtract an hour more from yesterday's time since we gained an extra hour while going off daylight savings time. If CW$tdst is 0 and CW$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 CW$ndst and CW$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).

Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is Y2K compliant (whatever that means). The programmers you've hired to use it, however, probably are not.

Long answer: The question belies a true understanding of the issue. Perl is just as Y2K compliant as your pencilno more, and no less. Can you use your pencil to write a non-Y2K-compliant memo? Of course you can. Is that the pencil's fault? Of course it isn't.

The date and time functions supplied with Perl (gmtime and localtime) supply adequate information to determine the year well beyond 2000 (2038 is when trouble strikes for 32-bit machines). The year returned by these functions when used in a list context is the year minus 1900. For years between 1910 and 1999 this happens to be a 2-digit decimal number. To avoid the year 2000 problem simply do not treat the year as a 2-digit number. It isn't.

When gmtime() and localtime() are used in scalar context they return a timestamp string that contains a fully-expanded year. For example, CW$timestamp = gmtime(1) sets CW$timestamp to Tue Nov 13 01:00:00 2001. There's no year 2000 problem here.

That doesn't mean that Perl can't be used to create non-Y2K compliant programs. It can. But so can your pencil. It's the fault of the user, not the language. At the risk of inflaming the NRA: ``Perl doesn't break Y2K, people do.'' See http://www.perl.org/about/y2k.html for a longer exposition.

The answer to this question is usually a regular expression, perhaps with auxiliary logic. See the more specific questions (numbers, mail addresses, etc.) for details.

It depends just what you mean by ``escape''. URL escapes are dealt with in perlfaq9. Shell escapes with the backslash (CW\) character are removed with

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

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

To turn CW"abbcccd" into CW"abccd":

    s/(.)\1/$1/g;       # add /s to include newlines

Here's a solution that turns abbcccd to abcd:

    y///cs;     # y == tr, but shorter :-)

This is documented in perlref. In general, this is fraught with quoting and readability problems, but it is possible. To interpolate a subroutine call (in list context) into a string:

    print "My sub returned @{[mysub(1,2,3)]} that time.\n";

This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single characters, a pattern like CW/x([^x]*)x/ will get the intervening bits in CW$1. For multiple ones, then something more like CW/alpha(.*?)omega/ would be needed. But none of these deals with nested patterns. For balanced expressions using CW(, CW{, CW[ or CW< as delimiters, use the CPAN module Regexp::Common, or see (??{ code }) in perlre. For other cases, you'll have to write a parser.

If you are serious about writing a parser, there are a number of modules or oddities that will make your life a lot easier. There are the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; and the byacc program. Starting from perl 5.8 the Text::Balanced is part of the standard distribution.

One simple destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time:

    while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
        # do something with $1
    }

A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work:

    # $_ contains the string to parse
    # BEGIN and END are the opening and closing markers for the
    # nested text.

    @( = ('(','');
    @) = (')','');
    ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
    @$ = (eval{/$re/},$@!~/unmatched/i);
    print join("\n",@$[0..$#$]) if( $$[-1] );

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

    $reversed = reverse $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 Perl distribution).

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

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.

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

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

To change part of a string, you can use the optional fourth argument 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";

You have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of CW"whoever" or CW"whomever" into CW"whosoever" or CW"whomsoever", case insensitively. These all assume that CW$_ 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 CW/g modifier in a CWwhile loop, keeping count of matches.

    $WANT = 3;
    $count = 0;
    $_ = "One fish two fish red fish blue fish";
    while (/(\w+)\s+fish\b/gi) {
        if (++$count == $WANT) {
            print "The third fish is a $1 one.\n";
        }
    }

That prints out: CW"The third fish is a red one." You can also use a repetition count and repeated pattern like this:

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

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 CWtr/// function like so:

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

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

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

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

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

To make the first letter of each word upper case:

        $line =~ s/\b(\w)/\U$1/g;

This has the strange effect of turning "CWdon't do it into CWDon'T 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
                   |      # or
                 (\s\w)   #preceded by whitespace
                   )
                /\U$1/xg;
    $string =~ /([\w']+)/\u\L$1/g;

To make the whole line upper case:

        $line = uc($line);

To force each word to be lower case, with the first letter upper case:

        $line =~ s/(\w+)/\u\L$1/g;

You can (and probably should) enable locale awareness of those characters by placing a CWuse locale pragma in your program. See perllocale for endless details on locales.

This is sometimes referred to as putting something into title case, but that's not quite accurate. Consider the proper capitalization of the movie Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, for example.

Damian Conway's Text::Autoformat module provides some smart case transformations:

    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";
    }

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

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

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

Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of Mastering Regular Expressions, to handle these for us. He suggests (assuming your string is contained in CW$text):

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

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

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

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

There's also a Text::CSV (Comma-Separated Values) module on CPAN.

Although the simplest approach would seem to be

    $string =~ s/^\s*(.*?)\s*$/$1/;

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+//;
    $string =~ s/\s+$//;

Or more nicely written as:

    for ($string) {
        s/^\s+//;
        s/\s+$//;
    }

This idiom takes advantage of the CWforeach loop's aliasing behavior to factor out common code. You can do this on several strings at once, or arrays, or even the values of a hash if you use a slice:

    # trim whitespace in the scalar, the array,
    # and all the values in the hash
    foreach ($scalar, @array, @hash{keys %hash}) {
        s/^\s+//;
        s/\s+$//;
    }

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

The simplest method uses the CWsprintf 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 CWpack function can only pad strings on the right with blanks and it will truncate the result to a maximum length of CW$pad_len.

    # Left padding a string with blanks (no truncation):
        $padded = sprintf("%${pad_len}s", $text);
        $padded = sprintf("%*s", $pad_len, $text);  # same thing

    # Right padding a string with blanks (no truncation):
        $padded = sprintf("%-${pad_len}s", $text);
        $padded = sprintf("%-*s", $pad_len, $text); # same thing

    # Left padding a number with 0 (no truncation):
        $padded = sprintf("%0${pad_len}d", $num);
        $padded = sprintf("%0*d", $pad_len, $num); # same thing

    # Right padding a string with blanks using pack (will truncate):
    $padded = pack("A$pad_len",$text);

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 CWx operator and combine that with CW$text. These methods do not truncate CW$text.

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

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

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

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

Use substr() or unpack(), both documented in perlfunc. If you prefer thinking in terms of columns instead of widths, you can use this kind of thing:

    # determine the unpack format needed to split Linux ps output
    # arguments are cut columns
    my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

    sub cut2fmt {
        my(@positions) = @_;
        my $template  = '';
        my $lastpos   = 1;
        for my $place (@positions) {
            $template .= "A" . ($place - $lastpos) . " ";
            $lastpos   = $place;
        }
        $template .= "A*";
        return $template;
    }

(contributed by brian d foy)

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

Let's assume that you have a string that contains placeholder variables.

    $text = 'this has a $foo in it and a $bar';

You can use a substitution with a double evaluation. The first /e turns CW$1 into CW$foo, and the second /e turns CW$foo into its value. You may want to wrap this in an CWeval: if you try to get the value of an undeclared variable while running under CWuse strict, you get a fatal error.

    eval { $text =~ s/(\$\w+)/$1/eeg };
    die if $@;

It's probably better in the general case to treat those variables as entries in some special hash. For example:

    %user_defs = (
        foo  => 23,
        bar  => 19,
    );
    $text =~ s/\$(\w+)/$user_defs{$1}/g;

If you get used to writing odd things like these:

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

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

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

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

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

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

Stringification also destroys arrays.

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

Check for these three things:

There must be no space after the << part.

There (probably) should be a semicolon at the end.

You can't (easily) have any space in front of the tag.

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

    # all in one
    ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
        your text
        goes here
    HERE_TARGET

But the HERE_TARGET must still be flush against the margin. If you want that indented also, you'll have to quote in the indentation.

    ($quote = <<'    FINIS') =~ s/^\s+//gm;
            ...we will have peace, when you and all your works have
            perished--and the works of your dark master to whom you
            would deliver us. You are a liar, Saruman, and a corrupter
            of men's hearts.  --Theoden in /usr/src/perl/taint.c
        FINIS
    $quote =~ s/\s+--/\n--/;

A nice general-purpose fixer-upper function for indented here documents follows. It expects to be called with a here document as its argument. It looks to see whether each line begins with a common substring, and if so, strips that substring off. Otherwise, it takes the amount of leading whitespace found on the first line and removes that much off each subsequent line.

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

This works with leading special strings, dynamically determined:

    $remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
        @@@ int
        @@@ runops() {
        @@@     SAVEI32(runlevel);
        @@@     runlevel++;
        @@@     while ( op = (*op->op_ppaddr)() );
        @@@     TAINT_NOT;
        @@@     return 0;
        @@@ }
    MAIN_INTERPRETER_LOOP

Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved:

    $poem = fix<<EVER_ON_AND_ON;
       Now far ahead the Road has gone,
          And I must follow, if I can,
       Pursuing it with eager feet,
          Until it joins some larger way
       Where many paths and errands meet.
          And whither then? I cannot say.
                --Bilbo in /usr/src/perl/pp_ctl.c
    EVER_ON_AND_ON

An array has a changeable length. A list does not. An array is something you can push or pop, while a list is a set of values. Some people make the distinction that a list is a value while an array is a variable. Subroutines are passed and return lists, you put things into list context, you initialize arrays with lists, and you foreach() across a list. CW@ 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 CW@_, and push/pop/shift only work on arrays.

As a side note, there's no such thing as a list in scalar context. When you say

    $scalar = (2, 5, 7, 9);

you're using the comma operator in scalar context, so it uses the scalar comma operator. There never was a list there at all! This causes the last value to be returned: 9. 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).

Sometimes it doesn't make a difference, but sometimes it does. For example, compare:

    $good[0] = `some program that outputs several lines`;

with

    @bad[0]  = `same program that outputs several lines`;

The CWuse warnings pragma and the -w flag will warn you about these matters.

There are several possible ways, depending on whether the array is ordered and whether you wish to preserve the ordering.

a)

If CW@in is sorted, and you want CW@out to be sorted: (this assumes all true values in the array)

    $prev = "not equal to $in[0]";
    @out = grep($_ ne $prev && ($prev = $_, 1), @in);

This is nice in that it doesn't use much extra memory, simulating uniq(1)'s behavior of removing only adjacent duplicates. The , 1 guarantees that the expression is true (so that grep picks it up) even if the CW$_ is 0, "", or undef.

b)

If you don't know whether CW@in is sorted:

    undef %saw;
    @out = grep(!$saw{$_}++, @in);

c)

Like (b), but CW@in contains only small integers:

    @out = grep(!$saw[$_]++, @in);

d)

A way to do (b) without any loops or greps:

    undef %saw;
    @saw{@in} = ();
    @out = sort keys %saw;  # remove sort if undesired

e)

Like (d), but CW@in contains only small positive integers:

    undef @ary;
    @ary[@in] = @in;
    @out = grep {defined} @ary;

But perhaps you should have been using a hash all along, eh?

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

That being said, there are several ways to approach this. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values.

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

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

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

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

Now you check whether CW$is_tiny_prime[$some_number].

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

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

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

Please do not use

    ($is_there) = grep $_ eq $whatever, @array;

or worse yet

    ($is_there) = grep /$whatever/, @array;

These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are regex characters in CW$whatever?). If you're only testing once, then use:

    $is_there = 0;
    foreach $elt (@array) {
        if ($elt eq $elt_to_find) {
            $is_there = 1;
            last;
        }
    }
    if ($is_there) { ... }

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

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

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

The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs.

    $are_equal = compare_arrays(\@frogs, \@toads);

    sub compare_arrays {
        my ($first, $second) = @_;
        no warnings;  # silence spurious -w undef complaints
        return 0 unless @$first == @$second;
        for (my $i = 0; $i < @$first; $i++) {
            return 0 if $first->[$i] ne $second->[$i];
        }
        return 1;
    }

For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module FreezeThaw:

    use FreezeThaw qw(cmpStr);
    @a = @b = ( "this", "that", [ "more", "stuff" ] );

    printf "a and b contain %s arrays\n",
        cmpStr(\@a, \@b) == 0
            ? "the same"
            : "different";

This approach also works for comparing hashes. Here we'll demonstrate two different answers:

    use FreezeThaw qw(cmpStr cmpStrHard);

    %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
    $a{EXTRA} = \%b;
    $b{EXTRA} = \%a;

    printf "a and b contain %s hashes\n",
        cmpStr(\%a, \%b) == 0 ? "the same" : "different";

    printf "a and b contain %s hashes\n",
        cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";

The first reports that both those the hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader.

To find the first array element which satisfies a condition, you can use the first() function in the List::Util module, which comes with 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 the same thing. Once you find the element, you stop the loop with last.

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

If you want the array index, you can iterate through the indices and check the array element at each index until you find one that satisfies the condition.

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

In general, you usually don't need a linked list in Perl, since with regular arrays, you can push and pop or shift and unshift at either end, or you can use splice to add and/or remove arbitrary number of elements at arbitrary points. Both pop and shift are both O(1) operations on Perl's dynamic arrays. In the absence of shifts and pops, push in general needs to reallocate on the order every log(N) times, and unshift will need to copy pointers each time.

If you really, really wanted, you could use structures as described in perldsc or perltoot and do just what the algorithm book tells you to do. For example, imagine a list node like this:

    $node = {
        VALUE => 42,
        LINK  => undef,
    };

You could walk the list this way:

    print "List: ";
    for ($node = $head;  $node; $node = $node->{LINK}) {
        print $node->{VALUE}, " ";
    }
    print "\n";

You could add to the list this way:

    my ($head, $tail);
    $tail = append($head, 1);       # grow a new head
    for $value ( 2 .. 10 ) {
        $tail = append($tail, $value);
    }

    sub append {
        my($list, $value) = @_;
        my $node = { VALUE => $value };
        if ($list) {
            $node->{LINK} = $list->{LINK};
            $list->{LINK} = $node;
        } else {
            $_[0] = $node;      # replace caller's version
        }
        return $node;
    }

But again, Perl's built-in are virtually always good enough.

Circular lists could be handled in the traditional fashion with linked lists, or you could just do something like this with an array:

    unshift(@array, pop(@array));  # the last shall be first
    push(@array, shift(@array));   # and vice versa

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 a Fisher-Yates shuffle.

    sub fisher_yates_shuffle {
        my $deck = shift;  # $deck is a reference to an array
        my $i = @$deck;
        while ($i--) {
            my $j = int rand ($i+1);
            @$deck[$i,$j] = @$deck[$j,$i];
        }
    }

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

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

Use CWfor/CWforeach:

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

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

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

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

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

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

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

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

Use the rand() function (see rand in perlfunc):

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

Or, simply: my CW$element = CW$array[ rand CW@array ];

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

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

For even faster execution, you could do:

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

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

        #!/usr/bin/perl -n
        # Fischer-Kause 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;

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

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

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

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

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

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;

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

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

This can be conveniently combined with precalculation of keys as given above.

See the sort article in the Far More Than You Ever Wanted To Know collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for more about this approach.

See also the question below on sorting hashes.

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

For example, this sets CW$vec to have bit N set if CW$ints[N] was set:

    $vec = '';
    foreach(@ints) { vec($vec,$_,1) = 1 }

Here's how, given a vector in CW$vec, you can get those bits into your CW@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 suggestion from Benjamin Goldberg:

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

Or use the CPAN module Bit::Vector:

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

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

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

    # vec demo
    $vector = "\xff\x0f\xef\xfe";
    print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
        unpack("N", $vector), "\n";
    $is_set = vec($vector, 23, 1);
    print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
    pvec($vector);

    set_vec(1,1,1);
    set_vec(3,1,1);
    set_vec(23,1,1);

    set_vec(3,1,3);
    set_vec(3,2,3);
    set_vec(3,4,3);
    set_vec(3,4,7);
    set_vec(3,8,3);
    set_vec(3,8,7);

    set_vec(0,32,17);
    set_vec(1,32,17);

    sub set_vec {
        my ($offset, $width, $value) = @_;
        my $vector = '';
        vec($vector, $offset, $width) = $value;
        print "offset=$offset width=$width value=$value\n";
        pvec($vector);
    }

    sub pvec {
        my $vector = shift;
        my $bits = unpack("b*", $vector);
        my $i = 0;
        my $BASE = 8;

        print "vector length in bytes: ", length($vector), "\n";
        @bytes = unpack("A8" x length($vector), $bits);
        print "bits are: @bytes\n\n";
    }

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

Use the each() function (see each in perlfunc) if you don't care whether it's sorted:

    while ( ($key, $value) = each %hash) {
        print "$key = $value\n";
    }

If you want it sorted, you'll have to use foreach() on the result of sorting the keys as shown in an earlier question.

(contributed by brian d foy)

The easy answer is Don't do that!

If you iterate through the hash with each(), you can delete the key most recently returned without worrying about it. If you delete or add other keys, the iterator may skip or double up on them since perl may rearrange the hash table. See the entry for CWeach() in perlfunc.

Create a reverse hash:

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

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

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

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;
     }

If you mean how many keys, then all you have to do is use the keys() function in a scalar context:

    $num_keys = keys %hash;

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

Internally, hashes are stored in a way that prevents you from imposing an order on key-value pairs. Instead, you have to sort a list of the keys or values:

    @keys = sort keys %hash;    # sorted by key
    @keys = sort {
                    $hash{$a} cmp $hash{$b}
            } 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, or if that fails, by straight ASCII comparison of the keys (well, possibly modified by your localesee perllocale).

    @keys = sort {
                $hash{$b} <=> $hash{$a}
                          ||
                length($b) <=> length($a)
                          ||
                      $a cmp $b
    } keys %hash;

You can look into using the DB_File module and tie() using the CW$DB_BTREE hash bindings as documented in In Memory Databases in DB_File. The Tie::IxHash module from CPAN might also be instructive. Hashes contain pairs of scalars: the first is the key, the second is the value. The key will be coerced to a string, although the value can be any kind of scalar: string, number, or reference. If a key CW$key is present in CW%hash, CWexists($hash{$key}) will return true. The value for a given key can be CWundef, in which case CW$hash{$key} will be CWundef while CWexists $hash{$key} will return true. This corresponds to (CW$key, CWundef) being in the hash.

Pictures help... here's the CW%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 $hash{'a'}                is true (Perl5 only)
        grep ($_ 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 $hash{'a'}                is true (Perl5 only)
        grep ($_ 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 $hash{'a'}                is FALSE (Perl5 only)
        grep ($_ eq 'a', keys %hash)     is FALSE

See, the whole entry is gone!

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

Using CWkeys %hash in scalar context returns the number of keys in the hash and resets the iterator associated with the hash. You may need to do this if you use CWlast to exit a loop early so that when you re-enter it, the hash iterator has been reset.

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

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

Or more succinctly:

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

Or if you really want to save space:

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

Either stringify the structure yourself (no fun), or else get the MLDBM (which uses Data::Dumper) module from CPAN and layer it on top of either DB_File or GDBM_File.

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,...)

If you say something like:

    somefunc($hash{"nonesuch key here"});

Then that element autovivifies; that is, it springs into existence whether you store something there or not. That's because functions get scalars passed in by reference. If somefunc() modifies CW$_[0], it has to be ready to write it back into the caller's version.

This has been fixed as of Perl5.004.

Normally, merely accessing a key's value for a nonexistent key does not cause that key to be forever there. This is different than awk's behavior.

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 the upcoming perlreftut. Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in perltoot.

You can't do this directly, but you could use the standard Tie::RefHash module distributed with Perl.

Perl is binary clean, so this shouldn't be a problem. For example, this works fine (assuming the files are found):

    if (`cat /vmunix` =~ /gzip/) {
        print "Your kernel is GNU-zip enabled!\n";
    }

On less elegant (read: Byzantine) systems, however, you have to play tedious games with text versus binary files. See binmode in perlfunc or perlopentut.

If you're concerned about 8-bit ASCII data, then see perllocale.

If you want to deal with multibyte characters, however, there are some gotchas. See the section on Regular Expressions.

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

   if (/\D/)            { print "has nondigits\n" }
   if (/^\d+$/)         { print "is a whole number\n" }
   if (/^-?\d+$/)       { print "is an integer\n" }
   if (/^[+-]?\d+$/)    { print "is a +/- integer\n" }
   if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
   if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
   if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                        { print "a C float\n" }

There are also some commonly used modules for the task. Scalar::Util (distributed with 5.8) provides access to perl's internal function CWlooks_like_number for determining whether a variable looks like a number. Data::Types exports functions that validate data types using both the above and other regular expressions. Thirdly, there is CWRegexp::Common which has regular expressions to match various types of numbers. Those three modules are available from the CPAN.

If you're on a POSIX system, Perl supports the CWPOSIX::strtod function. Its semantics are somewhat cumbersome, so here's a CWgetnum wrapper function for more convenient access. This function takes a string and returns the number it found, or CWundef for input that isn't a C float. The CWis_numeric function is a front end to CWgetnum 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 CPAN instead. The POSIX module (part of the standard Perl distribution) provides the CWstrtod and CWstrtol for converting strings to double and longs, respectively.

For some specific applications, you can use one of the DBM modules. See AnyDBM_File. More generically, you should consult the FreezeThaw or Storable modules from CPAN. Starting from Perl 5.8 Storable is part of the standard distribution. Here's one example using Storable's CWstore and CWretrieve functions:

    use Storable;
    store(\%hash, "filename");

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

The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great for printing out data structures. The Storable module on CPAN (or the 5.8 release of Perl), provides a function called CWdclone that recursively copies its argument.

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

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

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

Use the UNIVERSAL class (see UNIVERSAL).

Get the Business::CreditCard module from CPAN.

The kgbpack.c code in the PGPLOT module on CPAN does just this. If you're doing a lot of float or double processing, consider using the PDL module from CPAN insteadit makes number-crunching easy.

Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and other authors as noted. All rights reserved.

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

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.