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PERLFAQ4
!!!PERLFAQ4
NAME
DESCRIPTION
Data: Numbers
Data: Dates
Data: Strings
Data: Arrays
Data: Hashes (Associative Arrays)
Data: Misc
AUTHOR AND COPYRIGHT
----
!!NAME


perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
!!DESCRIPTION


The section of the FAQ answers questions
related to the manipulation of data as numbers, dates,
strings, arrays, hashes, and miscellaneous data
issues.
!!Data: Numbers


__Why am I getting long decimals (eg, 19.9499999999999)
instead of the numbers I should be getting (eg,
19.95)?__


The infinite set that a mathematician thinks of as the real
numbers can only be approximated on a computer, since the
computer only has a finite number of bits to store an
infinite number of, um, numbers.


Internally, your computer represents floating-point numbers
in binary. Floating-point numbers read in from a file or
appearing as literals in your program are converted from
their decimal floating-point representation (eg, 19.95) to
an internal binary representation.


However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly
represented as a decimal floating-point number. The
computer's binary representation of 19.95, therefore, isn't
exactly 19.95.


When a floating-point number gets printed, the binary
floating-point representation is converted back to decimal.
These decimal numbers are displayed in either the format you
specify with ''printf()'', or the current output format
for numbers. (See ``$#'' in perlvar if you use print.
$# has a different default value in Perl5 than it
did in Perl4. Changing $# yourself is
deprecated.)


This affects __all__ computer languages that represent
decimal floating-point numbers in binary, not just Perl.
Perl provides arbitrary-precision decimal numbers with the
Math::BigFloat module (part of the standard Perl
distribution), but mathematical operations are consequently
slower.


To get rid of the superfluous digits, just use a format (eg,
printf() to get the
required precision. See ``Floating-point Arithmetic'' in
perlop.


__Why isn't my octal data interpreted
correctly?__


Perl only understands octal and hex numbers as such when
they occur as literals in your program. If they are read in
from somewhere and assigned, no automatic conversion takes
place. You must explicitly use ''oct()'' or ''hex()''
if you want the values converted. ''oct()'' interprets
both hex (``0x350'') numbers and octal ones (``0350'' or
even without the leading ``0'', like ``377''), while
''hex()'' only converts hexadecimal ones, with or without
a leading ``0x'', like ``0x255'', ``3A'', ``ff'', or
``deadbeef''.


This problem shows up most often when people try using
''chmod()'', ''mkdir()'', ''umask()'', or
''sysopen()'', which all want permissions in
octal.


    chmod(644,  $file); # WRONG -- perl -w catches this
chmod(0644, $file); # right


__Does Perl have a__ ''round()'' __function? What
about__ ''ceil()'' __and__ ''floor()''__? Trig
functions?__


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


    printf(
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
    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.


__How do I convert bits into ints?__


To turn a string of 1s and 0s like 10110110 into a
scalar containing its binary value, use the ''pack()''
and ''unpack()'' functions (documented in ``pack'' in
perlfunc and ``unpack'' in perlfunc):


    $decimal = unpack('c', pack('B8', '10110110'));
This packs the string 10110110 into an eight bit binary structure. This is then unpacked as a character, which returns its ordinal value.


This does the same thing:


    $decimal = ord(pack('B8', '10110110'));
Here's an example of going the other way:


    $binary_string = unpack('B*',


__Why doesn't
__


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  is the bit pattern
00110011). The operators work with the binary form
of a number (the number 3 is treated as the bit
pattern 00000011).


So, saying 11  performs the ``and''
operation on numbers (yielding 1). Saying
 performs the
``and'' operation on strings (yielding
).


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


    if (
but a string consisting of two null bytes (the result of ) is not a false value in Perl. You need:


    if ( (


__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
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 .. 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
This situation has been fixed in Perl5.005. Use of .. in a for loop will iterate over the range, without creating the entire range.


    for my $i (5 .. 500_005) {
push(@results, some_func($i));
}
will not create a list of 500,000 integers.


__How can I output Roman numerals?__


Get the http://www.perl.com/CPAN/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 srand once at the start of your program to
seed the random number generator. 5.004 and later
automatically call srand at the beginning. Don't
call srand more than once--you make your numbers
less random, rather than more.


Computers are good at being predictable and bad at being
random (despite appearances caused by bugs in your programs
:-). http://www.perl.com/CPAN/doc/FMTEYEWTK/random ,
courtesy of Tom Phoenix, talks more about this. John von
Neumann said, ``Anyone who attempts to generate random
numbers by deterministic means is, of course, living in a
state of sin.''


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


__How do I find the
week-of-the-year/day-of-the-year?__


The day of the year is in the array returned by
''localtime()'' (see ``localtime'' in
perlfunc):


    $day_of_year = (localtime(time()))[[7];
or more legibly (in 5.004 or higher):


    use Time::localtime;
$day_of_year = localtime(time())-
You can find the week of the year by dividing this by 7:


    $week_of_year = int($day_of_year / 7);
Of course, this believes that weeks start at zero. The Date::Calc module from CPAN has a lot of date calculation functions, including day of the year, week of the year, and so on. Note that not all businesses consider ``week 1'' to be the same; for example, American businesses often consider the first week with a Monday in it to be Work Week #1, despite ISO 8601, which considers WW1 to be the first week with a Thursday in it.


__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, you'll find that the POSIX module's ''strftime()'' function has been extended in a non-standard way to use a %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.


__How can I compare two dates and find the
difference?__


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


__How can I take a string and turn it into epoch
seconds?__


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


__How can I find the Julian Day?__


Use the Time::JulianDay module (part of the Time-modules
bundle available from CPAN .)


Before you immerse yourself too deeply in this, be sure to
verify that it is the ''Julian'' Day you really want. Are
you really just interested in a way of getting serial days
so that they can do date arithmetic? If you are interested
in performing date arithmetic, this can be done using either
Date::Manip or Date::Calc, without converting to Julian Day
first.


There is too much confusion on this issue to cover in this
FAQ , but the term is applied (correctly) to
a calendar now supplanted by the Gregorian Calendar, with
the Julian Calendar failing to adjust properly for leap
years on centennial years (among other annoyances). The term
is also used (incorrectly) to mean: [[1] days in the
Gregorian Calendar; and [[2] days since a particular starting
time or `epoch', usually 1970 in the Unix world and 1980 in
the MS-DOS/Windows world. If you find that it is not the
first meaning that you really want, then check out the
Date::Manip and Date::Calc modules. (Thanks to David Cassell
for most of this text.)


__How do I find yesterday's date?__


The time() function returns the current time in
seconds since the epoch. Take twenty-four hours off
that:


    $yesterday = time() - ( 24 * 60 * 60 );
Then you can pass this to localtime() and get the individual year, month, day, hour, minute, seconds values.


Note very carefully that the code above assumes that your
days are twenty-four hours each. For most people, there are
two days a year when they aren't: the switch to and from
summer time throws this off. A solution to this issue is
offered by Russ Allbery.


    sub yesterday {
my $now  = defined $_[[0] ? $_[[0] : time;
my $then = $now - 60 * 60 * 24;
my $ndst = (localtime $now)[[8]


__Does Perl have a Year 2000 problem? Is Perl Y2K
compliant?__


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


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


The date and time functions supplied with Perl (gmtime and
localtime) supply adequate information to determine the year
well beyond 2000 (2038 is when trouble strikes for 32-bit
machines). The year returned by these functions when used in
a list context is the year minus 1900. For years between
1910 and 1999 this ''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, $timestamp =
gmtime(1005613200) sets $timestamp to ``Tue
Nov 13 01:00:00 2001''. There's no year 2000 problem
here.


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


__How do I validate input?__


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


__How do I unescape a string?__


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


    s/\(.)/$1/g;
This won't expand  or  or any other special escapes.


__How do I remove consecutive pairs of
characters?__


To turn  into
:


    s/(.)1/$1/g;       # add /s to include newlines
Here's a solution that turns ``abbcccd'' to ``abcd'':


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


__How do I expand function calls in a
string?__


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
If you prefer scalar context, similar chicanery is also useful for arbitrary expressions:


    print
Version 5.004 of Perl had a bug that gave list context to the expression in ${...}, but this is fixed in version 5.005.


See also ``How can I expand variables in text strings?'' in
this section of the FAQ .


__How do I find matching/nesting anything?__


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
/x([[^x]*)x/ will get the intervening bits in
$1. For multiple ones, then something more like
/alpha(.*?)omega/ would be needed. But none of
these deals with nested patterns, nor can they. For that
you'll have to write a parser.


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


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$1E$([[!$2]/gs;
@$ = (eval{/$re/},$@!~/unmatched/);
print join(


__How do I reverse a string?__


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


    $reversed = reverse $string;


__How do I expand tabs in a string?__


You can do it yourself:


    1 while $string =~ s/t+/' ' x (length($
Or you can just use the Text::Tabs module (part of the standard Perl distribution).


    use Text::Tabs;
@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(
The paragraphs you give to Text::Wrap should not contain embedded newlines. Text::Wrap doesn't justify the lines (flush-right).


__How can I access/change the first N letters of a
string?__


There are many ways. If you just want to grab a copy, use
''substr()'':


    $first_byte = substr($a, 0, 1);
If you want to modify part of a string, the simplest way is often to use ''substr()'' as an lvalue:


    substr($a, 0, 3) =
Although those with a pattern matching kind of thought process will likely prefer


    $a =~ s/^.../Tom/;


__How do I change the Nth occurrence of
something?__


You have to keep track of N yourself. For example, let's say
you want to change the fifth occurrence of
 or
 into
 or
, case insensitively. These
all assume that $_ contains the string to be
altered.


    $count = 0;
s{((whom?)ever)}{
++$count == 5           # is it the 5th?
?
In the more general case, you can use the /g modifier in a while loop, keeping count of matches.


    $WANT = 3;
$count = 0;
$_ =
That prints out:  You can also use a repetition count and repeated pattern like this:


    /(?:w+s+fishs+){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 a count of a certain single character (X) within a
string, you can use the tr/// function like
so:


    $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, tr/// 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 =


__How do I capitalize all the words on one
line?__


To make the first letter of each word upper
case:


        $line =~ s/b(w)/U$1/g;
This has the strange effect of turning don't do it`` into ''Don'T Do It


    $string =~ s/ (
(^w)    #at the beginning of the line
# or
(sw)   #preceded by whitespace
)
/U$1/xg;
$string =~ /([[w']+)/uL$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+)/uL$1/g;
You can (and probably should) enable locale awareness of those characters by placing a use 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.


__How can I split a [[character] delimited string except
when inside [[character]? (Comma-separated
files)__


Take the example case of trying to split a string that is
comma-separated into its different fields. (We'll pretend
you said comma-separated, not comma-delimited, which is
different and almost never what you mean.) You can't use
split(/,/) because you shouldn't split if the comma
is inside quotes. For example, take a data line like
this:


    SAR001,
Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of a highly recommended book on regular expressions, to handle these for us. He suggests (assuming your string is contained in $text):


     @new = ();
push(@new, $+) while $text =~ m{
If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, . Unescaping them is a task addressed earlier in this section.


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


    use Text::ParseWords;
@new = quotewords(
There's also a Text::CSV (Comma-Separated Values) module on CPAN .


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


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


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


__How do I pad a string with blanks or pad a number with
zeroes?__


(This answer contributed by Uri Guttman, with kibitzing from
Bart Lateur.)


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


The simplest method uses the sprintf function. It
can pad on the left or right with blanks and on the left
with zeroes and it will not truncate the result. The
pack function can only pad strings on the right
with blanks and it will truncate the result to a maximum
length of $pad_len.


    # Left padding a string with blanks (no truncation):
$padded = sprintf(
    # Right padding a string with blanks (no truncation):
$padded = sprintf(
    # Left padding a number with 0 (no truncation):
$padded = sprintf(
    # Right padding a string with blanks using pack (will truncate):
$padded = pack(
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 x operator and combine that with $text. These methods do not truncate $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 $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?__


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 .=


__How do I find the soundex value of a
string?__


Use the standard Text::Soundex module distributed with Perl.
Before you do so, you may want to determine whether
`soundex' is in fact what you think it is. Knuth's soundex
algorithm compresses words into a small space, and so it
does not necessarily distinguish between two words which you
might want to appear separately. For example, the last names
`Knuth' and `Kant' are both mapped to the soundex code K530.
If Text::Soundex does not do what you are looking for, you
might want to consider the String::Approx module available
at CPAN .


__How can I expand variables in text
strings?__


Let's assume that you have a string like:


    $text = 'this has a $foo in it and a $bar';
If those were both global variables, then this would suffice:


    $text =~ s/$(w+)/${$1}/g;  # no /e needed
But since they are probably lexicals, or at least, they could be, you'd have to do this:


    $text =~ s/($w+)/$1/eeg;
die if $@;                  # needed /ee, not /e
It's probably better in the general case to treat those variables as entries in some special hash. For example:


    %user_defs = (
foo  =
See also ``How do I expand function calls in a string?'' in this section of the FAQ .


__What's wrong with always quoting
``$vars''?__


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


If you get used to writing odd things like
these:


    print
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 =
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 ++ autoincrement operator or the ''syscall()'' function.


Stringification also destroys arrays.


    @lines = `command`;
print


__Why don't my __HERE documents
work?__


Check for these three things:


1. There must be no space after the


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


3. 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 =
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 =
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, $leader);  # common whitespace and common leading string
if (/^s*(?:([[^ws]+)(s*).*n)(?:s*12?.*n)+$/) {
($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
Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved:


    $poem = fix
!!Data: Arrays


__What is the difference between a list and an
array?__


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


__What is the difference between__ $array__[[1]
and__ @array__[[1]?__


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 use warnings pragma and the __-w__ flag will warn you about these matters.


__How can I remove duplicate elements from a list or
array?__


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


a)


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


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


b)


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


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


c)


Like (b), but @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 @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?


__How can I tell whether a list or array contains a certain
element?__


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


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


    @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
undef %is_blue;
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might have been a good idea to keep the blues all in a hash in the first place.


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


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


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


    @articles = ( 1..10, 150..2000, 2017 );
undef $read;
for (@articles) { vec($read,$_,1) = 1 }
Now check whether vec($read,$n,1) is true for some $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 $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) { ... }


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


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


    @union = @intersection = @difference = ();
%count = ();
foreach $element (@array1, @array2) { $count{$element}++ }
foreach $element (keys %count) {
push @union, $element;
push @{ $count{$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.


__How do I test whether two arrays or hashes are
equal?__


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


    $are_equal = compare_arrays(@frogs, @toads);
    sub compare_arrays {
my ($first, $second) = @_;
no warnings;  # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i
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 = (
    printf
This approach also works for comparing hashes. Here we'll demonstrate two different answers:


    use FreezeThaw qw(cmpStr cmpStrHard);
    %a = %b = (
    printf
    printf
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.


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


You can use this if you care about the index:


    for ($i= 0; $i
Now $found_index has what you want.


__How do I handle linked lists?__


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


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 =
You could walk the list this way:


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


__How do I handle circular lists?__


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


__How do I shuffle an array randomly?__


Use this:


    # fisher_yates_shuffle( @array ) :
# generate a random permutation of @array in place
sub fisher_yates_shuffle {
my $array = shift;
my $i;
for ($i = @$array; --$i; ) {
my $j = int rand ($i+1);
@$array[[$i,$j] = @$array[[$j,$i];
}
}
    fisher_yates_shuffle( @array );    # permutes @array in place
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.


__How do I process/modify each element of an
array?__


Use for/foreach:


    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
}
If you want to do the same thing to modify the values of the hash, you may not use the values function, oddly enough. You need a slice:


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


__How do I select a random element from an
array?__


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


    # at the top of the program:
srand;                      # not needed for 5.004 and later
    # then later on
$index   = rand @array;
$element = $array[[$index];
Make sure you ''only call srand once per program, if then''. If you are calling it more than once (such as before each call to rand), you're almost certainly doing something wrong.


__How do I permute N elements of a list?__


Here's a little program that generates all permutations of
all the words on each line of input. The algorithm embodied
in the ''permute()'' function should work on any
list:


    #!/usr/bin/perl -n
# tsc-permute: permute each word of input
permute([[split], [[]);
sub permute {
my @items = @{ $_[[0] };
my @perms = @{ $_[[1] };
unless (@items) {
print


__How do I sort an array by (anything)?__


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


    @list = sort { $a
The default sort function is cmp, string comparison, which would sort (1, 2, 10) into (1, 10, 2). , 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  { $_-
If you need to sort on several fields, the following paradigm is useful.


    @sorted = sort { field1($a)
This can be conveniently combined with precalculation of keys as given above.


See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for
more about this approach.


See also the question below on sorting hashes.


__How do I manipulate arrays of bits?__


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


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


    $vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
And here's how, given a vector in $vec, you can get those bits into your @ints array:


    sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/0// / length $vec
This method gets faster the more sparse the bit vector is. (Courtesy of Tim Bunce and Winfried Koenig.)


Here's a demo on how to use ''vec()'':


    # vec demo
$vector =
    set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);
    set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);
    set_vec(0,32,17);
set_vec(1,32,17);
    sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print
    sub pvec {
my $vector = shift;
my $bits = unpack(
        print


__Why does__ ''defined()'' __return true on empty
arrays and hashes?__


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.
!!Data: Hashes (Associative Arrays)


__How do I process an entire hash?__


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


    while ( ($key, $value) = each %hash) {
print
If you want it sorted, you'll have to use ''foreach()'' on the result of sorting the keys as shown in an earlier question.


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


Don't do that. :-)


[[lwall] In Perl 4, you were not allowed to modify a hash at
all while iterating over it. In Perl 5 you can delete from
it, but you still can't add to it, because that might cause
a doubling of the hash table, in which half the entries get
copied up to the new top half of the table, at which point
you've totally bamboozled the iterator code. Even if the
table doesn't double, there's no telling whether your new
entry will be inserted before or after the current iterator
position.


Either treasure up your changes and make them after the
iterator finishes or use keys to fetch all the old keys at
once, and iterate over the list of keys.


__How do I look up a hash element by
value?__


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;
}


__How can I know how many entries are in a
hash?__


If you mean how many keys, then all you have to do is take
the scalar sense of the ''keys()'' function:


    $num_keys = scalar keys %hash;
The ''keys()'' function also resets the iterator, which in void context is faster for tied hashes than would be iterating through the whole hash, one key-value pair at a time.


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


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


    @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 locale--see perllocale).


    @keys = sort {
$hash{$b}


__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 be
instructive.


__What's the difference between ``delete'' and ``undef''
with hashes?__


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


Pictures help... here's the %ary
table:


          keys  values
+------+------+
a     3
x     7
d     0
e     2
+------+------+
And these conditions hold


        $ary{'a'}                       is true
$ary{'d'}                       is false
defined $ary{'d'}               is true
defined $ary{'a'}               is true
exists $ary{'a'}                is true (Perl5 only)
grep ($_ eq 'a', keys %ary)     is true
If you now say


        undef $ary{'a'}
your table now reads:


          keys  values
+------+------+
a    undef
x     7
d     0
e     2
+------+------+
and these conditions now hold; changes in caps:


        $ary{'a'}                       is FALSE
$ary{'d'}                       is false
defined $ary{'d'}               is true
defined $ary{'a'}               is FALSE
exists $ary{'a'}                is true (Perl5 only)
grep ($_ eq 'a', keys %ary)     is true
Notice the last two: you have an undef value, but a defined key!


Now, consider this:


        delete $ary{'a'}
your table now reads:


          keys  values
+------+------+
x     7
d     0
e     2
+------+------+
and these conditions now hold; changes in caps:


        $ary{'a'}                       is false
$ary{'d'}                       is false
defined $ary{'d'}               is true
defined $ary{'a'}               is false
exists $ary{'a'}                is FALSE (Perl5 only)
grep ($_ eq 'a', keys %ary)     is FALSE
See, the whole entry is gone!


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


They may or may not implement the ''EXISTS
()'' and ''DEFINED ()'' methods
differently. For example, there isn't the concept of undef
with hashes that are tied to DBM* files. This means the
true/false tables above will give different results when
used on such a hash. It also means that exists and defined
do the same thing with a DBM* file, and what they end up
doing is not what they do with ordinary hashes.


__How do I reset an__ ''each()'' __operation part-way
through?__


Using keys %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
last to exit a loop early so that when you re-enter
it, the hash iterator has been reset.


__How can I get the unique keys from two
hashes?__


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


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


__How can I store a multidimensional array in a
DBM file?__


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.


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


Use the Tie::IxHash from CPAN .


    use Tie::IxHash;
tie(%myhash, Tie::IxHash);
for ($i=0; $i


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


If you say something like:


    somefunc($hash{
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 $_[[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.


__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   =
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.


__How can I use a reference as a hash key?__


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


__How do I handle binary data correctly?__


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
On less elegant (read: Byzantine) systems, however, you have to play tedious games with ``text'' versus ``binary'' files. See ``binmode'' in perlfunc or perlopentut. Most of these ancient-thinking systems are curses out of Microsoft, who seem to be committed to putting the backward into backward compatibility.


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


If you want to deal with multibyte characters, however,
there are some 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.


   if (/D/)            { print
If you're on a POSIX system, Perl's supports the POSIX::strtod function. Its semantics are somewhat cumbersome, so here's a getnum wrapper function for more convenient access. This function takes a string and returns the number it found, or undef for input that isn't a C float. The is_numeric function is a front end to getnum if you just want to say, ``Is this a float?''


    sub getnum {
use POSIX qw(strtod);
my $str = shift;
$str =~ s/^s+//;
$str =~ s/s+$//;
$! = 0;
my($num, $unparsed) = strtod($str);
if (($str eq '')  ($unparsed != 0)  $!) {
return undef;
} else {
return $num;
}
}
    sub is_numeric { defined getnum($_[[0]) }
Or you could check out the String::Scanf module on CPAN instead. The POSIX module (part of the standard Perl distribution) provides the strtod and strtol for converting strings to double and longs, respectively.


__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 FreezeThaw, Storable, or
Class::Eroot modules from CPAN . Here's one
example using Storable's store and
retrieve functions:


    use Storable;
store(%hash,
    # later on...
$href = retrieve(


__How do I print out or copy a recursive data
structure?__


The Data::Dumper module on CPAN (or the 5.005
release of Perl) is great for printing out data structures.
The Storable module, found on CPAN , provides
a function called dclone that recursively copies
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 that you wanted to copy.


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


__How do I define methods for every
class/object?__


Use the UNIVERSAL class (see
UNIVERSAL ).


__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 kgbpack.c code in the PGPLOT module on
CPAN does just this. If you're doing a lot of
float or double processing, consider using the
PDL module from CPAN
instead--it makes number-crunching easy.
!!AUTHOR AND COPYRIGHT


Copyright (c) 1997-1999 Tom Christiansen and Nathan
Torkington. All rights reserved.


When included as part of the Standard Version of Perl, or as
part of its complete documentation whether printed or
otherwise, this work may be distributed only under the terms
of Perl's Artistic License. Any distribution of this file or
derivatives thereof ''outside'' of that package require
that special arrangements be made with copyright
holder.


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.
----
One page links to perlfaq4(1):
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