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PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
NAME
perlfaq4 - Data Manipulation
DESCRIPTION
This section of the FAQ answers questions related to manipulating 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)?
For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know About
Floating-Point Arithmetic" (http://docs.sun.com/source/806-3568/ncg_goldberg.html).
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 shows 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;
Why is int() broken?
Your "int()" is most probably working just fine. It's the numbers that aren't quite what you think.
First, see the answer to "Why am I getting long decimals (eg, 19.9499999999999) instead of the
numbers I should be getting (eg, 19.95)?".
For example, this
print int(0.6/0.2-2), "\n";
will in most computers print 0, not 1, because even such 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.
Why isn't my octal data interpreted correctly?
(contributed by brian d foy)
You're probably trying to convert a string to a number, which Perl only converts as a decimal number.
When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest
of the digits are in base 10:
my $string = '0644';
print $string + 0; # prints 644
print $string + 44; # prints 688, certainly not octal!
This problem usually involves one of the Perl built-ins that has the same name a Unix command that
uses octal numbers as arguments on the command line. In this example, "chmod" on the command line
knows that its first argument is octal because that's what it does:
%prompt> chmod 644 file
If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as
octal numbers either by prefixing the digits with a 0 or using "oct":
chmod( 0644, $file); # right, has leading zero
chmod( oct(644), $file ); # also correct
The problem comes in when you take your numbers from something that Perl thinks is a string, such as
a command line argument in @ARGV:
chmod( $ARGV[0], $file); # wrong, even if "0644"
chmod( oct($ARGV[0]), $file ); # correct, treat string as octal
You can always check the value you're using by printing it in octal notation to ensure it matches
what you think it should be. Print it in octal and decimal format:
printf "0%o %d", $number, $number;
Does Perl have a round() function? What about ceil() and floor()? Trig functions?
Remember that "int()" merely truncates toward 0. For rounding to 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.
How do I convert between numeric representations/bases/radixes?
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 later in perlfaq4 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(32); # 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(32);
$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 = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the
inclined reader.
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on whether they're used on numbers or strings.
The operators treat a string as a series of bits and work with that (the string "3" is the bit
pattern 00110011). The operators work with the binary form of a number (the number 3 is treated as
the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers (yielding 3). Saying "11" & "3" performs
the "and" operation on strings (yielding "1").
Most problems with "&" and "|" arise because the programmer thinks they have a number but really it's
a string. The rest arise because the programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of "\020\020" & "\101\101") is not a false
value in Perl. You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
How do I multiply matrices?
Use the "Math::Matrix" or "Math::MatrixReal" modules (available from CPAN) or the "PDL" extension
(also available from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect the results, use:
@results = map { my_func($_) } @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 < 500_005; $i++) {
push(@results, some_func($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.cpan.org/modules/by-module/Roman <http://www.cpan.org/modules/by-module/Roman>
module.
Why aren't my random numbers random?
If you're using a version of Perl before 5.004, you must call "srand" once at the start of your
program to seed the random number generator.
BEGIN { srand() if $] < 5.004 }
5.004 and later automatically call "srand" at the beginning. Don't call "srand" more than once--you
make your numbers less random, rather than more.
Computers are good at being predictable and bad at being random (despite appearances caused by bugs
in your programs :-). see the random article in the "Far More Than You Ever Wanted To Know"
collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy of Tom Phoenix, talks more
about this. John von Neumann said, "Anyone who attempts to generate random numbers by deterministic
means is, of course, living in a state of sin."
If you want numbers that are more random than "rand" with "srand" provides, you should also check out
the "Math::TrulyRandom" module from CPAN. It uses the imperfections in your system's timer to
generate random numbers, but this takes quite a while. If you want a better pseudorandom generator
than comes with your operating system, look at "Numerical Recipes in C" at <http://www.nr.com/>.
How do I get a random number between X and Y?
To get a random number between two values, you can use the "rand()" built-in to get a random number
between 0 and 1. From there, you shift that into the range that you want.
"rand($x)" returns a number such that "0 <= 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 $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )
Hence you derive the following simple function to abstract that. It selects a random integer between
the two given integers (inclusive), For example: "random_int_between(50,120)".
sub random_int_between {
my($min, $max) = @_;
# Assumes that the two arguments are integers themselves!
return $min if $min == $max;
($min, $max) = ($max, $min) if $min > $max;
return $min + int rand(1 + $max - $min);
}
Data: Dates
How do I find the day or week of the year?
The "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 "POSIX"'s "mktime" to get a time in epoch seconds for the
argument to "localtime".
use POSIX qw/mktime strftime/;
my $week_of_year = strftime "%W",
localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
The "Date::Calc" module provides two functions 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 );
How do I find the current century or millennium?
Use the following simple functions:
sub get_century {
return int((((localtime(shift || time))[5] + 1999))/100);
}
sub get_millennium {
return 1+int((((localtime(shift || time))[5] + 1899))/1000);
}
On some systems, the "POSIX" module's "strftime()" function has 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?
(contributed by brian d foy)
You could just store all your dates as a number and then subtract. Life isn't always that simple
though. If you want to work with formatted dates, the "Date::Manip", "Date::Calc", or "DateTime"
modules can help you.
How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same format, you can split it up and pass the
parts to "timelocal" in the 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?
(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
How do I find yesterday's date?
(contributed by brian d foy)
Use one of the Date modules. The "DateTime" module makes it simple, and give you the same time of
day, only the day before.
use DateTime;
my $yesterday = DateTime->now->subtract( days => 1 );
print "Yesterday was $yesterday\n";
You can also use the "Date::Calc" module using its "Today_and_Now" function.
use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
print "@date_time\n";
Most people try to use the time rather than the calendar to figure out dates, but that assumes that
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. Let the modules do the work.
If you absolutely must do it yourself (or can't use one of the modules), here's a solution using
"Time::Local", which comes with Perl:
# contributed by Gunnar Hjalmarsson
use Time::Local;
my $today = timelocal 0, 0, 12, ( localtime )[3..5];
my ($d, $m, $y) = ( localtime $today-86400 )[3..5];
printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;
In this case, you measure the day starting at noon, and subtract 24 hours. Even if the length of the
calendar day is 23 or 25 hours, you'll still end up on the previous calendar day, although not at
noon. Since you don't care about the time, the one hour difference doesn't matter and you end up with
the previous date.
Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
(contributed by brian d foy)
Perl itself never had a Y2K problem, although that never stopped people from creating Y2K problems on
their own. See the documentation for "localtime" for its proper use.
Starting with Perl 5.11, "localtime" and "gmtime" can handle dates past 03:14:08 January 19, 2038,
when a 32-bit based time would overflow. You still might get a warning on a 32-bit "perl":
% perl5.11.2 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
Integer overflow in hexadecimal number at -e line 1.
Wed Nov 1 19:42:39 5576711
On a 64-bit "perl", you can get even larger dates for those really long running projects:
% perl5.11.2 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
Thu Nov 2 00:42:39 5576711
You're still out of luck if you need to keep tracking of decaying protons though.
Data: Strings
How do I validate input?
(contributed by brian d foy)
There are many ways to ensure that values are what you expect or want to accept. Besides the specific
examples that we cover in the perlfaq, you can also look at the modules with "Assert" and "Validate"
in their names, along with other modules such as "Regexp::Common".
Some modules have validation for particular types of input, such as "Business::ISBN",
"Business::CreditCard", "Email::Valid", and "Data::Validate::IP".
How do I unescape a string?
It depends just what you mean by "escape". URL escapes are dealt with in perlfaq9. Shell escapes
with the backslash ("\") character are removed with
s/\\(.)/$1/g;
This won't expand "\n" or "\t" or any other special escapes.
How do I remove consecutive pairs of characters?
(contributed by brian d foy)
You can use the substitution operator to find pairs of characters (or runs of characters) and replace
them with a single instance. In this substitution, we find a character in "(.)". The memory
parentheses store the matched character in the back-reference "\1" and we use that to require that
the same thing immediately follow it. We replace that part of the string with the character in $1.
s/(.)\1/$1/g;
We can also use the transliteration operator, "tr///". In this example, the search list side of our
"tr///" contains nothing, but the "c" option complements that so it contains everything. The
replacement list also contains nothing, so the transliteration is almost a no-op since it won't do
any replacements (or more exactly, replace the character with itself). However, the "s" option
squashes duplicated and consecutive characters in the string so a character does not show up next to
itself
my $str = 'Haarlem'; # in the Netherlands
$str =~ tr///cs; # Now Harlem, like in New York
How do I expand function calls in a string?
(contributed by brian d foy)
This is documented in perlref, and although it's not the easiest thing to read, it does work. In each
of these examples, we call the function inside the braces used to dereference a reference. If we have
more than one return value, we can construct and dereference an anonymous array. In this case, we
call the function in list context.
print "The time values are @{ [localtime] }.\n";
If we want to call the function in scalar context, we have to do a bit more work. We can really have
any code we like inside the braces, so we simply have to end with the scalar reference, although how
you do that is up to you, and you can use code inside the braces. Note that the use of parens creates
a list context, so we need "scalar" to force the scalar context on the function:
print "The time is ${\(scalar localtime)}.\n"
print "The time is ${ my $x = localtime; \$x }.\n";
If your function already returns a reference, you don't need to create the reference yourself.
sub timestamp { my $t = localtime; \$t }
print "The time is ${ timestamp() }.\n";
The "Interpolation" module can also do a lot of magic for you. You can specify a variable name, in
this case "E", to set up a tied hash that does the interpolation for you. It has several other
methods to do this as well.
use Interpolation E => 'eval';
print "The time values are $E{localtime()}.\n";
In most cases, it is probably easier to simply use string concatenation, which also forces scalar
context.
print "The time is " . localtime() . ".\n";
How do I find matching/nesting anything?
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. For balanced expressions using "(", "{", "[" or "<" 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] );
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 $string =~ s/\t+/' ' x (length($&) * 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);
How do I reformat a paragraph?
Use "Text::Wrap" (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
The paragraphs you give to "Text::Wrap" should not contain 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.
How can I access or change N characters of a string?
You can access the first characters of a string with substr(). To get the first character, for
example, start at position 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";
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 "whoever" or "whomever" into "whosoever" or "whomsoever", case insensitively. These all assume
that $_ contains the string to be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;
In the more general case, you can use the "/g" modifier in a "while" loop, keeping count of matches.
$WANT = 3;
$count = 0;
$_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
}
}
That prints out: "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;
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 = "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, "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:
$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($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;
How do I capitalize all the words on one line?
(contributed by brian d foy)
Damian Conway's Text::Autoformat handles all of the thinking for you.
use Text::Autoformat;
my $x = "Dr. Strangelove or: How I Learned to Stop ".
"Worrying and Love the Bomb";
print $x, "\n";
for my $style (qw( sentence title highlight )) {
print autoformat($x, { case => $style }), "\n";
}
How do you want to capitalize those words?
FRED AND BARNEY'S LODGE # all uppercase
Fred And Barney's Lodge # title case
Fred and Barney's Lodge # highlight case
It's not as easy a problem as it looks. How many words do you think are in there? Wait for it... wait
for it.... If you answered 5 you're right. Perl words are groups of "\w+", but that's not what you
want to capitalize. How is Perl supposed to know not to capitalize that "s" after the apostrophe? You
could try a regular expression:
$string =~ s/ (
(^\w) #at the beginning of the line
| # or
(\s\w) #preceded by whitespace
)
/\U$1/xg;
$string =~ s/([\w']+)/\u\L$1/g;
Now, what if you don't want to capitalize that "and"? Just use Text::Autoformat and get on with the
next problem. :)
How can I split a [character] delimited string except when inside [character]?
Several modules can handle this sort of parsing--"Text::Balanced", "Text::CSV", "Text::CSV_XS", and
"Text::ParseWords", among others.
Take the example case of trying to split a string that is comma-separated into its different fields.
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,"","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 $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, "like \"this\"".
Alternatively, the "Text::ParseWords" module (part of the standard Perl distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
How do I strip blank space from the beginning/end of a string?
(contributed by brian d foy)
A substitution can do this for you. For a single line, you want to replace all the leading or
trailing whitespace with nothing. You can do that with a pair of substitutions.
s/^\s+//;
s/\s+$//;
You can also write that as a single substitution, although it turns out the combined statement is
slower than the separate ones. That might not matter to you, though.
s/^\s+|\s+$//g;
In this regular expression, the alternation matches either at the beginning or the end of the string
since the anchors have a lower precedence than the alternation. With the "/g" flag, the substitution
makes all possible matches, so it gets both. Remember, the trailing newline matches the "\s+", and
the "$" anchor can match to the physical end of the string, so the newline disappears too. Just add
the newline to the output, which has the added benefit of preserving "blank" (consisting entirely of
whitespace) lines which the "^\s+" would remove all by itself.
while( <> )
{
s/^\s+|\s+$//g;
print "$_\n";
}
For a multi-line string, you can apply the regular expression to each logical line in the string by
adding the "/m" flag (for "multi-line"). With the "/m" flag, the "$" matches before an embedded
newline, so it doesn't remove it. It still removes the newline at the end of the string.
$string =~ s/^\s+|\s+$//gm;
Remember that lines consisting entirely of whitespace will disappear, since the first part of the
alternation can match the entire string and replace it with nothing. If need to keep embedded blank
lines, you have to do a little more work. Instead of matching any whitespace (since that includes a
newline), just match the other whitespace.
$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
How do I pad a string with blanks or pad a number with zeroes?
In the following examples, $pad_len is the length to which you 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):
$padded = sprintf("%${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 "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?
(contributed by brian d foy)
If you know the columns that contain the data, you can use "substr" to extract a single column.
my $column = substr( $line, $start_column, $length );
You can use "split" if the columns are separated by whitespace or some other delimiter, as long as
whitespace or the delimiter cannot appear as part of the data.
my $line = ' fred barney betty ';
my @columns = split /\s+/, $line;
# ( '', 'fred', 'barney', 'betty' );
my $line = 'fred||barney||betty';
my @columns = split /\|/, $line;
# ( 'fred', '', 'barney', '', 'betty' );
If you want to work with comma-separated values, don't do this since that format is a bit more
complicated. Use one of the modules that handle that format, such as "Text::CSV", "Text::CSV_XS", or
"Text::CSV_PP".
If you want to break apart an entire line of fixed columns, you can use "unpack" with the A (ASCII)
format. By using a number after the format specifier, you can denote the column width. See the "pack"
and "unpack" entries in perlfunc for more details.
my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
Note that spaces in the format argument to "unpack" do not denote literal spaces. If you have space
separated data, you may want "split" instead.
How do I find the soundex value of a string?
(contributed by brian d foy)
You can use the Text::Soundex module. If you want to do fuzzy or close matching, you might also try
the "String::Approx", and "Text::Metaphone", and "Text::DoubleMetaphone" modules.
How can I expand variables in text strings?
(contributed by brian d foy)
If you can avoid it, don't, or if you can use a templating system, such as "Text::Template" or
"Template" Toolkit, do that instead. You might even be able to get the job done with "sprintf" or
"printf":
my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
However, for the one-off simple case where I don't want to pull out a full templating system, I'll
use a string that has two Perl scalar variables in it. In this example, I want to expand $foo and
$bar to their variable's values:
my $foo = 'Fred';
my $bar = 'Barney';
$string = 'Say hello to $foo and $bar';
One way I can do this involves the substitution operator and a double "/e" flag. The first "/e"
evaluates $1 on the replacement side and turns it into $foo. The second /e starts with $foo and
replaces it with its value. $foo, then, turns into 'Fred', and that's finally what's left in the
string:
$string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
The "/e" will also silently ignore violations of strict, replacing undefined variable names with the
empty string. Since I'm using the "/e" flag (twice even!), I have all of the same security problems I
have with "eval" in its string form. If there's something odd in $foo, perhaps something like "@{[
system "rm -rf /" ]}", then I could get myself in trouble.
To get around the security problem, I could also pull the values from a hash instead of evaluating
variable names. Using a single "/e", I can check the hash to ensure the value exists, and if it
doesn't, I can replace the missing value with a marker, in this case "???" to signal that I missed
something:
my $string = 'This has $foo and $bar';
my %Replacements = (
foo => 'Fred',
);
# $string =~ s/\$(\w+)/$Replacements{$1}/g;
$string =~ s/\$(\w+)/
exists $Replacements{$1} ? $Replacements{$1} : '???'
/eg;
print $string;
What's wrong with always quoting "$vars"?
The problem is that those double-quotes 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 "$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 "++" autoincrement operator or the
syscall() function.
Stringification also destroys arrays.
@lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right
Why don't my <<HERE documents work?
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
Data: Arrays
What is the difference between a list and an array?
(contributed by brian d foy)
A list is a fixed collection of scalars. An array is a variable that holds a variable collection of
scalars. An array can supply its collection for list operations, so list operations also work on
arrays:
# slices
( 'dog', 'cat', 'bird' )[2,3];
@animals[2,3];
# iteration
foreach ( qw( dog cat bird ) ) { ... }
foreach ( @animals ) { ... }
my @three = grep { length == 3 } qw( dog cat bird );
my @three = grep { length == 3 } @animals;
# supply an argument list
wash_animals( qw( dog cat bird ) );
wash_animals( @animals );
Array operations, which change the scalars, reaaranges them, or adds or subtracts some scalars, only
work on arrays. These can't work on a list, which is fixed. Array operations include "shift",
"unshift", "push", "pop", and "splice".
An array can also change its length:
$#animals = 1; # truncate to two elements
$#animals = 10000; # pre-extend to 10,001 elements
You can change an array element, but you can't change a list element:
$animals[0] = 'Rottweiler';
qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!
foreach ( @animals ) {
s/^d/fr/; # works fine
}
foreach ( qw( dog cat bird ) ) {
s/^d/fr/; # Error! Modification of read only value!
}
However, if the list element is itself a variable, it appears that you can change a list element.
However, the list element is the variable, not the data. You're not changing the list element, but
something the list element refers to. The list element itself doesn't change: it's still the same
variable.
You also have to be careful about context. You can assign an array to a scalar to get the number of
elements in the array. This only works for arrays, though:
my $count = @animals; # only works with arrays
If you try to do the same thing with what you think is a list, you get a quite different result.
Although it looks like you have a list on the righthand side, Perl actually sees a bunch of scalars
separated by a comma:
my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets bird
Since you're assigning to a scalar, the righthand side is in scalar context. The comma operator (yes,
it's an operator!) in scalar context evaluates its lefthand side, throws away the result, and
evaluates it's righthand side and returns the result. In effect, that list-lookalike assigns to
$scalar it's rightmost value. Many people mess this up becuase they choose a list-lookalike whose
last element is also the count they expect:
my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentally
What is the difference between $array[1] and @array[1]?
(contributed by brian d foy)
The difference is the sigil, that special character in front of the array name. The "$" sigil means
"exactly one item", while the "@" sigil means "zero or more items". The "$" gets you a single scalar,
while the "@" gets you a list.
The confusion arises because people incorrectly assume that the sigil denotes the variable type.
The $array[1] is a single-element access to the array. It's going to return the item in index 1 (or
undef if there is no item there). If you intend to get exactly one element from the array, this is
the form you should use.
The @array[1] is an array slice, although it has only one index. You can pull out multiple elements
simultaneously by specifying additional indices as a list, like @array[1,4,3,0].
Using a slice on the lefthand side of the assignment supplies list context to the righthand side.
This can lead to unexpected results. For instance, if you want to read a single line from a
filehandle, assigning to a scalar value is fine:
$array[1] = <STDIN>;
However, in list context, the line input operator returns all of the lines as a list. The first line
goes into @array[1] and the rest of the lines mysteriously disappear:
@array[1] = <STDIN>; # most likely not what you want
Either the "use warnings" pragma or the -w flag will warn you when you use an array slice with a
single index.
How can I remove duplicate elements from a list or array?
(contributed by brian d foy)
Use a hash. When you think the words "unique" or "duplicated", think "hash keys".
If you don't care about the order of the elements, you could just create the hash then extract the
keys. It's not important how you create that hash: just that you use "keys" to get the unique
elements.
my %hash = map { $_, 1 } @array;
# or a hash slice: @hash{ @array } = ();
# or a foreach: $hash{$_} = 1 foreach ( @array );
my @unique = keys %hash;
If you want to use a module, try the "uniq" function from "List::MoreUtils". In list context it
returns the unique elements, preserving their order in the list. In scalar context, it returns the
number of unique elements.
use List::MoreUtils qw(uniq);
my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
You can also go through each element and skip the ones you've seen before. Use a hash to keep track.
The first time the loop sees an element, that element has no key in %Seen. The "next" statement
creates the key and immediately uses its value, which is "undef", so the loop continues to the "push"
and increments the value for that key. The next time the loop sees that same element, its key exists
in the hash and the value for that key is true (since it's not 0 or "undef"), so the next skips that
iteration and the loop goes to the next element.
my @unique = ();
my %seen = ();
foreach my $elem ( @array )
{
next if $seen{ $elem }++;
push @unique, $elem;
}
You can write this more briefly using a grep, which does the same thing.
my %seen = ();
my @unique = grep { ! $seen{ $_ }++ } @array;
How can I tell whether a certain element is contained in a list or array?
(portions of this answer contributed by Anno Siegel and brian d foy)
Hearing the word "in" is an indication that you probably should 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. In Perl 5.10 and later, you can use the
smart match operator to check that an item is contained in an array or a hash:
use 5.010;
if( $item ~~ @array )
{
say "The array contains $item"
}
if( $item ~~ %hash )
{
say "The hash contains $item"
}
With earlier versions of Perl, you have to do a bit more work. If 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 $is_blue{$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 $is_tiny_prime[$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.
These methods guarantee fast individual tests but require a re-organization of the original list or
array. They only pay off if you have to test multiple values against the same array.
If you are testing only once, the standard module "List::Util" exports the function "first" for this
purpose. It works by stopping once it finds the element. It's written in C for speed, and its Perl
equivalent looks like this subroutine:
sub first (&@) {
my $code = shift;
foreach (@_) {
return $_ if &{$code}();
}
undef;
}
If speed is of little concern, the common idiom uses grep in scalar context (which returns the number
of items that passed its condition) to traverse the entire list. This does have the benefit of
telling you how many matches it found, though.
my $is_there = grep $_ eq $whatever, @array;
If you want to actually extract the matching elements, simply use grep in list context.
my @matches = grep $_ eq $whatever, @array;
How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes that 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.
How do I test whether two arrays or hashes are equal?
With Perl 5.10 and later, the smart match operator can give you the answer with the least amount of
work:
use 5.010;
if( @array1 ~~ @array2 )
{
say "The arrays are the same";
}
if( %hash1 ~~ %hash2 ) # doesn't check values!
{
say "The hash keys are the same";
}
The following code works for single-level arrays. It uses 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.
How do I find the first array element for which a condition is true?
To find the first array element which satisfies a condition, you 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 ( @array ) {
if( /Perl/ ) { $found = $_; 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;
}
}
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 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.
How do I handle circular lists?
(contributed by brian d foy)
If you want to cycle through an array endlessly, you can increment the index modulo the number of
elements in the array:
my @array = qw( a b c );
my $i = 0;
while( 1 ) {
print $array[ $i++ % @array ], "\n";
last if $i > 20;
}
You can also use "Tie::Cycle" to use a scalar that always has the next element of the circular array:
use Tie::Cycle;
tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
print $cycle; # FFFFFF
print $cycle; # 000000
print $cycle; # FFFF00
The "Array::Iterator::Circular" creates an iterator object for circular arrays:
use Array::Iterator::Circular;
my $color_iterator = Array::Iterator::Circular->new(
qw(red green blue orange)
);
foreach ( 1 .. 20 ) {
print $color_iterator->next, "\n";
}
How do I shuffle an array randomly?
If you either have Perl 5.8.0 or later installed, or if you 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; # $deck is a reference to an array
return unless @$deck; # must not be empty!
my $i = @$deck;
while (--$i) {
my $j = int rand ($i+1);
@$deck[$i,$j] = @$deck[$j,$i];
}
}
# shuffle my mpeg collection
#
my @mpeg = <audio/*/*.mp3>;
fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
print @mpeg;
Note that the above implementation shuffles an array in place, unlike the "List::Util::shuffle()"
which takes a list and 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.
How do I process/modify each element of an array?
Use "for"/"foreach":
for (@lines) {
s/foo/bar/; # change that word
tr/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
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 "values" function.
As of Perl 5.6 the values are not copied, so if you modify $orbit (in this case), you modify the
value.
for $orbit ( values %orbits ) {
($orbit **= 3) *= (4/3) * 3.14159;
}
Prior to perl 5.6 "values" returned copies of the values, so older perl code often contains
constructions such as @orbits{keys %orbits} instead of "values %orbits" where the hash is to be
modified.
How do I select a random element from an array?
Use the "rand()" function (see "rand" in perlfunc):
$index = rand @array;
$element = $array[$index];
Or, simply:
my $element = $array[ rand @array ];
How do I permute N elements of a list?
Use the "List::Permutor" module on CPAN. If the list is actually 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-Krause ordered permutation generator
sub permute (&@) {
my $code = shift;
my @idx = 0..$#_;
while ( $code->(@_[@idx]) ) {
my $p = $#idx;
--$p while $idx[$p-1] > $idx[$p];
my $q = $p or return;
push @idx, reverse splice @idx, $p;
++$q while $idx[$p-1] > $idx[$q];
@idx[$p-1,$q]=@idx[$q,$p-1];
}
}
permute { print "@_\n" } split;
The "Algorithm::Loops" module also provides the "NextPermute" and "NextPermuteNum" functions which
efficiently find all unique permutations of an array, even if it contains duplicate values, modifying
it in-place: if its elements are in reverse-sorted order then the array is reversed, making it
sorted, and it returns false; otherwise the next permutation is returned.
"NextPermute" uses string order and "NextPermuteNum" numeric order, so you can enumerate all the
permutations of 0..9 like this:
use Algorithm::Loops qw(NextPermuteNum);
my @list= 0..9;
do { print "@list\n" } while NextPermuteNum @list;
How do I sort an array by (anything)?
Supply a comparison function to sort() (described in "sort" in perlfunc):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would sort "(1, 2, 10)" into "(1, 10, 2)".
"<=>", used above, is the numerical comparison operator.
If you have a complicated function needed to pull out the part you want to sort on, then don't do it
inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the
same element. Here's an example of how to pull out the first word after the first number on each
item, and then sort those words case-insensitively.
@idx = ();
for (@data) {
($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 { $_->[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 later in perlfaq4 on sorting hashes.
How do I manipulate arrays of bits?
Use "pack()" and "unpack()", or else "vec()" and the bitwise operations.
For example, you don't have to store individual bits in an array (which would mean that you're
wasting a lot of space). To convert an array of bits to a string, use "vec()" to set the right bits.
This sets $vec to have bit N set only if $ints[N] was set:
@ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
$vec = '';
foreach( 0 .. $#ints ) {
vec($vec,$_,1) = 1 if $ints[$_];
}
The string $vec only takes up as many bits as it needs. For instance, if you had 16 entries in @ints,
$vec only needs two bytes to store them (not counting the scalar variable overhead).
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 > 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";
}
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?
(contributed by brian d foy)
There are a couple of ways that you can process an entire hash. You can get a list of keys, then go
through each key, or grab a one key-value pair at a time.
To go through all of the keys, use the "keys" function. This extracts all of the keys of the hash and
gives them back to you as a list. You can then get the value through the particular key you're
processing:
foreach my $key ( keys %hash ) {
my $value = $hash{$key}
...
}
Once you have the list of keys, you can process that list before you process the hash elements. For
instance, you can sort the keys so you can process them in lexical order:
foreach my $key ( sort keys %hash ) {
my $value = $hash{$key}
...
}
Or, you might want to only process some of the items. If you only want to deal with the keys that
start with "text:", you can select just those using "grep":
foreach my $key ( grep /^text:/, keys %hash ) {
my $value = $hash{$key}
...
}
If the hash is very large, you might not want to create a long list of keys. To save some memory, you
can grab one key-value pair at a time using "each()", which returns a pair you haven't seen yet:
while( my( $key, $value ) = each( %hash ) ) {
...
}
The "each" operator returns the pairs in apparently random order, so if ordering matters to you,
you'll have to stick with the "keys" method.
The "each()" operator can be a bit tricky though. You can't add or delete keys of the hash while
you're using it without possibly skipping or re-processing some pairs after Perl internally rehashes
all of the elements. Additionally, a hash has only one iterator, so if you use "keys", "values", or
"each" on the same hash, you can reset the iterator and mess up your processing. See the "each" entry
in perlfunc for more details.
How do I merge two hashes?
(contributed by brian d foy)
Before you decide to merge two hashes, you have to decide what to do if both hashes contain keys that
are the same and if you want to leave the original hashes as they were.
If you want to preserve the original hashes, copy one hash (%hash1) to a new hash (%new_hash), then
add the keys from the other hash (%hash2 to the new hash. Checking that the key already exists in
%new_hash gives you a chance to decide what to do with the duplicates:
my %new_hash = %hash1; # make a copy; leave %hash1 alone
foreach my $key2 ( keys %hash2 )
{
if( exists $new_hash{$key2} )
{
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else
{
$new_hash{$key2} = $hash2{$key2};
}
}
If you don't want to create a new hash, you can still use this looping technique; just change the
%new_hash to %hash1.
foreach my $key2 ( keys %hash2 )
{
if( exists $hash1{$key2} )
{
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else
{
$hash1{$key2} = $hash2{$key2};
}
}
If you don't care that one hash overwrites keys and values from the other, you could just use a hash
slice to add one hash to another. In this case, values from %hash2 replace values from %hash1 when
they have keys in common:
@hash1{ keys %hash2 } = values %hash2;
What happens if I add or remove keys from a hash while iterating over it?
(contributed by brian d foy)
The easy answer is "Don't do that!"
If you iterate through the hash with each(), you can delete the 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 "each()" in perlfunc.
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?
(contributed by brian d foy)
This is very similar to "How do I process an entire hash?", also in perlfaq4, but a bit simpler in
the common cases.
You can use the "keys()" built-in function in scalar context to find out have many entries you have
in a hash:
my $key_count = keys %hash; # must be scalar context!
If you want to find out how many entries have a defined value, that's a bit different. You have to
check each value. A "grep" is handy:
my $defined_value_count = grep { defined } values %hash;
You can use that same structure to count the entries any way that you like. If you want the count of
the keys with vowels in them, you just test for that instead:
my $vowel_count = grep { /[aeiou]/ } keys %hash;
The "grep" in scalar context returns the count. If you want the list of matching items, just use it
in list context instead:
my @defined_values = grep { defined } values %hash;
The "keys()" function also resets the iterator, which means that you may see strange results if you
use this between uses of other hash operators such as "each()".
How do I sort a hash (optionally by value instead of key)?
(contributed by brian d foy)
To sort a hash, start with the keys. In this example, we give the list of keys to the sort function
which then compares them ASCIIbetically (which might be affected by your locale settings). The output
list has the keys in ASCIIbetical order. Once we have the keys, we can go through them to create a
report which lists the keys in ASCIIbetical order.
my @keys = sort { $a cmp $b } keys %hash;
foreach my $key ( @keys )
{
printf "%-20s %6d\n", $key, $hash{$key};
}
We could get more fancy in the "sort()" block though. Instead of comparing the keys, we can compute a
value with them and use that value as the comparison.
For instance, to make our report order case-insensitive, we use the "\L" sequence in a double-quoted
string to make everything lowercase. The "sort()" block then compares the lowercased values to
determine in which order to put the keys.
my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
Note: if the computation is expensive or the hash has many elements, you may want to look at the
Schwartzian Transform to cache the computation results.
If we want to sort by the hash value instead, we use the hash key to look it up. We still get out a
list of keys, but this time they are ordered by their value.
my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
From there we can get more complex. If the hash values are the same, we can provide a secondary sort
on the hash key.
my @keys = sort {
$hash{$a} <=> $hash{$b}
or
"\L$a" cmp "\L$b"
} keys %hash;
How can I always keep my hash sorted?
You can look into using the "DB_File" module and "tie()" using the $DB_BTREE hash bindings as
documented in "In Memory Databases" in DB_File. The "Tie::IxHash" module from CPAN might also be
instructive. Although this does keep your hash sorted, you might not like the slow down you suffer
from the tie interface. Are you sure you need to do this? :)
What's the difference between "delete" and "undef" with hashes?
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 $key is present in %hash, "exists($hash{$key})" will return true. The value for a given key
can be "undef", in which case $hash{$key} will be "undef" while "exists $hash{$key}" will return
true. This corresponds to ($key, "undef") being in the hash.
Pictures help... Here's the %hash table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
And these conditions hold
$hash{'a'} is true
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is true
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true
If you now say
undef $hash{'a'}
your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $hash{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl 5 only)
grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
Why don't my tied hashes make the defined/exists distinction?
This depends on the tied hash's implementation of EXISTS(). For example, there isn't the concept of
undef with 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.
How do I reset an each() operation part-way through?
(contributed by brian d foy)
You can use the "keys" or "values" functions to reset "each". To simply reset the iterator used by
"each" without doing anything else, use one of them in void context:
keys %hash; # resets iterator, nothing else.
values %hash; # resets iterator, nothing else.
See the documentation for "each" in perlfunc.
How can I get the unique keys from two hashes?
First you extract the keys from the hashes into lists, then 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 my %myhash, 'Tie::IxHash';
for (my $i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
my @keys = keys %myhash;
# @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a hash create it?
(contributed by brian d foy)
Are you using a really old version of Perl?
Normally, accessing a hash key's value for a nonexistent key will not create the key.
my %hash = ();
my $value = $hash{ 'foo' };
print "This won't print\n" if exists $hash{ 'foo' };
Passing $hash{ 'foo' } to a subroutine used to be a special case, though. Since you could assign
directly to $_[0], Perl had to be ready to make that assignment so it created the hash key ahead of
time:
my_sub( $hash{ 'foo' } );
print "This will print before 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
# $_[0] = 'bar'; # create hash key in case you do this
1;
}
Since Perl 5.004, however, this situation is a special case and Perl creates the hash key only when
you make the assignment:
my_sub( $hash{ 'foo' } );
print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
$_[0] = 'bar';
}
However, if you want the old behavior (and think carefully about that because it's a weird side
effect), you can pass a hash slice instead. Perl 5.004 didn't make this a special case:
my_sub( @hash{ qw/foo/ } );
How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Usually a hash ref, perhaps like this:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};
References are documented in perlref and 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?
(contributed by brian d foy and Ben Morrow)
Hash keys are strings, so you can't really use a reference as the key. When you try to do that, perl
turns the reference into its stringified form (for instance, "HASH(0xDEADBEEF)"). From there you
can't get back the reference from the stringified form, at least without doing some extra work on
your own.
Remember that the entry in the hash will still be there even if the referenced variable goes out of
scope, and that it is entirely possible for Perl to subsequently allocate a different variable at the
same address. This will mean a new variable might accidentally be associated with the value for an
old.
If you have Perl 5.10 or later, and you just want to store a value against the reference for lookup
later, you can use the core Hash::Util::Fieldhash module. This will also handle renaming the keys if
you use multiple threads (which causes all variables to be reallocated at new addresses, changing
their stringification), and garbage-collecting the entries when the referenced variable goes out of
scope.
If you actually need to be able to get a real reference back from each hash entry, you can use the
Tie::RefHash module, which does the required work for you.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so it can handle binary data just fine. On Windows or DOS, however, you have
to use "binmode" for binary files to avoid conversions for line endings. In general, you should use
"binmode" any time you want to work with binary data.
Also see "binmode" in perlfunc or perlopentut.
If you're concerned about 8-bit textual 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 "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 "looks_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 "Regexp::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 "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 the 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" or "Storable" modules from CPAN. Starting from Perl
5.8 "Storable" is part of the standard distribution. Here's one example using "Storable"'s "store"
and "retrieve" functions:
use Storable;
store(\%hash, "filename");
# later on...
$href = retrieve("filename"); # by ref
%hash = %{ retrieve("filename") }; # direct to hash
How do I print out or copy a recursive data structure?
The "Data::Dumper" module on CPAN (or the 5.005 release of Perl) is great for printing out data
structures. The "Storable" module on CPAN (or the 5.8 release of Perl), 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?
(contributed by Ben Morrow)
You can use the "UNIVERSAL" class (see UNIVERSAL). However, please be very careful to consider the
consequences of doing this: adding methods to every object is very likely to have unintended
consequences. If possible, it would be better to have all your object inherit from some common base
class, or to use an object system like Moose that supports roles.
How do I verify a credit card checksum?
Get the "Business::CreditCard" module from CPAN.
How do I pack arrays of doubles or floats for XS code?
The arrays.h/arrays.c code in the "PGPLOT" module on CPAN does just this. If you're doing a lot of
float or double processing, consider using the "PDL" module from CPAN instead--it makes number-crunching numbercrunching
crunching easy.
See <http://search.cpan.org/dist/PGPLOT> for the code.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2010 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.
perl v5.12.5 2012-11-03 PERLFAQ4(1)
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