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PERLFAQ4(1)
NAME
perlfaq4 - Data Manipulation ($Revision: 1.61 $, $Date: 2005/03/11 16:27:53
$)
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)?
Internally, your computer represents floating-point numbers in binary.
Digital (as in powers of two) computers cannot store all numbers exactly.
Some real numbers lose precision in the process. This is a problem with
how computers store numbers and affects all computer languages, not just
Perl.
perlnumber show the gory details of number representations and conversions.
To limit the number of decimal places in your numbers, you can use the
printf or sprintf function. See the "Floating Point Arithmetic" for more
details.
printf "%.2f", 10/3;
my $number = sprintf "%.2f", 10/3;
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 above item "Why am I getting long decimals (eg,
19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?".
For example, this
print int(0.6/0.2-2), "\n";
will in most computers print 0, not 1, because even such simple numbers as
0.6 and 0.2 cannot be presented exactly by floating-point numbers. What
you think in the above as 'three' is really more like
2.9999999999999995559.
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. Octal literals in perl must start with a leading
"0" and hexadecimal literals must start with a leading "0x". If they are
read in from somewhere and assigned, no automatic conversion takes place.
You must explicitly use oct() or hex() if you want the values converted to
decimal. oct() interprets hex ("0x350"), octal ("0350" or even without the
leading "0", like "377") and binary ("0b1010") numbers, while hex() only
converts hexadecimal ones, with or without a leading "0x", like "0x255",
"3A", "ff", or "deadbeef". The inverse mapping from decimal to octal can
be done with either the "%o" or "%O" sprintf() formats.
This problem shows up most often when people try using chmod(), mkdir(),
umask(), or sysopen(), which by widespread tradition typically take
permissions in octal.
chmod(644, $file); # WRONG
chmod(0644, $file); # right
Note the mistake in the first line was specifying the decimal literal 644,
rather than the intended octal literal 0644. The problem can be seen with:
printf("%#o",644); # prints 01204
Surely you had not intended "chmod(01204, $file);" - did you? If you want
to use numeric literals as arguments to chmod() et al. then please try to
express them as octal constants, that is with a leading zero and with the
following digits restricted to the set 0..7.
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 below use the Bit::Vector module from CPAN. The
reason you might choose Bit::Vector over the perl built in functions is
that it works with numbers of ANY size, that it is optimized for speed on
some operations, and for at least some programmers the notation might be
familiar.
How do I convert hexadecimal into decimal
Using perl's built in conversion of 0x notation:
$dec = 0xDEADBEEF;
Using the hex function:
$dec = hex("DEADBEEF");
Using pack:
$dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
Using the CPAN module Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new_Hex(32, "DEADBEEF");
$dec = $vec->to_Dec();
How do I convert from decimal to hexadecimal
Using sprintf:
$hex = sprintf("%X", 3735928559); # upper case A-F
$hex = sprintf("%x", 3735928559); # lower case a-f
Using unpack:
$hex = unpack("H*", pack("N", 3735928559));
Using Bit::Vector:
use Bit::Vector;
$vec = Bit::Vector->new_Dec(32, -559038737);
$hex = $vec->to_Hex();
And Bit::Vector supports odd bit counts:
use Bit::Vector;
$vec = Bit::Vector->new_Dec(33, 3735928559);
$vec->Resize(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 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 $] < 5.004 }
5.004 and later automatically call "srand" at the beginning. Don't call
"srand" more than once---you make your numbers less random, rather than
more.
Computers are good at being predictable and bad at being random (despite
appearances caused by bugs in your programs :-). see the random article in
the "Far More Than You Ever Wanted To Know" collection in
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of Tom Phoenix,
talks more about this. John von Neumann said, ``Anyone who attempts to
generate random numbers by deterministic means is, of course, living in a
state of sin.''
If you want numbers that are more random than "rand" with "srand" provides,
you should also check out the Math::TrulyRandom module from CPAN. It uses
the imperfections in your system's timer to generate random numbers, but
this takes quite a while. If you want a better pseudorandom generator than
comes with your operating system, look at ``Numerical Recipes in C'' at
http://www.nr.com/ .
How do I get a random number between X and Y?
"rand($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 );
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_in(50,120)".
sub random_int_in ($$) {
my($min, $max) = @_;
# Assumes that the two arguments are integers themselves!
return $min if $min == $max;
($min, $max) = ($max, $min) if $min > $max;
return $min + int rand(1 + $max - $min);
}
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 the Time::Local module to get a
time in epoch seconds for the argument to localtime.
use POSIX qw/strftime/;
use Time::Local;
my $week_of_year = strftime "%W",
localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
The Date::Calc module provides two functions for to calculate these.
use Date::Calc;
my $day_of_year = Day_of_Year( 1987, 12, 18 );
my $week_of_year = Week_of_Year( 1987, 12, 18 );
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?
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?
(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?
If you only need to find the date (and not the same time), you can use the
Date::Calc module.
use Date::Calc qw(Today Add_Delta_Days);
my @date = Add_Delta_Days( Today(), -1 );
print "@date\n";
Most people try to use the time rather than the calendar to figure out
dates, but that assumes that your days are twenty-four hours each. For
most people, there are two days a year when they aren't: the switch to and
from summer time throws this off. Russ Allbery offers this solution.
sub yesterday {
my $now = defined $_[0] ? $_[0] : time;
my $then = $now - 60 * 60 * 24;
my $ndst = (localtime $now)[8] > 0;
my $tdst = (localtime $then)[8] > 0;
$then - ($tdst - $ndst) * 60 * 60;
}
Should give you "this time yesterday" in seconds since epoch relative to
the first argument or the current time if no argument is given and suitable
for passing to localtime or whatever else you need to do with it. $ndst is
whether we're currently in daylight savings time; $tdst is whether the
point 24 hours ago was in daylight savings time. If $tdst and $ndst are
the same, a boundary wasn't crossed, and the correction will subtract 0.
If $tdst is 1 and $ndst is 0, subtract an hour more from yesterday's time
since we gained an extra hour while going off daylight savings time. If
$tdst is 0 and $ndst is 1, subtract a negative hour (add an hour) to
yesterday's time since we lost an hour.
All of this is because during those days when one switches off or onto DST,
a "day" isn't 24 hours long; it's either 23 or 25.
The explicit settings of $ndst and $tdst are necessary because localtime
only says it returns the system tm struct, and the system tm struct at
least on Solaris doesn't guarantee any particular positive value (like,
say, 1) for isdst, just a positive value. And that value can potentially
be negative, if DST information isn't available (this sub just treats those
cases like no DST).
Note that between 2am and 3am on the day after the time zone switches off
daylight savings time, the exact hour of "yesterday" corresponding to the
current hour is not clearly defined. Note also that if used between 2am
and 3am the day after the change to daylight savings time, the result will
be between 3am and 4am of the previous day; it's arguable whether this is
correct.
This sub does not attempt to deal with leap seconds (most things don't).
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://www.perl.org/about/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 "\n" or "\t" or any other special escapes.
How do I remove consecutive pairs of characters?
To turn "abbcccd" into "abccd":
s/(.)\1/$1/g; # add /s to include newlines
Here's a solution that turns "abbcccd" to "abcd":
y///cs; # y == tr, but shorter :-)
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 "My sub returned @{[mysub(1,2,3)]} that time.\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?
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"". Sometimes you might want this. Other times you might need a more
thorough solution (Suggested by brian d foy):
$string =~ s/ (
(^\w) #at the beginning of the line
| # or
(\s\w) #preceded by whitespace
)
/\U$1/xg;
$string =~ /([\w']+)/\u\L$1/g;
To make the whole line upper case:
$line = uc($line);
To force each word to be lower case, with the first letter upper case:
$line =~ s/(\w+)/\u\L$1/g;
You can (and probably should) enable locale awareness of those characters
by placing a "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.
Damian Conway's Text::Autoformat module provides some smart case
transformations:
use Text::Autoformat;
my $x = "Dr. Strangelove or: How I Learned to Stop ".
"Worrying and Love the Bomb";
print $x, "\n";
for my $style (qw( sentence title highlight ))
{
print autoformat($x, { case => $style }), "\n";
}
How can I split a [character] delimited string except when inside
[character]?
Several modules can handle this sort of pasing---Text::Balanced, Text::CSV,
Text::CSV_XS, and Text::ParseWords, among others.
Take the example case of trying to split a string that is comma-separated
into its different fields. You can't use "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);
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?
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?
Use substr() or unpack(), both documented in perlfunc. If you prefer
thinking in terms of columns instead of widths, you can use this kind of
thing:
# determine the unpack format needed to split Linux ps output
# arguments are cut columns
my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
sub cut2fmt {
my(@positions) = @_;
my $template = '';
my $lastpos = 1;
for my $place (@positions) {
$template .= "A" . ($place - $lastpos) . " ";
$lastpos = $place;
}
$template .= "A*";
return $template;
}
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?
Let's assume that you have a string that contains placeholder variables.
$text = 'this has a $foo in it and a $bar';
You can use a substitution with a double evaluation. The first /e turns $1
into $foo, and the second /e turns $foo into its value. You may want to
wrap this in an "eval": if you try to get the value of an undeclared
variable while running under "use strict", you get a fatal error.
eval { $text =~ s/(\$\w+)/$1/eeg };
die if $@;
It's probably better in the general case to treat those variables as
entries in some special hash. For example:
%user_defs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
What's wrong with always quoting "$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?
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 = "not equal to $in[0]";
@out = grep($_ ne $prev && ($prev = $_, 1), @in);
This is nice in that it doesn't use much extra memory, simulating
uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
guarantees that the expression is true (so that grep picks it up) even
if the $_ is 0, "", or undef.
b) If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$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 certain element is contained in a list or array?
Hearing the word "in" is an indication that you probably should have used a
hash, not a list or array, to store your data. Hashes are designed to
answer this question quickly and efficiently. Arrays aren't.
That being said, there are several ways to approach this. If you are going
to make this query many times over arbitrary string values, the fastest way
is probably to invert the original array and maintain a hash whose keys are
the first array's values.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
%is_blue = ();
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$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 $n.
Please do not use
($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} > 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?
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 my $element ( @array )
{
if( /Perl/ ) { $found = $element; last }
}
If you want the array index, you can iterate through the indices and check
the array element at each index until you find one that satisfies the
condition.
my( $found, $index ) = ( undef, -1 );
for( $i = 0; $i < @array; $i++ )
{
if( $array[$i] =~ /Perl/ )
{
$found = $array[$i];
$index = $i;
last;
}
}
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:
$node = {
VALUE => 42,
LINK => undef,
};
You could walk the list this way:
print "List: ";
for ($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?
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?
If you either have Perl 5.8.0 or later installed, or if you have Scalar-
List-Utils 1.03 or later installed, you can say:
use List::Util 'shuffle';
@shuffled = shuffle(@list);
If not, you can use a Fisher-Yates shuffle.
sub fisher_yates_shuffle {
my $deck = shift; # $deck is a reference to an array
my $i = @$deck;
while ($i--) {
my $j = int rand ($i+1);
@$deck[$i,$j] = @$deck[$j,$i];
}
}
# shuffle my mpeg collection
#
my @mpeg = <audio/*/*.mp3>;
fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
print @mpeg;
Note that the above implementation shuffles an array in place, unlike the
List::Util::shuffle() which takes a list and returns a new shuffled list.
You've probably seen shuffling algorithms that work using splice, randomly
picking another element to swap the current element with
srand;
@new = ();
@old = 1 .. 10; # just a demo
while (@old) {
push(@new, splice(@old, rand @old, 1));
}
This is bad because splice is already O(N), and since you do it N times,
you just invented a quadratic algorithm; that is, O(N**2). This does not
scale, although Perl is so efficient that you probably won't notice this
until you have rather largish arrays.
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;
$_ *= (4/3) * 3.14159; # this will be constant folded
}
which can also be done with map() which is made to transform one list into
another:
@volumes = map {$_ ** 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-Kause ordered permutation generator
sub permute (&@) {
my $code = shift;
my @idx = 0..$#_;
while ( $code->(@_[@idx]) ) {
my $p = $#idx;
--$p while $idx[$p-1] > $idx[$p];
my $q = $p or return;
push @idx, reverse splice @idx, $p;
++$q while $idx[$p-1] > $idx[$q];
@idx[$p-1,$q]=@idx[$q,$p-1];
}
}
permute {print"@_\n"} split;
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 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 }
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), $bits);
print "bits are: @bytes\n\n";
}
Why does defined() return true on empty arrays and hashes?
The short story is that you should probably only use defined on scalars or
functions, not on aggregates (arrays and hashes). See "defined" in
perlfunc in the 5.004 release or later of Perl for more detail.
Data: Hashes (Associative Arrays)
How do I process an entire hash?
Use the each() function (see "each" in perlfunc) if you don't care whether
it's sorted:
while ( ($key, $value) = each %hash) {
print "$key = $value\n";
}
If you want it sorted, you'll have to use foreach() on the result of
sorting the keys as shown in an earlier question.
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?
If you mean how many keys, then all you have to do is use the keys()
function in a scalar context:
$num_keys = keys %hash;
The keys() function also resets the iterator, which means that you may see
strange results if you use this between uses of other hash operators such
as each().
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} <=> $hash{$a}
||
length($b) <=> length($a)
||
$a cmp $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.
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 $hash{'a'} is true (Perl5 only)
grep ($_ eq 'a', keys %hash) is true
If you now say
undef $hash{'a'}
your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl5 only)
grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $hash{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl5 only)
grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
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?
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 my %myhash, 'Tie::IxHash';
for (my $i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
my @keys = keys %myhash;
# @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a hash create it?
If you say something like:
somefunc($hash{"nonesuch key here"});
Then that element "autovivifies"; that is, it springs into existence
whether you store something there or not. That's because functions get
scalars passed in by reference. If somefunc() modifies $_[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:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};
References are documented in perlref and the upcoming perlreftut. Examples
of complex data structures are given in perldsc and perllol. Examples of
structures and object-oriented classes are in perltoot.
How can I use a reference as a hash key?
You can't do this directly, but you could use the standard Tie::RefHash
module distributed with Perl.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so this shouldn't be a problem. For example, this
works fine (assuming the files are found):
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On less elegant (read: Byzantine) systems, however, you have to play
tedious games with "text" versus "binary" files. See "binmode" in perlfunc
or perlopentut.
If you're concerned about 8-bit ASCII data, then see perllocale.
If you want to deal with multibyte characters, however, there are some
gotchas. See the section on Regular Expressions.
How do I determine whether a scalar is a number/whole/integer/float?
Assuming that you don't care about IEEE notations like "NaN" or "Infinity",
you probably just want to use a regular expression.
if (/\D/) { print "has nondigits\n" }
if (/^\d+$/) { 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?
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) 1997-2005 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it under
the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are hereby
placed into the public domain. You are permitted and encouraged to use
this code in your own programs for fun or for profit as you see fit. A
simple comment in the code giving credit would be courteous but is not
required.
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