PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)NAMEperlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date:
1999/01/08 04:26:39 $)
DESCRIPTION
The section of the FAQ answers question related to the
manipulation of data as numbers, dates, strings, arrays,
hashes, and miscellaneous data issues.
Data: Numbers
Why am I getting long decimals (eg, 19.9499999999999)
instead of the numbers I should be getting (eg, 19.95)?
The infinite set that a mathematician thinks of as the
real numbers can only be approximate on a computer, since
the computer only has a finite number of bits to store an
infinite number of, um, numbers.
Internally, your computer represents floating-point
numbers in binary. Floating-point numbers read in from a
file or appearing as literals in your program are
converted from their decimal floating-point representation
(eg, 19.95) to the internal binary representation.
However, 19.95 can't be precisely represented as a binary
floating-point number, just like 1/3 can't be exactly
represented as a decimal floating-point number. The
computer's binary representation of 19.95, therefore,
isn't exactly 19.95.
When a floating-point number gets printed, the binary
floating-point representation is converted back to
decimal. These decimal numbers are displayed in either
the format you specify with printf(), or the current
output format for numbers (see the section on $# in the
perlvar manpage if you use print. $# has a different
default value in Perl5 than it did in Perl4. Changing $#
yourself is deprecated.
This affects all computer languages that represent decimal
floating-point numbers in binary, not just Perl. Perl
provides arbitrary-precision decimal numbers with the
Math::BigFloat module (part of the standard Perl
distribution), but mathematical operations are
consequently slower.
To get rid of the superfluous digits, just use a format
(eg, printf("%.2f", 19.95)) to get the required precision.
See the section on Floating-point Arithmetic in the perlop
manpage.
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Why isn't my octal data interpreted correctly?
Perl only understands octal and hex numbers as such when
they occur as literals in your program. If they are read
in from somewhere and assigned, no automatic conversion
takes place. You must explicitly use oct() or hex() if
you want the values converted. oct() interprets both hex
("0x350") numbers and octal ones ("0350" or even without
the leading "0", like "377"), while hex() only converts
hexadecimal ones, with or without a leading "0x", like
"0x255", "3A", "ff", or "deadbeef".
This problem shows up most often when people try using
chmod(), mkdir(), umask(), or sysopen(), which all want
permissions in octal.
chmod(644, $file); # WRONG -- perl -w catches this
chmod(0644, $file); # right
Does Perl have a round() function? What about ceil() and
floor()? Trig functions?
Remember that int() merely truncates toward 0. For
rounding to a certain number of digits, sprintf() or
printf() is usually the easiest route.
printf("%.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
$floor = floor(3.5); # 3
In 5.000 to 5.003 Perls, trigonometry was done in the
Math::Complex module. With 5.004, the Math::Trig module
(part of the standard perl distribution) implements the
trigonometric functions. Internally it uses the
Math::Complex module and some functions can break out from
the real axis into the complex plane, for example the
inverse sine of 2.
Rounding in financial applications can have serious
implications, and the rounding method used should be
specified precisely. In these cases, it probably pays not
to trust whichever system rounding is being used by Perl,
but to instead implement the rounding function you need
yourself.
To see why, notice how you'll still have an issue on half-
way-point alternation:
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for ($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 bits into ints?
To turn a string of 1s and 0s like 10110110 into a scalar
containing its binary value, use the pack() function
(documented in the section on pack in the perlfunc
manpage):
$decimal = pack('B8', '10110110');
Here's an example of going the other way:
$binary_string = join('', unpack('B*', "\x29"));
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on
whether they're used on numbers or strings. The operators
treat a string as a series of bits and work with that (the
string "3" is the bit pattern 00110011). The operators
work with the binary form of a number (the number 3 is
treated as the bit pattern 00000011).
So, saying 11 & 3 performs the "and" operation on numbers
(yielding 1). Saying "11" & "3" performs the "and"
operation on strings (yielding "1").
Most problems with & and | arise because the programmer
thinks they have a number but really it's a string. The
rest arise because the programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of
"\020\020" & "\101\101") is not a false value in Perl.
You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
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How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules
(available from CPAN) or the PDL extension (also available
from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and
collect the results, use:
@results = map { my_func($_) } @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));
}
How can I output Roman numerals?
Get the http://www.perl.com/CPAN/modules/by-module/Roman
module.
Why aren't my random numbers random?
If you're using a version of Perl before 5.004, you must
call srand once at the start of your program to seed the
random number generator. 5.004 and later automatically
call srand at the beginning. Don't call srand more than
once--you make your numbers less random, rather than more.
Computers are good at being predictable and bad at being
random (despite appearances caused by bugs in your
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programs :-).
http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of
Tom Phoenix, talks more about this.. John von Neumann
said, ``Anyone who attempts to generate random numbers by
deterministic means is, of course, living in a state of
sin.''
If you want numbers that are more random than rand with
srand provides, you should also check out the
Math::TrulyRandom module from CPAN. It uses the
imperfections in your system's timer to generate random
numbers, but this takes quite a while. If you want a
better pseudorandom generator than comes with your
operating system, look at ``Numerical Recipes in C'' at
http://www.nr.com/ .
Data: Dates
How do I find the week-of-the-year/day-of-the-year?
The day of the year is in the array returned by
localtime() (see the section on localtime in the perlfunc
manpage):
$day_of_year = (localtime(time()))[7];
or more legibly (in 5.004 or higher):
use Time::localtime;
$day_of_year = localtime(time())->yday;
You can find the week of the year by dividing this by 7:
$week_of_year = int($day_of_year / 7);
Of course, this believes that weeks start at zero. The
Date::Calc module from CPAN has a lot of date calculation
functions, including day of the year, week of the year,
and so on. Note that not all businesses consider ``week
1'' to be the same; for example, American businesses often
consider the first week with a Monday in it to be Work
Week #1, despite ISO 8601, which considers WW1 to be the
first week with a Thursday in it.
How 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 use one of the Date::Manip and Date::Calc
modules from CPAN.
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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?
Neither Date::Manip nor Date::Calc deal with Julian days.
Instead, there is an example of Julian date calculation
that should help you in Time::JulianDay (part of the Time-
modules bundle) which can be found at
http://www.perl.com/CPAN/modules/by-module/Time/.
How do I find yesterday's date?
The time() function returns the current time in seconds
since the epoch. Take one day off that:
$yesterday = time() - ( 24 * 60 * 60 );
Then you can pass this to localtime() and get the
individual year, month, day, hour, minute, seconds values.
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 an array 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.
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That doesn't mean that Perl can't be used to create non-
Y2K compliant programs. It can. But so can your pencil.
It's the fault of the user, not the language. At the risk
of inflaming the NRA: ``Perl doesn't break Y2K, people
do.'' See http://language.perl.com/news/y2k.html for a
longer exposition.
Data: Strings
How do I validate input?
The answer to this question is usually a regular
expression, perhaps with auxiliary logic. See the more
specific questions (numbers, mail addresses, etc.) for
details.
How do I unescape a string?
It depends just what you mean by ``escape''. URL escapes
are dealt with in the perlfaq9 manpage. 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;
How do I expand function calls in a string?
This is documented in the perlref manpage. 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";
If you prefer scalar context, similar chicanery is also
useful for arbitrary expressions:
print "That yields ${\($n + 5)} widgets\n";
Version 5.004 of Perl had a bug that gave list context to
the expression in ${...}, but this is fixed in version
5.005.
See also ``How can I expand variables in text strings?''
in this section of the FAQ.
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How do I find matching/nesting anything?
This isn't something that can be done in one regular
expression, no matter how complicated. To find something
between two single characters, a pattern like /x([^x]*)x/
will get the intervening bits in $1. For multiple ones,
then something more like /alpha(.*?)omega/ would be
needed. But none of these deals with nested patterns, nor
can they. For that you'll have to write a parser.
If you are serious about writing a parser, there are a
number of modules or oddities that will make your life a
lot easier. There is the CPAN module Parse::RecDescent,
the standard module Text::Balanced, the byacc program, the
CPAN module Parse::Yapp, and Mark-Jason Dominus's
excellent py tool at http://www.plover.com/~mjd/perl/py/ .
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/);
print join("\n",@$[0..$#$]) if( $$[-1] );
How do I reverse a string?
Use reverse() in scalar context, as documented in the
reverse entry in the perlfunc manpage.
$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;
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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).
How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
substr():
$first_byte = substr($a, 0, 1);
If you want to modify part of a string, the simplest way
is often to use substr() as an lvalue:
substr($a, 0, 3) = "Tom";
Although those with a pattern matching kind of thought
process will likely prefer:
$a =~ s/^.../Tom/;
How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, let's
say you want to change the fifth occurrence of "whoever"
or "whomever" into "whosoever" or "whomsoever", case
insensitively.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}igex;
In the more general case, you can use the /g modifier in a
while loop, keeping count of matches.
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$WANT = 3;
$count = 0;
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
# Warning: don't `last' out of this loop
}
}
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 charcters 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";
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$1/g;
This has the strange effect of turning "don't do it" into
"Don'T Do It". Sometimes you might want this, instead
(Suggested by Brian Foy):
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$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 the perllocale manpage for endless details
on locales.
This is sometimes referred to as putting something into
"title case", but that's not quite accurate. Consdier the
proper capitalization of the movie Dr. Strangelove or: How
I Learned to Stop Worrying and Love the Bomb, for example.
How can I split a [character] delimited string except when
inside [character]? (Comma-separated files)
Take the example case of trying to split a string that is
comma-separated into its different fields. (We'll pretend
you said comma-separated, not comma-delimited, which is
different and almost never what you mean.) You can't use
split(/,/) because you shouldn't split if the comma is
inside quotes. For example, take a data line like this:
SAR001,"","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 a highly recommended book on regular
expressions, to handle these for us. He suggests
(assuming your string is contained in $text):
@new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a
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quotation-mark-delimited field, escape them with
backslashes (eg, "like \"this\"". Unescaping them is a
task addressed earlier in this section.
Alternatively, the Text::ParseWords module (part of the
standard perl distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
There's also a Text::CSV 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/;
This is unnecessarily slow, destructive, and fails with
embedded newlines. It is much better faster to do this 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 slide:
# trim whitespace in the scalar, the array,
# and all the values in the hash
foreach ($scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+$//;
}
How do I pad a string with blanks or pad a number with
zeroes?
(This answer contributed by Uri Guttman)
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
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character. You can use a single character string constant
instead of the $pad_char variable if you know what it is
in advance.
The simplest method use the sprintf function. It can pad
on the left or right with blanks and on the left with
zeroes.
# Left padding with blank:
$padded = sprintf( "%${pad_len}s", $text ) ;
# Right padding with blank:
$padded = sprintf( "%${pad_len}s", $text ) ;
# Left padding with 0:
$padded = sprintf( "%0${pad_len}d", $num ) ;
If you need to pad with a character other than blank or
zero you can use one of the following methods.
These methods generate a pad string with the x operator
and concatenate that with the original text.
Left and right padding with any character:
$padded = $pad_char x ( $pad_len - length( $text ) ) . $text ;
$padded = $text . $pad_char x ( $pad_len - length( $text ) ) ;
Or you can left or right pad $text directly:
$text .= $pad_char x ( $pad_len - length( $text ) ) ;
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ) ;
How do I extract selected columns from a string?
Use substr() or unpack(), both documented in the perlfunc
manpage. 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);
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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?
Use the standard Text::Soundex module distributed with
perl.
How can I expand variables in text strings?
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
If those were both global variables, then this would
suffice:
$text =~ s/\$(\w+)/${$1}/g; # no /e needed
But since they are probably lexicals, or at least, they
could be, you'd have to do this:
$text =~ s/(\$\w+)/$1/eeg;
die if $@; # needed /ee, not /e
It's probably better in the general case to treat those
variables as entries in some special hash. For example:
%user_defs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
See also ``How do I expand function calls in a string?''
in this section of the FAQ.
What's wrong with always quoting ""$vars""?
The problem is that those double-quotes force
stringification, coercing numbers and references into
strings, even when you don't want them to be. Think of it
this way: double-quote expansion is used to produce new
strings. If you already have a string, why do you need
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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:
1. There must be no space after the << part.
2. There (probably) should be a semicolon at the end.
3. You can't (easily) have any space in front of the tag.
If you want to indent the text in the here document, you
can do this:
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# 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 off. Otherwise, it takes the amount of
leading white space found on the first line and removes
that much off each subsequent line.
sub fix {
local $_ = shift;
my ($white, $leader); # common white space and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}
This works with leading special strings, dynamically
determined:
$remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
@@@ while ( op = (*op->op_ppaddr)() ) ;
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading white space, with
remaining indentation correctly preserved:
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$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 @_,
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
evaluates the left hand side, then evaluates and returns
the left hand side. 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,
which makes 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 -w flag will warn you about these matters.
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How can I extract just the unique elements of an 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)
$prev = 'nonesuch';
@out = grep($_ ne $prev && ($prev = $_), @in);
This is nice in that it doesn't use much extra memory,
simulating uniq(1)'s behavior of removing only
adjacent duplicates. It's less nice in that it won't
work with false values like undef, 0, or ""; "0 but
true" is ok, though.
b) If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$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 = @ary;
But perhaps you should have been using a hash all along,
eh?
How can I tell whether a list or array contains a certain
element?
Hearing the word "in" is an 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.
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That being said, there are several ways to approach this.
If you are going to make this query many times over
arbitrary string values, the fastest way is probably to
invert the original array and keep an associative array
lying about whose keys are the first array's values.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
undef %is_blue;
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might
have been a good idea to keep the blues all in a hash in
the first place.
If the values are all small integers, you could use a
simple indexed array. This kind of an array will take up
less space:
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
undef @is_tiny_prime;
for (@primes) { $is_tiny_prime[$_] = 1; }
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 regexp characters in $whatever?). If
you're only testing once, then use:
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$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;
}
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) = @_;
local $^W = 0; # silence spurious -w undef complaints
return 0 unless @$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";
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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;
$b{EXTRA} = \%a;
printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
The first reports that both those the hashes contain the
same data, while the second reports that they do not.
Which you prefer is left as an exercise to the reader.
How do I find the first array element for which a
condition is true?
You can use this if you care about the index:
for ($i= 0; $i < @array; $i++) {
if ($array[$i] eq "Waldo") {
$found_index = $i;
last;
}
}
Now $found_index has what you want.
How do I handle linked lists?
In general, you usually don't need a linked list in Perl,
since with regular arrays, you can push and pop or shift
and unshift at either end, or you can use splice to add
and/or remove arbitrary number of elements at arbitrary
points. Both pop and shift are both O(1) operations on
perl's dynamic arrays. In the absence of shifts and pops,
push in general needs to reallocate on the order every
log(N) times, and unshift will need to copy pointers each
time.
If you really, really wanted, you could use structures as
described in the perldsc manpage or the perltoot manpage
and do just what the algorithm book tells you to do. For
example, imagine a list node like this:
$node = {
VALUE => 42,
LINK => undef,
};
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PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
You could walk the list this way:
print "List: ";
for ($node = $head; $node; $node = $node->{LINK}) {
print $node->{VALUE}, " ";
}
print "\n";
You could grow the list this way:
my ($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?
Use this:
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PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
# fisher_yates_shuffle( \@array ) :
# generate a random permutation of @array in place
sub fisher_yates_shuffle {
my $array = shift;
my $i;
for ($i = @$array; --$i; ) {
my $j = int rand ($i+1);
next if $i == $j;
@$array[$i,$j] = @$array[$j,$i];
}
}
fisher_yates_shuffle( \@array ); # permutes @array in place
You've probably seen shuffling algorithms that works using
splice, randomly picking another element to swap the
current element with:
srand;
@new = ();
@old = 1 .. 10; # just a demo
while (@old) {
push(@new, splice(@old, rand @old, 1));
}
This is bad because splice is already O(N), and since you
do it N times, you just invented a quadratic algorithm;
that is, O(N**2). This does not scale, although Perl is
so efficient that you probably won't notice this until you
have rather largish arrays.
How do I process/modify each element of an array?
Use for/foreach:
for (@lines) {
s/foo/bar/; # change that word
y/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
for (@volumes = @radii) { # @volumes has changed parts
$_ **= 3;
$_ *= (4/3) * 3.14159; # this will be constant folded
}
If you want to do the same thing to modify the values of
the hash, you may not use the values function, oddly
enough. You need a slice:
for $orbit ( @orbits{keys %orbits} ) {
($orbit **= 3) *= (4/3) * 3.14159;
}
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How do I select a random element from an array?
Use the rand() function (see the rand entry in the
perlfunc manpage):
# at the top of the program:
srand; # not needed for 5.004 and later
# then later on
$index = rand @array;
$element = $array[$index];
Make sure you only call srand once per program, if then.
If you are calling it more than once (such as before each
call to rand), you're almost certainly doing something
wrong.
How do I permute N elements of a list?
Here's a little program that generates all permutations of
all the words on each line of input. The algorithm
embodied in the permute() function should work on any
list:
#!/usr/bin/perl -n
# tsc-permute: permute each word of input
permute([split], []);
sub permute {
my @items = @{ $_[0] };
my @perms = @{ $_[1] };
unless (@items) {
print "@perms\n";
} else {
my(@newitems,@newperms,$i);
foreach $i (0 .. $#items) {
@newitems = @items;
@newperms = @perms;
unshift(@newperms, splice(@newitems, $i, 1));
permute([@newitems], [@newperms]);
}
}
}
How do I sort an array by (anything)?
Supply a comparison function to sort() (described in the
sort entry in the perlfunc manpage):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which
would sort (1, 2, 10) into (1, 10, 2). <=>, used above,
is the numerical comparison operator.
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If you have a complicated function needed to pull out the
part you want to sort on, then don't do it inside the sort
function. Pull it out first, because the sort BLOCK can
be called many times for the same element. Here's an
example of how to pull out the first word after the first
number on each item, and then sort those words case-
insensitively.
@idx = ();
for (@data) {
($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 http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for
more about this approach.
See also the question below on sorting hashes.
How do I manipulate arrays of bits?
Use pack() and unpack(), or else vec() and the bitwise
operations.
For example, this sets $vec to have bit N set if $ints[N]
was set:
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
And here's how, given a vector in $vec, you can get those
bits into your @ints array:
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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.)
Here's a demo on how to use 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);
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PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
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), $bits);
print "bits are: @bytes\n\n";
}
Why does defined() return true on empty arrays and hashes?
The short story is that you should probably only use
defined on scalars or functions, not on aggregates (arrays
and hashes). See the defined entry in the perlfunc
manpage in the 5.004 release or later of Perl for more
detail.
Data: Hashes (Associative Arrays)
How do I process an entire hash?
Use the each() function (see the each entry in the
perlfunc manpage) if you don't care whether it's sorted:
while ( ($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?
Don't do that.
How do I look up a hash element by value?
Create a reverse hash:
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PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
%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.
How can I know how many entries are in a hash?
If you mean how many keys, then all you have to do is take
the scalar sense of the keys() function:
$num_keys = scalar keys %hash;
In void context it just resets the iterator, which is
faster for tied hashes.
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, and if that
fails, by straight ASCII comparison of the keys (well,
possibly modified by your locale -- see the perllocale
manpage).
@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 the section
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PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
on In Memory Databases in the DB_File manpage. The
Tie::IxHash module from CPAN might also be instructive.
What's the difference between ""delete"" and ""undef""
with hashes?
Hashes are pairs of scalars: the first is the key, the
second is the value. The key will be coerced to a string,
although the value can be any kind of scalar: string,
number, or reference. If a key $key is present in the
array, exists($key) will return true. The value for a
given key can be undef, in which case $array{$key} will be
undef while $exists{$key} will return true. This
corresponds to ($key, undef) being in the hash.
Pictures help... here's the %ary table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
And these conditions hold
$ary{'a'} is true
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is true
exists $ary{'a'} is true (perl5 only)
grep ($_ eq 'a', keys %ary) is true
If you now say
undef $ary{'a'}
your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
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$ary{'a'} is FALSE
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is FALSE
exists $ary{'a'} is true (perl5 only)
grep ($_ eq 'a', keys %ary) is true
Notice the last two: you have an undef value, but a
defined key!
Now, consider this:
delete $ary{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$ary{'a'} is false
$ary{'d'} is false
defined $ary{'d'} is true
defined $ary{'a'} is false
exists $ary{'a'} is FALSE (perl5 only)
grep ($_ eq 'a', keys %ary) is FALSE
See, the whole entry is gone!
Why don't my tied hashes make the defined/exists
distinction?
They may or may not implement the EXISTS() and DEFINED()
methods differently. For example, there isn't the concept
of undef with hashes that are tied to DBM* files. This
means the true/false tables above will give different
results when used on such a hash. It also means that
exists and defined do the same thing with a DBM* file, and
what they end up doing is not what they do with ordinary
hashes.
How do I reset an each() operation part-way through?
Using keys %hash in scalar context returns the number of
keys in the hash and resets the iterator associated with
the hash. You may need to do this if you use last to exit
a loop early so that when you re-enter it, the hash
iterator has been reset.
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How can I get the unique keys from two hashes?
First you extract the keys from the hashes into arrays,
and then solve the uniquifying the array problem described
above. For example:
%seen = ();
for $element (keys(%foo), keys(%bar)) {
$seen{$element}++;
}
@uniq = keys %seen;
Or more succinctly:
@uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
%seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
@uniq = keys %seen;
How can I store a multidimensional array in a DBM file?
Either stringify the structure yourself (no fun), or else
get the MLDBM (which uses Data::Dumper) module from CPAN
and layer it on top of either DB_File or GDBM_File.
How can I make my hash remember the order I put elements
into it?
Use the Tie::IxHash from CPAN.
use Tie::IxHash;
tie(%myhash, Tie::IxHash);
for ($i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
@keys = keys %myhash;
# @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a
hash create it?
If you say something like:
somefunc($hash{"nonesuch key here"});
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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:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};
References are documented in the perlref manpage and the
upcoming the perlreftut manpage. Examples of complex data
structures are given in the perldsc manpage and the
perllol manpage. Examples of structures and object-
oriented classes are in the perltoot manpage.
How can I use a reference as a hash key?
You can't do this directly, but you could use the standard
Tie::Refhash module distributed with perl.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so this shouldn't be a problem. For
example, this works fine (assuming the files are found):
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On some legacy systems, however, you have to play tedious
games with "text" versus "binary" files. See the section
on binmode in the perlfunc manpage, or the upcoming the
perlopentut manpage manpage.
If you're concerned about 8-bit ASCII data, then see the
perllocale manpage.
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If you want to deal with multibyte characters, however,
there are some gotchas. See the section on Regular
Expressions.
How do I determine whether a scalar is a
number/whole/integer/float?
Assuming that you don't care about IEEE notations like
"NaN" or "Infinity", you probably just want to use a
regular expression.
if (/\D/) { print "has nondigits\n" }
if (/^\d+$/) { 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" }
if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
{ print "a C float" }
If you're on a POSIX system, Perl's supports the
POSIX::strtod function. Its semantics are somewhat
cumbersome, so here's a getnum wrapper function for more
convenient access. This function takes a string and
returns the number it found, or undef for input that isn't
a C float. The is_numeric function is a front end to
getnum if you just want to say, ``Is this a float?''
sub getnum {
use POSIX qw(strtod);
my $str = shift;
$str =~ s/^\s+//;
$str =~ s/\s+$//;
$! = 0;
my($num, $unparsed) = strtod($str);
if (($str eq '') || ($unparsed != 0) || $!) {
return undef;
} else {
return $num;
}
}
sub is_numeric { defined &getnum }
Or you could check out String::Scanf which can be found at
http://www.perl.com/CPAN/modules/by-module/String/. The
POSIX module (part of the standard Perl distribution)
provides the strtol and strtod 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 the AnyDBM_File manpage. More generically,
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you should consult the FreezeThaw, Storable, or
Class::Eroot modules from CPAN. Here's one example using
Storable's store and retrieve functions:
use Storable;
store(\%hash, "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, found on CPAN, provides a function called
dclone that recursively copies its argument.
use Storable qw(dclone);
$r2 = dclone($r1);
Where $r1 can be a reference to any kind of data structure
you'd like. It will be deeply copied. Because dclone
takes and returns references, you'd have to add extra
punctuation if you had a hash of arrays that you wanted to
copy.
%newhash = %{ dclone(\%oldhash) };
How do I define methods for every class/object?
Use the UNIVERSAL class (see the UNIVERSAL manpage).
How do I verify a credit card checksum?
Get the Business::CreditCard module from CPAN.
How do I pack arrays of doubles or floats for XS code?
The kgbpack.c code in the PGPLOT module on CPAN does just
this. If you're doing a lot of float or double
processing, consider using the PDL module from CPAN
instead--it makes number-crunching easy.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan
Torkington. All rights reserved.
When included as part of the Standard Version of Perl, or
as part of its complete documentation whether printed or
otherwise, this work may be distributed only under the
terms of Perl's Artistic Licence. Any distribution of
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this file or derivatives thereof outside of that package
require that special arrangements be made with copyright
holder.
Irrespective of its distribution, all code examples in
this file are hereby placed into the public domain. You
are permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but
is not required.
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