perlfaq4 - Data Manipulation ($Revision: 1.7 $, $Date:
2004/04/07 21:33:08 $)
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), "0;
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
$floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the
Math::Complex module. With 5.004, the Math::Trig module
(part of the standard Perl distribution) implements the
trigonometric functions. Internally it uses the Math::Complex
module and some functions can break out from the real
axis into the complex plane, for example the inverse sine
of 2.
Rounding in financial applications can have serious implications,
and the rounding method used should be specified
precisely. In these cases, it probably pays not to trust
whichever system rounding is being used by Perl, but to
instead implement the rounding function you need yourself.
To see why, notice how you'll still have an issue on halfway-point
alternation:
for ($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 representa- [Toc] [Back]
tions/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 = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin ->
hex, etc.) are left as an exercise to the inclined
reader.
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on
whether they're used on numbers or strings. The operators
treat a string as a series of bits and work with that (the
string "3" is the bit pattern 00110011). The operators
work with the binary form of a number (the number 3 is
treated as the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers
(yielding 3). Saying "11" & "3" performs the "and"
operation on strings (yielding "1").
Most problems with "&" and "|" arise because the programmer
thinks they have a number but really it's a string.
The rest arise because the programmer says:
if (" 20 20" & "101101") {
# ...
}
but a string consisting of two null bytes (the result of
"" 20 20" & "101101"") is not a false value in Perl.
You need:
if ( (" 20 20" & "101101") !~ /[^ 00]/) {
# ...
}
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules (available
from CPAN) or the PDL extension (also available from
CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect
the results, use:
@results = map { my_func($_) } @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 week. 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?
Use the Time::JulianDay module (part of the Time-modules
bundle available from CPAN.)
Before you immerse yourself too deeply in this, be sure to
verify that it is the Julian Day you really want. Are you
interested in a way of getting serial days so that you
just can tell how many days they are apart or so that you
can do also other date arithmetic? If you are interested
in performing date arithmetic, this can be done using modules
Date::Manip or Date::Calc.
There is too many details and much confusion on this issue
to cover in this FAQ, but the term is applied (correctly)
to a calendar now supplanted by the Gregorian Calendar,
with the Julian Calendar failing to adjust properly for
leap years on centennial years (among other annoyances).
The term is also used (incorrectly) to mean: [1] days in
the Gregorian Calendar; and [2] days since a particular
starting time or `epoch', usually 1970 in the Unix world
and 1980 in the MS-DOS/Windows world. If you find that it
is not the first meaning that you really want, then check
out the Date::Manip and Date::Calc modules. (Thanks to
David Cassell for most of this text.)
How do I find yesterday's date?
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 "@date0;
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 $tdst are necessary
because localtime only says it returns the system tm
struct, and the system tm struct at least on Solaris
doesn't guarantee any particular positive value (like,
say, 1) for isdst, just a positive value. And that value
can potentially be negative, if DST information isn't
available (this sub just treats those cases like no DST).
Note that between 2am and 3am on the day after the time
zone switches off daylight savings time, the exact hour of
"yesterday" corresponding to the current hour is not
clearly defined. Note also that if used between 2am and
3am the day after the change to daylight savings time, the
result will be between 3am and 4am of the previous day;
it's arguable whether this is correct.
This sub does not attempt to deal with leap seconds (most
things don't).
Does Perl have a Year 2000 problem? Is Perl Y2K compli-
ant?
Short answer: No, Perl does not have a Year 2000 problem.
Yes, Perl is Y2K compliant (whatever that means). The
programmers you've hired to use it, however, probably are
not.
Long answer: The question belies a true understanding of
the issue. Perl is just as Y2K compliant as your pencil--no
more, and no less. Can you use your pencil to
write a non-Y2K-compliant memo? Of course you can. Is
that the pencil's fault? Of course it isn't.
The date and time functions supplied with Perl (gmtime and
localtime) supply adequate information to determine the
year well beyond 2000 (2038 is when trouble strikes for
32-bit machines). The year returned by these functions
when used in a list context is the year minus 1900. For
years between 1910 and 1999 this happens to be a 2-digit
decimal number. To avoid the year 2000 problem simply do
not treat the year as a 2-digit number. It isn't.
When gmtime() and localtime() are used in scalar context
they return a timestamp string that contains a fullyexpanded
year. For example, "$timestamp =
gmtime(1005613200)" sets $timestamp to "Tue Nov 13
01:00:00 2001". There's no year 2000 problem here.
That doesn't mean that Perl can't be used to create
non-Y2K compliant programs. It can. But so can your pencil.
It's the fault of the user, not the language. At
the risk of inflaming the NRA: ``Perl doesn't break Y2K,
people do.'' See http://language.perl.com/news/y2k.html
for a longer exposition.
Data: Strings
How do I validate input?
The answer to this question is usually a regular expression,
perhaps with auxiliary logic. See the more specific
questions (numbers, mail addresses, etc.) for details.
How do I unescape a string?
It depends just what you mean by ``escape''. URL escapes
are dealt with in perlfaq9. Shell escapes with the backslash
("
s/\(.)/$1/g;
This won't expand "0 or "" 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.0;
See also ``How can I expand variables in text strings?''
in this section of the FAQ.
How do I find matching/nesting anything?
This isn't something that can be done in one regular
expression, no matter how complicated. To find something
between two single characters, a pattern like
"/x([^x]*)x/" will get the intervening bits in $1. For
multiple ones, then something more like
"/alpha(.*?)omega/" would be needed. But none of these
deals with nested patterns. 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$2]/gs;
@$ = (eval{/$re/},$@!~/unmatched/i);
print join("0,@$[0..$#$]) 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 $string =~ s/+/' ' x (length($&) * 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("", ' ', @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 sub-
str(). 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;
$_ = "One fish two fish red fish blue fish";
while (/(96
if (++$count == $WANT) {
print "The third fish is a $1 one.0;
}
}
That prints out: "The third fish is a red one." You can
also use a repetition count and repeated pattern like
this:
/(?:216
How can I count the number of occurrences of a substring [Toc] [Back]
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 =~ /-+/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 =~ /-+/g;
How do I capitalize all the words on one line?
To make the first letter of each word upper case:
1
$line =~ s/6
8
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/ (
(^792
| # or
(24
)
/U$1/xg;
$string =~ /([120
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/(72
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, "0;
for my $style (qw( sentence title highlight ))
{
print autoformat($x, { case => $style }), "0;
}
How can I split a [character] delimited string except when
inside [character]?
Several modules can handle this sort of pasing---Text::Balanced,
Text::CVS, Text::CVS_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 $text =~ m{
"([^
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a quotation-mark-delimited
field, escape them with backslashes
(eg, "like
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
$string =~ s/^(.*?)$/$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/^/;
$string =~ s/;
Or more nicely written as:
for ($string) {
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/;
}
How do I pad a string with blanks or pad a number with [Toc] [Back]
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?
Use the standard Text::Soundex module distributed with
Perl. Before you do so, you may want to determine whether
`soundex' is in fact what you think it is. Knuth's
soundex algorithm compresses words into a small space, and
so it does not necessarily distinguish between two words
which you might want to appear separately. For example,
the last names `Knuth' and `Kant' are both mapped to the
soundex code K530. If Text::Soundex does not do what you
are looking for, you might want to consider the
String::Approx module available at CPAN.
How can I expand variables in text strings?
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
If those were both global variables, then this would suffice:
$text =~ s/+)/${$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,
);
$text =~ s/+)/$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 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
($VAR = <<HERE_TARGET) =~ s/^/gm;
your text
goes here
HERE_TARGET
But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote
in the indentation.
($quote = <<' FINIS') =~ s/^/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/-/0/;
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, $leader); # common whitespace and
common leading string
if (/^(?:([^96?:12?.*0+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(/, '');
}
s/^?$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 [Toc] [Back]
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 lapislazuli/;
%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 arrays0,
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 hashes0,
cmpStr(a, b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes0,
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 condi- [Toc] [Back]
tion 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 $node->{VALUE}, " ";
}
print "0;
You could add to the list this way:
my ($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)0;
}
For even faster execution, you could do:
use Algorithm::Permute;
my @array = 'a'..'d';
Algorithm::Permute::permute {
print "next permutation: (@array)0;
} @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 Pro-
gramming 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"@_0} 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 compar
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