PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)
NAME
perlfaq4 - Data Manipulation ($Revision: 1.73 $, $Date: 2005/12/31
00:54:37 $)
DESCRIPTION
This section of the FAQ answers questions related to manipulating num
bers, 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 lan
guages, not just Perl.
perlnumber show the gory details of number representations and conver
sions.
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. Its the numbers that
arent quite what you think.
First, see the above item "Why am I getting long decimals (eg,
19.9499999999999) instead of the numbers I should be getting (eg,
19.95)?".
For example, this
print int(0.6/0.2-2), "\n";
will in most computers print 0, not 1, because even such simple numbers
as 0.6 and 0.2 cannot be presented exactly by floating-point numbers.
What you think in the above as three is really more like
2.9999999999999995559.
Why isnt 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 conver
sion 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 cer
tain 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
$floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
module. With 5.004, the Math::Trig module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the Math::Complex module and some functions can break out from the
real axis into the complex plane, for example the inverse sine of 2.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these
cases, it probably pays not to trust whichever system rounding is being
used by Perl, but to instead implement the rounding function you need
yourself.
To see why, notice how youll still have an issue on half-way-point
alternation:
for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
0.8 0.8 0.9 0.9 1.0 1.0
Dont blame Perl. Its the same as in C. IEEE says we have to do
this. Perl numbers whose absolute values are integers under 2**31 (on
32 bit machines) will work pretty much like mathematical integers.
Other numbers are not guaranteed.
How do I convert between numeric representations/bases/radixes?
As always with Perl there is more than one way to do it. Below are a
few examples of approaches to making common conversions between number
representations. This is intended to be representational rather than
exhaustive.
Some of the examples below use the Bit::Vector module from CPAN. The
reason you might choose Bit::Vector over the perl built in functions is
that it works with numbers of ANY size, that it is optimized for speed
on some operations, and for at least some programmers the notation
might be familiar.
How do I convert hexadecimal into decimal
Using perls 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 Perls 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 nota
tion:
$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 = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
are left as an exercise to the inclined reader.
Why doesnt & work the way I want it to?
The behavior of binary arithmetic operators depends on whether theyre
used on numbers or strings. The operators treat a string as a series
of bits and work with that (the string "3" is the bit pattern
00110011). The operators work with the binary form of a number (the
number 3 is treated as the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers (yielding
3). Saying "11" & "3" performs the "and" operation on strings (yield
ing "1").
Most problems with "&" and "|" arise because the programmer thinks they
have a number but really its a string. The rest arise because the
programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of ""\020\020" &
"\101\101"") is not a false value in Perl. You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
or the PDL extension (also available from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect the
results, use:
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the results:
foreach $iterator (@array) {
some_func($iterator);
}
To call a function on each integer in a (small) range, you can use:
@results = map { some_func($_) } (5 .. 25);
but you should be aware that the ".." operator creates an array of all
integers in the range. This can take a lot of memory for large ranges.
Instead use:
@results = ();
for ($i=5; $i < 500_005; $i++) {
push(@results, some_func($i));
}
This situation has been fixed in Perl5.005. Use of ".." in a "for" loop
will iterate over the range, without creating the entire range.
for my $i (5 .. 500_005) {
push(@results, some_func($i));
}
will not create a list of 500,000 integers.
How can I output Roman numerals?
Get the http://www.cpan.org/modules/by-module/Roman module.
Why arent my random numbers random?
If youre 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 $] < 5.004 }
5.004 and later automatically call "srand" at the beginning. Dont
call "srand" more than once---you make your numbers less random, rather
than more.
Computers are good at being predictable and bad at being random
(despite appearances caused by bugs in your programs :-). see the ran
dom article in the "Far More Than You Ever Wanted To Know" collection
in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of Tom
Phoenix, talks more about this. John von Neumann said, "Anyone who
attempts to generate random numbers by deterministic means is, of
course, living in a state of sin."
If you want numbers that are more random than "rand" with "srand" pro
vides, you should also check out the Math::TrulyRandom module from
CPAN. It uses the imperfections in your systems timer to generate
random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
"Numerical Recipes in C" at http://www.nr.com/ .
How do I get a random number between X and Y?
"rand($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 ran
dom number between 0 and 5 that you can then add to 10.
my $number = 10 + int rand( 15-10+1 );
Hence you derive the following simple function to abstract that. It
selects a random integer between the two given integers (inclusive),
For example: "random_int_in(50,120)".
sub random_int_in ($$) {
my($min, $max) = @_;
# Assumes that the two arguments are integers themselves!
return $min if $min == $max;
($min, $max) = ($max, $min) if $min > $max;
return $min + int rand(1 + $max - $min);
}
Data: Dates
How do I find the day or week of the year?
The localtime function returns the day of the year. Without an argu
ment 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 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 modules strftime() function has been
extended in a non-standard way to use a %C format, which they sometimes
claim is the "century". It isnt, because on most such systems, this
is only the first two digits of the four-digit year, and thus cannot be
used to reliably determine the current century or millennium.
How can I compare two dates and find the difference?
(contributed by brian d foy)
You could just store all your dates as a number and then subtract. Life
isnt always that simple though. If you want to work with formatted
dates, the Date::Manip, Date::Calc, or DateTime modules can help you.
How can I take a string and turn it into epoch seconds?
If its a regular enough string that it always has the same format, you
can split it up and pass the parts to "timelocal" in the standard
Time::Local module. Otherwise, you should look into the Date::Calc and
Date::Manip modules from CPAN.
How can I find the Julian Day?
(contributed by brian d foy and Dave Cross)
You can use the Time::JulianDay module available on CPAN. Ensure that
you really want to find a Julian day, though, as many people have dif
ferent ideas about Julian days. See http://www.her
metic.ch/cal_stud/jdn.htm for instance.
You can also try the DateTime module, which can convert a date/time to
a Julian Day.
$ perl -MDateTime -leprint DateTime->today->jd
2453401.5
Or the modified Julian Day
$ perl -MDateTime -leprint DateTime->today->mjd
53401
Or even the day of the year (which is what some people think of as a
Julian day)
$ perl -MDateTime -leprint DateTime->today->doy
31
How do I find yesterdays date?
(contributed by brian d foy)
Use one of the Date modules. The "DateTime" module makes it simple, and
give you the same time of day, only the day before.
use DateTime;
my $yesterday = DateTime->now->subtract( days => 1 );
print "Yesterday was $yesterday\n";
You can also use the "Date::Calc" module using its Today_and_Now func
tion.
use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
print "@date\n";
Most people try to use the time rather than the calendar to figure out
dates, but that assumes that days are twenty-four hours each. For most
people, there are two days a year when they arent: the switch to and
from summer time throws this off. Let the modules do the work.
Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
Y2K compliant (whatever that means). The programmers youve hired to
use it, however, probably are not.
Long answer: The question belies a true understanding of the issue.
Perl is just as Y2K compliant as your pencil--no more, and no less.
Can you use your pencil to write a non-Y2K-compliant memo? Of course
you can. Is that the pencils fault? Of course it isnt.
The date and time functions supplied with Perl (gmtime and localtime)
supply adequate information to determine the year well beyond 2000
(2038 is when trouble strikes for 32-bit machines). The year returned
by these functions when used in a list context is the year minus 1900.
For years between 1910 and 1999 this happens to be a 2-digit decimal
number. To avoid the year 2000 problem simply do not treat the year as
a 2-digit number. It isnt.
When gmtime() and localtime() are used in scalar context they return a
timestamp string that contains a fully-expanded year. For example,
"$timestamp = gmtime(1005613200)" sets $timestamp to "Tue Nov 13
01:00:00 2001". Theres no year 2000 problem here.
That doesnt mean that Perl cant be used to create non-Y2K compliant
programs. It can. But so can your pencil. Its the fault of the
user, not the language. At the risk of inflaming the NRA: "Perl
doesnt break Y2K, people do." See http://www.perl.org/about/y2k.html
for a longer exposition.
Data: Strings
How do I validate input?
(contributed by brian d foy)
There are many ways to ensure that values are what you expect or want
to accept. Besides the specific examples that we cover in the perlfaq,
you can also look at the modules with "Assert" and "Validate" in their
names, along with other modules such as "Regexp::Common".
Some modules have validation for particular types of input, such as
"Business::ISBN", "Business::CreditCard", "Email::Valid", and
"Data::Validate::IP".
How do I unescape a string?
It depends just what you mean by "escape". URL escapes are dealt with
in perlfaq9. Shell escapes with the backslash ("\") character are
removed with
s/\\(.)/$1/g;
This wont expand "\n" or "\t" or any other special escapes.
How do I remove consecutive pairs of characters?
(contributed by brian d foy)
You can use the substitution operator to find pairs of characters (or
runs of characters) and replace them with a single instance. In this
substitution, we find a character in "(.)". The memory parentheses
store the matched character in the back-reference "\1" and we use that
to require that the same thing immediately follow it. We replace that
part of the string with the character in $1.
s/(.)\1/$1/g;
We can also use the transliteration operator, "tr///". In this example,
the search list side of our "tr///" contains nothing, but the "c"
option complements that so it contains everything. The replacement list
also contains nothing, so the transliteration is almost a no-op since
it wont do any replacements (or more exactly, replace the character
with itself). However, the "s" option squashes duplicated and consecu
tive characters in the string so a character does not show up next to
itself
my $str = Haarlem; # in the Netherlands
$str =~ tr///cs; # Now Harlem, like in New York
How do I expand function calls in a string?
(contributed by brian d foy)
This is documented in perlref, and although its not the easiest thing
to read, it does work. In each of these examples, we call the function
inside the braces used to dereference a reference. If we have a more
than one return value, we can construct and dereference an anonymous
array. In this case, we call the function in list context.
print "The time values are @{ [localtime] }.\n";
If we want to call the function in scalar context, we have to do a bit
more work. We can really have any code we like inside the braces, so we
simply have to end with the scalar reference, although how you do that
is up to you, and you can use code inside the braces.
print "The time is ${\(scalar localtime)}.\n"
print "The time is ${ my $x = localtime; \$x }.\n";
If your function already returns a reference, you dont need to create
the reference yourself.
sub timestamp { my $t = localtime; \$t }
print "The time is ${ timestamp() }.\n";
The "Interpolation" module can also do a lot of magic for you. You can
specify a variable name, in this case "E", to set up a tied hash that
does the interpolation for you. It has several other methods to do this
as well.
use Interpolation E => eval;
print "The time values are $E{localtime()}.\n";
In most cases, it is probably easier to simply use string concatena
tion, which also forces scalar context.
print "The time is " . localtime . ".\n";
How do I find matching/nesting anything?
This isnt something that can be done in one regular expression, no
matter how complicated. To find something between two single charac
ters, 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 mod
ule Regexp::Common, or see "(??{ code })" in perlre. For other cases,
youll have to write a parser.
If you are serious about writing a parser, there are a number of mod
ules 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 Perls regular
expression engine do it for you. This is courtesy Dean Inada, and
rather has the nature of an Obfuscated Perl Contest entry, but it
really does work:
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
@( = ((,);
@) = (),);
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
@$ = (eval{/$re/},$@!~/unmatched/i);
print join("\n",@$[0..$#$]) if( $$[-1] );
How do I reverse a string?
Use reverse() in scalar context, as documented in "reverse" in perl
func.
$reversed = reverse $string;
How do I expand tabs in a string?
You can do it yourself:
1 while $string =~ s/\t+/ x (length($&) * 8 - length($) % 8)/e;
Or you can just use the Text::Tabs module (part of the standard Perl
distribution).
use Text::Tabs;
@expanded_lines = expand(@lines_with_tabs);
How do I reformat a paragraph?
Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", , @paragraphs);
The paragraphs you give to Text::Wrap should not contain embedded new
lines. Text::Wrap doesnt justify the lines (flush-right).
Or use the CPAN module Text::Autoformat. Formatting files can be eas
ily 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 capa
bilities.
How can I access or change N characters of a string?
You can access the first characters of a string with substr(). To get
the first character, for example, start at position 0 and grab the
string of length 1.
$string = "Just another Perl Hacker";
$first_char = substr( $string, 0, 1 ); # J
To change part of a string, you can use the optional fourth argument
which is the replacement string.
substr( $string, 13, 4, "Perl 5.8.0" );
You can also use substr() as an lvalue.
substr( $string, 13, 4 ) = "Perl 5.8.0";
How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, lets say you want
to change the fifth occurrence of "whoever" or "whomever" into "whoso
ever" or "whomsoever", case insensitively. These all assume that $_
contains the string to be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;
In the more general case, you can use the "/g" modifier in a "while"
loop, keeping count of matches.
$WANT = 3;
$count = 0;
$_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
}
}
That prints out: "The third fish is a red one." You can also use a
repetition count and repeated pattern like this:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
How can I count the number of occurrences of a substring within a
string?
There are a number of ways, with varying efficiency. If you want a
count of a certain single character (X) within a string, you can use
the "tr///" function like so:
$string = "ThisXlineXhasXsomeXxsXinXit";
$count = ($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///" wont work. What you can do is wrap a while()
loop around a global pattern match. For example, lets count negative
integers:
$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
Another version uses a global match in list context, then assigns the
result to a scalar, producing a count of the number of matches.
$count = () = $string =~ /-\d+/g;
How do I capitalize all the words on one line?
To make the first letter of each word upper case:
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning ""dont do it"" into ""DonT Do
It"". Sometimes you might want this. Other times you might need a
more thorough solution (Suggested by brian d foy):
$string =~ s/ (
(^\w) #at the beginning of the line
| # or
(\s\w) #preceded by whitespace
)
/\U$1/xg;
$string =~ /([\w]+)/\u\L$1/g;
To make the whole line upper case:
$line = uc($line);
To force each word to be lower case, with the first letter upper case:
$line =~ s/(\w+)/\u\L$1/g;
You can (and probably should) enable locale awareness of those charac
ters 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 thats 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 Conways Text::Autoformat module provides some smart case trans
formations:
use Text::Autoformat;
my $x = "Dr. Strangelove or: How I Learned to Stop ".
"Worrying and Love the Bomb";
print $x, "\n";
for my $style (qw( sentence title highlight ))
{
print autoformat($x, { case => $style }), "\n";
}
How can I split a [character] delimited string except when inside
[character]?
Several modules can handle this sort of pasing---Text::Balanced,
Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
Take the example case of trying to split a string that is comma-sepa
rated into its different fields. You cant use "split(/,/)" because you
shouldnt 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 Expres
sions, to handle these for us. He suggests (assuming your string is
contained in $text):
@new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ,;
If you want to represent quotation marks inside a quotation-mark-delim
ited field, escape them with backslashes (eg, "like \"this\"".
Alternatively, the Text::ParseWords module (part of the standard Perl
distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
Theres also a Text::CSV (Comma-Separated Values) module on CPAN.
How do I strip blank space from the beginning/end of a string?
(contributed by brian d foy)
A substitution can do this for you. For a single line, you want to
replace all the leading or trailing whitespace with nothing. You can do
that with a pair of substitutions.
s/^\s+//;
s/\s+$//;
You can also write that as a single substitution, although it turns out
the combined statement is slower than the separate ones. That might not
matter to you, though.
s/^\s+|\s+$//g;
In this regular expression, the alternation matches either at the
beginning or the end of the string since the anchors have a lower
precedence than the alternation. With the "/g" flag, the substitution
makes all possible matches, so it gets both. Remember, the trailing
newline matches the "\s+", and the "$" anchor can match to the physi
cal end of the string, so the newline disappears too. Just add the new
line to the output, which has the added benefit of preserving "blank"
(consisting entirely of whitespace) lines which the "^\s+" would remove
all by itself.
while( <> )
{
s/^\s+|\s+$//g;
print "$_\n";
}
For a multi-line string, you can apply the regular expression to each
logical line in the string by adding the "/m" flag (for "multi-line").
With the "/m" flag, the "$" matches before an embedded newline, so it
doesnt remove it. It still removes the newline at the end of the
string.
$string =~ s/^\s+|\s+$//gm;
Remember that lines consisting entirely of whitespace will disappear,
since the first part of the alternation can match the entire string and
replace it with nothing. If need to keep embedded blank lines, you have
to do a little more work. Instead of matching any whitespace (since
that includes a newline), just match the other whitespace.
$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
How do I pad a string with blanks or pad a number with zeroes?
In the following examples, $pad_len is the length to which you wish to
pad the string, $text or $num contains the string to be padded, and
$pad_char contains the padding character. You can use a single charac
ter 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 trun
cate the result. The "pack" function can only pad strings on the right
with blanks and it will truncate the result to a maximum length of
$pad_len.
# Left padding a string with blanks (no truncation):
$padded = sprintf("%${pad_len}s", $text);
$padded = sprintf("%*s", $pad_len, $text); # same thing
# Right padding a string with blanks (no truncation):
$padded = sprintf("%-${pad_len}s", $text);
$padded = sprintf("%-*s", $pad_len, $text); # same thing
# Left padding a number with 0 (no truncation):
$padded = sprintf("%0${pad_len}d", $num);
$padded = sprintf("%0*d", $pad_len, $num); # same thing
# Right padding a string with blanks using pack (will truncate):
$padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you can
use one of the following methods. They all generate a pad string with
the "x" operator and combine that with $text. These methods do not
truncate $text.
Left and right padding with any character, creating a new string:
$padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
$padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying $text directly:
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
$text .= $pad_char x ( $pad_len - length( $text ) );
How do I extract selected columns from a string?
Use substr() or unpack(), both documented in perlfunc. If you prefer
thinking in terms of columns instead of widths, you can use this kind
of thing:
# determine the unpack format needed to split Linux ps output
# arguments are cut columns
my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
sub cut2fmt {
my(@positions) = @_;
my $template = ;
my $lastpos = 1;
for my $place (@positions) {
$template .= "A" . ($place - $lastpos) . " ";
$lastpos = $place;
}
$template .= "A*";
return $template;
}
How do I find the soundex value of a string?
(contributed by brian d foy)
You can use the Text::Soundex module. If you want to do fuzzy or close
matching, you might also try the String::Approx, and Text::Metaphone,
and Text::DoubleMetaphone modules.
How can I expand variables in text strings?
Lets assume that you have a string that contains placeholder vari
ables.
$text = this has a $foo in it and a $bar;
You can use a substitution with a double evaluation. The first /e
turns $1 into $foo, and the second /e turns $foo into its value. You
may want to wrap this in an "eval": if you try to get the value of an
undeclared variable while running under "use strict", you get a fatal
error.
eval { $text =~ s/(\$\w+)/$1/eeg };
die if $@;
Its 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{$1}/g;
Whats wrong with always quoting "$vars"?
The problem is that those double-quotes force stringification-- coerc
ing numbers and references into strings--even when you dont 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
Youll be in trouble. Those should (in 99.8% of the cases) be the sim
pler and more direct:
print $var;
$new = $old;
somefunc($var);
Otherwise, besides slowing you down, youre 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 num
ber, 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 dont my <op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading whitespace, with remaining indenta
tion correctly preserved:
$poem = fix< 1 ? \@intersection : \@difference }, $element;
}
Note that this is the symmetric difference, that is, all elements in
either A or in B but not in both. Think of it as an xor operation.
How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a stringwise
comparison, and does not distinguish defined versus undefined empty
strings. Modify if you have other needs.
$are_equal = compare_arrays(\@frogs, \@toads);
sub compare_arrays {
my ($first, $second) = @_;
no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
}
return 1;
}
For multilevel structures, you may wish to use an approach more like
this one. It uses the CPAN module FreezeThaw:
use FreezeThaw qw(cmpStr);
@a = @b = ( "this", "that", [ "more", "stuff" ] );
printf "a and b contain %s arrays\n",
cmpStr(\@a, \@b) == 0
? "the same"
: "different";
This approach also works for comparing hashes. Here well demonstrate
two different answers:
use FreezeThaw qw(cmpStr cmpStrHard);
%a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
$a{EXTRA} = \%b;
$b{EXTRA} = \%a;
printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
The first reports that both those the hashes contain the same data,
while the second reports that they do not. Which you prefer is left as
an exercise to the reader.
How do I find the first array element for which a condition is true?
To find the first array element which satisfies a condition, you can
use the first() function in the List::Util module, which comes with
Perl 5.8. This example finds the first element that contains "Perl".
use List::Util qw(first);
my $element = first { /Perl/ } @array;
If you cannot use List::Util, you can make your own loop to do the same
thing. Once you find the element, you stop the loop with last.
my $found;
foreach ( @array )
{
if( /Perl/ ) { $found = $_; last }
}
If you want the array index, you can iterate through the indices and
check the array element at each index until you find one that satisfies
the condition.
my( $found, $index ) = ( undef, -1 );
for( $i = 0; $i < @array; $i++ )
{
if( $array[$i] =~ /Perl/ )
{
$found = $array[$i];
$index = $i;
last;
}
}
How do I handle linked lists?
In general, you usually dont 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) opera
tions on Perls dynamic arrays. In the absence of shifts and pops,
push in general needs to reallocate on the order every log(N) times,
and unshift will need to copy pointers each time.
If you really, really wanted, you could use structures as described in
perldsc or perltoot and do just what the algorithm book tells you to
do. For example, imagine a list node like this:
$node = {
VALUE => 42,
LINK => undef,
};
You could walk the list this way:
print "List: ";
for ($node = $head; $node; $node = $node->{LINK}) {
print $node->{VALUE}, " ";
}
print "\n";
You could add to the list this way:
my ($head, $tail);
$tail = append($head, 1); # grow a new head
for $value ( 2 .. 10 ) {
$tail = append($tail, $value);
}
sub append {
my($list, $value) = @_;
my $node = { VALUE => $value };
if ($list) {
$node->{LINK} = $list->{LINK};
$list->{LINK} = $node;
} else {
$_[0] = $node; # replace callers version
}
return $node;
}
But again, Perls 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 =
