PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1)NAMEperlfaq4 - Data Manipulation
DESCRIPTION
This section of the FAQ answers questions related to manipu-
lating numbers, dates, strings, arrays, hashes, and miscel-
laneous data issues.
Data: Numbers
Why am I getting long decimals (eg, 19.9499999999999)
instead of the numbers I should be getting (eg, 19.95)?
Internally, your computer represents floating-point numbers
in binary. Digital (as in powers of two) computers cannot
store all numbers exactly. Some real numbers lose precision
in the process. This is a problem with how computers store
numbers and affects all computer languages, not just Perl.
perlnumber show the gory details of number representations
and conversions.
To limit the number of decimal places in your numbers, you
can use the printf or sprintf function. See the "Floating
Point Arithmetic" for more details.
printf "%.2f", 10/3;
my $number = sprintf "%.2f", 10/3;
Why is int() broken?
Your int() is most probably working just fine. It's the
numbers that aren't quite what you think.
First, see the above item "Why am I getting long decimals
(eg, 19.9499999999999) instead of the numbers I should be
getting (eg, 19.95)?".
For example, this
print int(0.6/0.2-2), "\n";
will in most computers print 0, not 1, because even such
simple numbers as 0.6 and 0.2 cannot be presented exactly by
floating-point numbers. What you think in the above as
'three' is really more like 2.9999999999999995559.
Why isn't my octal data interpreted correctly?
Perl only understands octal and hex numbers as such when
they occur as literals in your program. Octal literals in
perl must start with a leading "0" and hexadecimal literals
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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 con-
stants, 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 usu-
ally 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 mathemati-
cal and trigonometric functions.
use POSIX;
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the
Math::Complex module. With 5.004, the Math::Trig module
(part of the standard Perl distribution) implements the tri-
gonometric functions. Internally it uses the Math::Complex
module and some functions can break out from the real axis
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into the complex plane, for example the inverse sine of 2.
Rounding in financial applications can have serious implica-
tions, and the rounding method used should be specified pre-
cisely. In these cases, it probably pays not to trust
whichever system rounding is being used by Perl, but to
instead implement the rounding function you need yourself.
To see why, notice how you'll still have an issue on half-
way-point alternation:
for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
0.8 0.8 0.9 0.9 1.0 1.0
Don't blame Perl. It's the same as in C. IEEE says we have
to do this. Perl numbers whose absolute values are integers
under 2**31 (on 32 bit machines) will work pretty much like
mathematical integers. Other numbers are not guaranteed.
How do I convert between numeric
representations/bases/radixes?
As always with Perl there is more than one way to do it.
Below are a few examples of approaches to making common
conversions between number representations. This is
intended to be representational rather than exhaustive.
Some of the examples below use the Bit::Vector module from
CPAN. The reason you might choose Bit::Vector over the perl
built in functions is that it works with numbers of ANY
size, that it is optimized for speed on some operations, and
for at least some programmers the notation might be fami-
liar.
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:
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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:
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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 = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin ->
hex, etc.) are left as an exercise to the inclined
reader.
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Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on
whether they're used on numbers or strings. The operators
treat a string as a series of bits and work with that (the
string "3" is the bit pattern 00110011). The operators work
with the binary form of a number (the number 3 is treated as
the bit pattern 00000011).
So, saying "11 & 3" performs the "and" operation on numbers
(yielding 3). Saying "11" & "3" performs the "and" opera-
tion on strings (yielding "1").
Most problems with "&" and "|" arise because the programmer
thinks they have a number but really it's a string. The
rest arise because the programmer says:
if ("\020\020" & "\101\101") {
# ...
}
but a string consisting of two null bytes (the result of
""\020\020" & "\101\101"") is not a false value in Perl.
You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules (available
from CPAN) or the PDL extension (also available from CPAN).
How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect
the results, use:
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore
the results:
foreach $iterator (@array) {
some_func($iterator);
}
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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 $] < 5.004 }
5.004 and later automatically call "srand" at the beginning.
Don't call "srand" more than once---you make your numbers
less random, rather than more.
Computers are good at being predictable and bad at being
random (despite appearances caused by bugs in your programs
:-). see the random article in the "Far More Than You Ever
Wanted To Know" collection in
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
Tom Phoenix, talks more about this. John von Neumann said,
"Anyone who attempts to generate random numbers by deter-
ministic means is, of course, living in a state of sin."
If you want numbers that are more random than "rand" with
"srand" provides, you should also check out the
Math::TrulyRandom module from CPAN. It uses the
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imperfections in your system's timer to generate random
numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating sys-
tem, look at "Numerical Recipes in C" at http://www.nr.com/
.
How do I get a random number between X and Y?
"rand($x)" returns a number such that "0 <= rand($x) < $x".
Thus what you want to have perl figure out is a random
number in the range from 0 to the difference between your X
and Y.
That is, to get a number between 10 and 15, inclusive, you
want a random number between 0 and 5 that you can then add
to 10.
my $number = 10 + int rand( 15-10+1 );
Hence you derive the following simple function to abstract
that. It selects a random integer between the two given
integers (inclusive), For example: "random_int_in(50,120)".
sub random_int_in ($$) {
my($min, $max) = @_;
# Assumes that the two arguments are integers themselves!
return $min if $min == $max;
($min, $max) = ($max, $min) if $min > $max;
return $min + int rand(1 + $max - $min);
}
Data: Dates
How do I find the day or week of the year?
The localtime function returns the day of the year. Without
an argument localtime uses the current time.
$day_of_year = (localtime)[7];
The POSIX module can also format a date as the day of the
year or week of the year.
use POSIX qw/strftime/;
my $day_of_year = strftime "%j", localtime;
my $week_of_year = strftime "%W", localtime;
To get the day of year for any date, use the Time::Local
module to get a time in epoch seconds for the argument to
localtime.
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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 module's strftime() function has
been extended in a non-standard way to use a %C format,
which they sometimes claim is the "century". It isn't,
because on most such systems, this is only the first two
digits of the four-digit year, and thus cannot be used to
reliably determine the current century or millennium.
How can I compare two dates and find the difference?
(contributed by brian d foy)
You could just store all your dates as a number and then
subtract. Life isn't always that simple though. If you want
to work with formatted dates, the Date::Manip, Date::Calc,
or DateTime modules can help you.
How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same
format, you can split it up and pass the parts to "timelo-
cal" 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)
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You can use the Time::JulianDay module available on CPAN.
Ensure that you really want to find a Julian day, though, as
many people have different ideas about Julian days. See
http://www.hermetic.ch/cal_stud/jdn.htm for instance.
You can also try the DateTime module, which can convert a
date/time to a Julian Day.
$ perl -MDateTime -le'print DateTime->today->jd'
2453401.5
Or the modified Julian Day
$ perl -MDateTime -le'print DateTime->today->mjd'
53401
Or even the day of the year (which is what some people think
of as a Julian day)
$ perl -MDateTime -le'print DateTime->today->doy'
31
How do I find yesterday's date?
(contributed by brian d foy)
Use one of the Date modules. The "DateTime" module makes it
simple, and give you the same time of day, only the day
before.
use DateTime;
my $yesterday = DateTime->now->subtract( days => 1 );
print "Yesterday was $yesterday\n";
You can also use the "Date::Calc" module using its
Today_and_Now function.
use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
print "@date\n";
Most people try to use the time rather than the calendar to
figure out dates, but that assumes that days are twenty-four
hours each. For most people, there are two days a year when
they aren't: the switch to and from summer time throws this
off. Let the modules do the work.
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Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
Short answer: No, Perl does not have a Year 2000 problem.
Yes, Perl is Y2K compliant (whatever that means). The pro-
grammers you've hired to use it, however, probably are not.
Long answer: The question belies a true understanding of the
issue. Perl is just as Y2K compliant as your pencil--no
more, and no less. Can you use your pencil to write a
non-Y2K-compliant memo? Of course you can. Is that the
pencil's fault? Of course it isn't.
The date and time functions supplied with Perl (gmtime and
localtime) supply adequate information to determine the year
well beyond 2000 (2038 is when trouble strikes for 32-bit
machines). The year returned by these functions when used
in a list context is the year minus 1900. For years between
1910 and 1999 this happens to be a 2-digit decimal number.
To avoid the year 2000 problem simply do not treat the year
as a 2-digit number. It isn't.
When gmtime() and localtime() are used in scalar context
they return a timestamp string that contains a fully-
expanded year. For example, "$timestamp =
gmtime(1005613200)" sets $timestamp to "Tue Nov 13 01:00:00
2001". There's no year 2000 problem here.
That doesn't mean that Perl can't be used to create non-Y2K
compliant programs. It can. But so can your pencil. It's
the fault of the user, not the language. At the risk of
inflaming the NRA: "Perl doesn't break Y2K, people do." See
http://www.perl.org/about/y2k.html for a longer exposition.
Data: Strings
How do I validate input?
(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
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("\") character are removed with
s/\\(.)/$1/g;
This won't expand "\n" or "\t" or any other special escapes.
How do I remove consecutive pairs of characters?
(contributed by brian d foy)
You can use the substitution operator to find pairs of char-
acters (or runs of characters) and replace them with a sin-
gle instance. In this substitution, we find a character in
"(.)". The memory parentheses store the matched character in
the back-reference "\1" and we use that to require that the
same thing immediately follow it. We replace that part of
the string with the character in $1.
s/(.)\1/$1/g;
We can also use the transliteration operator, "tr///". In
this example, the search list side of our "tr///" contains
nothing, but the "c" option complements that so it contains
everything. The replacement list also contains nothing, so
the transliteration is almost a no-op since it won't do any
replacements (or more exactly, replace the character with
itself). However, the "s" option squashes duplicated and
consecutive characters in the string so a character does not
show up next to itself
my $str = 'Haarlem'; # in the Netherlands
$str =~ tr///cs; # Now Harlem, like in New York
How do I expand function calls in a string?
(contributed by brian d foy)
This is documented in perlref, and although it's not the
easiest thing to read, it does work. In each of these exam-
ples, 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.
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print "The time is ${\(scalar localtime)}.\n"
print "The time is ${ my $x = localtime; \$x }.\n";
If your function already returns a reference, you don't need
to create the reference yourself.
sub timestamp { my $t = localtime; \$t }
print "The time is ${ timestamp() }.\n";
The "Interpolation" module can also do a lot of magic for
you. You can specify a variable name, in this case "E", to
set up a tied hash that does the interpolation for you. It
has several other methods to do this as well.
use Interpolation E => 'eval';
print "The time values are $E{localtime()}.\n";
In most cases, it is probably easier to simply use string
concatenation, which also forces scalar context.
print "The time is " . localtime . ".\n";
How do I find matching/nesting anything?
This isn't something that can be done in one regular expres-
sion, 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 some-
thing 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
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Dean Inada, and rather has the nature of an Obfuscated Perl
Contest entry, but it really does work:
# $_ contains the string to parse
# BEGIN and END are the opening and closing markers for the
# nested text.
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
@$ = (eval{/$re/},$@!~/unmatched/i);
print join("\n",@$[0..$#$]) if( $$[-1] );
How do I reverse a string?
Use reverse() in scalar context, as documented in "reverse"
in perlfunc.
$reversed = reverse $string;
How do I expand tabs in a string?
You can do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
Or you can just use the Text::Tabs module (part of the stan-
dard Perl distribution).
use Text::Tabs;
@expanded_lines = expand(@lines_with_tabs);
How do I reformat a paragraph?
Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
The paragraphs you give to Text::Wrap should not contain
embedded newlines. Text::Wrap doesn't justify the lines
(flush-right).
Or use the CPAN module Text::Autoformat. Formatting files
can be easily done by making a shell alias, like so:
alias fmt="perl -i -MText::Autoformat -n0777 \
-e 'print autoformat $_, {all=>1}' $*"
See the documentation for Text::Autoformat to appreciate its
many capabilities.
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How can I access or change N characters of a string?
You can access the first characters of a string with
substr(). To get the first character, for example, start at
position 0 and grab the string of length 1.
$string = "Just another Perl Hacker";
$first_char = substr( $string, 0, 1 ); # 'J'
To change part of a string, you can use the optional fourth
argument which is the replacement string.
substr( $string, 13, 4, "Perl 5.8.0" );
You can also use substr() as an lvalue.
substr( $string, 13, 4 ) = "Perl 5.8.0";
How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, let's
say you want to change the fifth occurrence of "whoever" or
"whomever" into "whosoever" or "whomsoever", case insensi-
tively. 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;
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How can I count the number of occurrences of a substring
within a string?
There are a number of ways, with varying efficiency. If you
want a count of a certain single character (X) within a
string, you can use the "tr///" function like so:
$string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
print "There are $count X characters in the string";
This is fine if you are just looking for a single character.
However, if you are trying to count multiple character sub-
strings within a larger string, "tr///" won't work. What
you can do is wrap a while() loop around a global pattern
match. For example, let's count negative integers:
$string = "-9 55 48 -2 23 -76 4 14 -44";
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
Another version uses a global match in list context, then
assigns the result to a scalar, producing a count of the
number of matches.
$count = () = $string =~ /-\d+/g;
How do I capitalize all the words on one line?
To make the first letter of each word upper case:
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning ""don't do it"" into
""Don'T Do It"". Sometimes you might want this. Other
times you might need a more thorough solution (Suggested by
brian d foy):
$string =~ s/ (
(^\w) #at the beginning of the line
| # or
(\s\w) #preceded by whitespace
)
/\U$1/xg;
$string =~ /([\w']+)/\u\L$1/g;
To make the whole line upper case:
$line = uc($line);
To force each word to be lower case, with the first letter
upper case:
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$line =~ s/(\w+)/\u\L$1/g;
You can (and probably should) enable locale awareness of
those characters by placing a "use locale" pragma in your
program. See perllocale for endless details on locales.
This is sometimes referred to as putting something into
"title case", but that's not quite accurate. Consider the
proper capitalization of the movie Dr. Strangelove or: How I
Learned to Stop Worrying and Love the Bomb, for example.
Damian Conway's Text::Autoformat module provides some smart
case transformations:
use Text::Autoformat;
my $x = "Dr. Strangelove or: How I Learned to Stop ".
"Worrying and Love the Bomb";
print $x, "\n";
for my $style (qw( sentence title highlight ))
{
print autoformat($x, { case => $style }), "\n";
}
How can I split a [character] delimited string except when
inside [character]?
Several modules can handle this sort of
pasing---Text::Balanced, Text::CSV, Text::CSV_XS, and
Text::ParseWords, among others.
Take the example case of trying to split a string that is
comma-separated into its different fields. You can't use
"split(/,/)" because you shouldn't split if the comma is
inside quotes. For example, take a data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly com-
plex problem. Thankfully, we have Jeffrey Friedl, author of
Mastering Regular Expressions, to handle these for us. He
suggests (assuming your string is contained in $text):
@new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
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If you want to represent quotation marks inside a
quotation-mark-delimited field, escape them with backslashes
(eg, "like \"this\"".
Alternatively, the Text::ParseWords module (part of the
standard Perl distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
There's also a Text::CSV (Comma-Separated Values) module on
CPAN.
How do I strip blank space from the beginning/end of a
string?
(contributed by brian d foy)
A substitution can do this for you. For a single line, you
want to replace all the leading or trailing whitespace with
nothing. You can do that with a pair of substitutions.
s/^\s+//;
s/\s+$//;
You can also write that as a single substitution, although
it turns out the combined statement is slower than the
separate ones. That might not matter to you, though.
s/^\s+|\s+$//g;
In this regular expression, the alternation matches either
at the beginning or the end of the string since the anchors
have a lower precedence than the alternation. With the "/g"
flag, the substitution makes all possible matches, so it
gets both. Remember, the trailing newline matches the "\s+",
and the "$" anchor can match to the physical end of the
string, so the newline disappears too. Just add the newline
to the output, which has the added benefit of preserving
"blank" (consisting entirely of whitespace) lines which the
"^\s+" would remove all by itself.
while( <> )
{
s/^\s+|\s+$//g;
print "$_\n";
}
For a multi-line string, you can apply the regular expres-
sion to each logical line in the string by adding the "/m"
flag (for "multi-line"). With the "/m" flag, the "$" matches
before an embedded newline, so it doesn't remove it. It
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still removes the newline at the end of the string.
$string =~ s/^\s+|\s+$//gm;
Remember that lines consisting entirely of whitespace will
disappear, since the first part of the alternation can match
the entire string and replace it with nothing. If need to
keep embedded blank lines, you have to do a little more
work. Instead of matching any whitespace (since that
includes a newline), just match the other whitespace.
$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
How do I pad a string with blanks or pad a number with
zeroes?
In the following examples, $pad_len is the length to which
you wish to pad the string, $text or $num contains the
string to be padded, and $pad_char contains the padding
character. You can use a single character string constant
instead of the $pad_char variable if you know what it is in
advance. And in the same way you can use an integer in place
of $pad_len if you know the pad length in advance.
The simplest method uses the "sprintf" function. It can pad
on the left or right with blanks and on the left with zeroes
and it will not truncate the result. The "pack" function can
only pad strings on the right with blanks and it will trun-
cate 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 $text.
Left and right padding with any character, creating a new
string:
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$padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
$padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying $text
directly:
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
$text .= $pad_char x ( $pad_len - length( $text ) );
How do I extract selected columns from a string?
Use substr() or unpack(), both documented in perlfunc. If
you prefer thinking in terms of columns instead of widths,
you can use this kind of thing:
# determine the unpack format needed to split Linux ps output
# arguments are cut columns
my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
sub cut2fmt {
my(@positions) = @_;
my $template = '';
my $lastpos = 1;
for my $place (@positions) {
$template .= "A" . ($place - $lastpos) . " ";
$lastpos = $place;
}
$template .= "A*";
return $template;
}
How do I find the soundex value of a string?
(contributed by brian d foy)
You can use the Text::Soundex module. If you want to do
fuzzy or close matching, you might also try the
String::Approx, and Text::Metaphone, and
Text::DoubleMetaphone modules.
How can I expand variables in text strings?
Let's assume that you have a string that contains place-
holder variables.
$text = 'this has a $foo in it and a $bar';
You can use a substitution with a double evaluation. The
first /e turns $1 into $foo, and the second /e turns $foo
into its value. You may want to wrap this in an "eval": if
you try to get the value of an undeclared variable while
running under "use strict", you get a fatal error.
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eval { $text =~ s/(\$\w+)/$1/eeg };
die if $@;
It's probably better in the general case to treat those
variables as entries in some special hash. For example:
%user_defs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
What's wrong with always quoting "$vars"?
The problem is that those double-quotes force
stringification-- coercing numbers and references into
strings--even when you don't want them to be strings. Think
of it this way: double-quote expansion is used to produce
new strings. If you already have a string, why do you need
more?
If you get used to writing odd things like these:
print "$var"; # BAD
$new = "$old"; # BAD
somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases)
be the simpler and more direct:
print $var;
$new = $old;
somefunc($var);
Otherwise, besides slowing you down, you're going to break
code when the thing in the scalar is actually neither a
string nor a number, but a reference:
func(\@array);
sub func {
my $aref = shift;
my $oref = "$aref"; # WRONG
}
You can also get into subtle problems on those few opera-
tions 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.
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@lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right
Why don't my <<HERE documents work?
Check for these three things:
There must be no space after the << part.
There (probably) should be a semicolon at the end.
You can't (easily) have any space in front of the tag.
If you want to indent the text in the here document, you can
do this:
# all in one
($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGET
But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote in the
indentation.
($quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s+--/\n--/;
A nice general-purpose fixer-upper function for indented
here documents follows. It expects to be called with a here
document as its argument. It looks to see whether each line
begins with a common substring, and if so, strips that sub-
string off. Otherwise, it takes the amount of leading whi-
tespace found on the first line and removes that much off
each subsequent line.
sub fix {
local $_ = shift;
my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}
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This works with leading special strings, dynamically deter-
mined:
$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.
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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?
(contributed by brian d foy)
Use a hash. When you think the words "unique" or "dupli-
cated", think "hash keys".
If you don't care about the order of the elements, you could
just create the hash then extract the keys. It's not impor-
tant how you create that hash: just that you use "keys" to
get the unique elements.
my %hash = map { $_, 1 } @array;
# or a hash slice: @hash{ @array } = ();
# or a foreach: $hash{$_} = 1 foreach ( @array );
my @unique = keys %hash;
You can also go through each element and skip the ones
you've seen before. Use a hash to keep track. The first time
the loop sees an element, that element has no key in %Seen.
The "next" statement creates the key and immediately uses
its value, which is "undef", so the loop continues to the
"push" and increments the value for that key. The next time
the loop sees that same element, its key exists in the hash
and the value for that key is true (since it's not 0 or
undef), so the next skips that iteration and the loop goes
to the next element.
my @unique = ();
my %seen = ();
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foreach my $elem ( @array )
{
next if $seen{ $elem }++;
push @unique, $elem;
}
You can write this more briefly using a grep, which does the
same thing.
my %seen = ();
my @unique = grep { ! $seen{ $_ }++ } @array;
How can I tell whether a certain element is contained in a
list or array?
(portions of this answer contributed by Anno Siegel)
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 arbi-
trary string values, the fastest way is probably to invert
the original array and maintain a hash whose keys are the
first array's values.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
%is_blue = ();
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might
have been a good idea to keep the blues all in a hash in the
first place.
If the values are all small integers, you could use a simple
indexed array. This kind of an array will take up less
space:
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
@is_tiny_prime = ();
for (@primes) { $is_tiny_prime[$_] = 1 }
# or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings,
you can save quite a lot of space by using bit strings
instead:
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@articles = ( 1..10, 150..2000, 2017 );
undef $read;
for (@articles) { vec($read,$_,1) = 1 }
Now check whether "vec($read,$n,1)" is true for some $n.
These methods guarantee fast individual tests but require a
re-organization of the original list or array. They only
pay off if you have to test multiple values against the same
array.
If you are testing only once, the standard module List::Util
exports the function "first" for this purpose. It works by
stopping once it finds the element. It's written in C for
speed, and its Perl equivalant looks like this subroutine:
sub first (&@) {
my $code = shift;
foreach (@_) {
return $_ if &{$code}();
}
undef;
}
If speed is of little concern, the common idiom uses grep in
scalar context (which returns the number of items that
passed its condition) to traverse the entire list. This does
have the benefit of telling you how many matches it found,
though.
my $is_there = grep $_ eq $whatever, @array;
If you want to actually extract the matching elements, sim-
ply use grep in list context.
my @matches = grep $_ eq $whatever, @array;
How do I compute the difference of two arrays? How do I
compute the intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes
that each element is unique in a given array:
@union = @intersection = @difference = ();
%count = ();
foreach $element (@array1, @array2) { $count{$element}++ }
foreach $element (keys %count) {
push @union, $element;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
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Note that this is the symmetric difference, that is, all
elements in either A or in B but not in both. Think of it
as an xor operation.
How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a
stringwise comparison, and does not distinguish defined
versus undefined empty strings. Modify if you have other
needs.
$are_equal = compare_arrays(\@frogs, \@toads);
sub compare_arrays {
my ($first, $second) = @_;
no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
}
return 1;
}
For multilevel structures, you may wish to use an approach
more like this one. It uses the CPAN module FreezeThaw:
use FreezeThaw qw(cmpStr);
@a = @b = ( "this", "that", [ "more", "stuff" ] );
printf "a and b contain %s arrays\n",
cmpStr(\@a, \@b) == 0
? "the same"
: "different";
This approach also works for comparing hashes. Here we'll
demonstrate two different answers:
use FreezeThaw qw(cmpStr cmpStrHard);
%a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
$a{EXTRA} = \%b;
$b{EXTRA} = \%a;
printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
The first reports that both those the hashes contain the
same data, while the second reports that they do not. Which
you prefer is left as an exercise to the reader.
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How do I find the first array element for which a condition
is true?
To find the first array element which satisfies a condition,
you can use the first() function in the List::Util module,
which comes with Perl 5.8. This example finds the first
element that contains "Perl".
use List::Util qw(first);
my $element = first { /Perl/ } @array;
If you cannot use List::Util, you can make your own loop to
do the same thing. Once you find the element, you stop the
loop with last.
my $found;
foreach ( @array )
{
if( /Perl/ ) { $found = $_; last }
}
If you want the array index, you can iterate through the
indices and check the array element at each index until you
find one that satisfies the condition.
my( $found, $index ) = ( undef, -1 );
for( $i = 0; $i < @array; $i++ )
{
if( $array[$i] =~ /Perl/ )
{
$found = $array[$i];
$index = $i;
last;
}
}
How do I handle linked lists?
In general, you usually don't need a linked list in Perl,
since with regular arrays, you can push and pop or shift and
unshift at either end, or you can use splice to add and/or
remove arbitrary number of elements at arbitrary points.
Both pop and shift are both O(1) operations on Perl's
dynamic arrays. In the absence of shifts and pops, push in
general needs to reallocate on the order every log(N) times,
and unshift will need to copy pointers each time.
If you really, really wanted, you could use structures as
described in perldsc or perltoot and do just what the algo-
rithm book tells you to do. For example, imagine a list
node like this:
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$node = {
VALUE => 42,
LINK => undef,
};
You could walk the list this way:
print "List: ";
for ($node = $head; $node; $node = $node->{LINK}) {
print $node->{VALUE}, " ";
}
print "\n";
You could add to the list this way:
my ($head, $tail);
$tail = append($head, 1); # grow a new head
for $value ( 2 .. 10 ) {
$tail = append($tail, $value);
}
sub append {
my($list, $value) = @_;
my $node = { VALUE => $value };
if ($list) {
$node->{LINK} = $list->{LINK};
$list->{LINK} = $node;
} else {
$_[0] = $node; # replace caller's version
}
return $node;
}
But again, Perl's built-in are virtually always good enough.
How do I handle circular lists?
Circular lists could be handled in the traditional fashion
with linked lists, or you could just do something like this
with an array:
unshift(@array, pop(@array)); # the last shall be first
push(@array, shift(@array)); # and vice versa
How do I shuffle an array randomly?
If you either have Perl 5.8.0 or later installed, or if you
have Scalar-List-Utils 1.03 or later installed, you can say:
use List::Util 'shuffle';
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@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 effi-
cient that you probably won't notice this until you have
rather largish arrays.
How do I process/modify each element of an array?
Use "for"/"foreach":
for (@lines) {
s/foo/bar/; # change that word
tr/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
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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 actu-
ally an array, try the Algorithm::Permute module (also on
CPAN). It's written in XS code and is very efficient.
use Algorithm::Permute;
my @array = 'a'..'d';
my $p_iterator = Algorithm::Permute->new ( \@array );
while (my @perm = $p_iterator->next) {
print "next permutation: (@perm)\n";
}
For even faster execution, you could do:
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use Algorithm::Permute;
my @array = 'a'..'d';
Algorithm::Permute::permute {
print "next permutation: (@array)\n";
} @array;
Here's a little program that generates all permutations of
all the words on each line of input. The algorithm embodied
in the permute() function is discussed in Volume 4 (still
unpublished) of Knuth's The Art of Computer Programming and
will work on any list:
#!/usr/bin/perl -n
# Fischer-Kause ordered permutation generator
sub permute (&@) {
my $code = shift;
my @idx = 0..$#_;
while ( $code->(@_[@idx]) ) {
my $p = $#idx;
--$p while $idx[$p-1] > $idx[$p];
my $q = $p or return;
push @idx, reverse splice @idx, $p;
++$q while $idx[$p-1] > $idx[$q];
@idx[$p-1,$q]=@idx[$q,$p-1];
}
}
permute {print"@_\n"} split;
How do I sort an array by (anything)?
Supply a comparison function to sort() (described in "sort"
in perlfunc):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which
would sort "(1, 2, 10)" into "(1, 10, 2)". "<=>", used
above, is the numerical comparison operator.
If you have a complicated function needed to pull out the
part you want to sort on, then don't do it inside the sort
function. Pull it out first, because the sort BLOCK can be
called many times for the same element. Here's an example
of how to pull out the first word after the first number on
each item, and then sort those words case-insensitively.
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@idx = ();
for (@data) {
($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 { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following para-
digm is useful.
@sorted = sort { field1($a) <=> field1($b) ||
field2($a) cmp field2($b) ||
field3($a) cmp field3($b)
} @data;
This can be conveniently combined with precalculation of
keys as given above.
See the sort article in the "Far More Than You Ever Wanted
To Know" collection in
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for more about
this approach.
See also the question below on sorting hashes.
How do I manipulate arrays of bits?
Use pack() and unpack(), or else vec() and the bitwise
operations.
For example, this sets $vec to have bit N set if $ints[N]
was set:
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
Here's how, given a vector in $vec, you can get those bits
into your @ints array:
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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.)
You can make the while loop a lot shorter with this sugges-
tion from Benjamin Goldberg:
while($vec =~ /[^\0]+/g ) {
push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
}
Or use the CPAN module Bit::Vector:
$vector = Bit::Vector->new($num_of_bits);
$vector->Index_List_Store(@ints);
@ints = $vector->Index_List_Read();
Bit::Vector provides efficient methods for bit vector, sets
of small integers and "big int" math.
Here's a more extensive illustration using vec():
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# vec demo
$vector = "\xff\x0f\xef\xfe";
print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", $vector), "\n";
$is_set = vec($vector, 23, 1);
print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
pvec($vector);
set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);
set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);
set_vec(0,32,17);
set_vec(1,32,17);
sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print "offset=$offset width=$width value=$value\n";
pvec($vector);
}
sub pvec {
my $vector = shift;
my $bits = unpack("b*", $vector);
my $i = 0;
my $BASE = 8;
print "vector length in bytes: ", length($vector), "\n";
@bytes = unpack("A8" x length($vector), $bits);
print "bits are: @bytes\n\n";
}
Why does defined() return true on empty arrays and hashes?
The short story is that you should probably only use defined
on scalars or functions, not on aggregates (arrays and
hashes). See "defined" in perlfunc in the 5.004 release or
later of Perl for more detail.
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How do I process an entire hash?
Use the each() function (see "each" in perlfunc) if you
don't care whether it's sorted:
while ( ($key, $value) = each %hash) {
print "$key = $value\n";
}
If you want it sorted, you'll have to use foreach() on the
result of sorting the keys as shown in an earlier question.
What happens if I add or remove keys from a hash while
iterating over it?
(contributed by brian d foy)
The easy answer is "Don't do that!"
If you iterate through the hash with each(), you can delete
the key most recently returned without worrying about it.
If you delete or add other keys, the iterator may skip or
double up on them since perl may rearrange the hash table.
See the entry for "each()" in perlfunc.
How do I look up a hash element by value?
Create a reverse hash:
%by_value = reverse %by_key;
$key = $by_value{$value};
That's not particularly efficient. It would be more space-
efficient to use:
while (($key, $value) = each %by_key) {
$by_value{$value} = $key;
}
If your hash could have repeated values, the methods above
will only find one of the associated keys. This may or may
not worry you. If it does worry you, you can always reverse
the hash into a hash of arrays instead:
while (($key, $value) = each %by_key) {
push @{$key_list_by_value{$value}}, $key;
}
How can I know how many entries are in a hash?
If you mean how many keys, then all you have to do is use
the keys() function in a scalar context:
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$num_keys = keys %hash;
The keys() function also resets the iterator, which means
that you may see strange results if you use this between
uses of other hash operators such as each().
How do I sort a hash (optionally by value instead of key)?
(contributed by brian d foy)
To sort a hash, start with the keys. In this example, we
give the list of keys to the sort function which then com-
pares them ASCIIbetically (which might be affected by your
locale settings). The output list has the keys in ASCIIbeti-
cal order. Once we have the keys, we can go through them to
create a report which lists the keys in ASCIIbetical order.
my @keys = sort { $a cmp $b } keys %hash;
foreach my $key ( @keys )
{
printf "%-20s %6d\n", $key, $hash{$value};
}
We could get more fancy in the "sort()" block though.
Instead of comparing the keys, we can compute a value with
them and use that value as the comparison.
For instance, to make our report order case-insensitive, we
use the "\L" sequence in a double-quoted string to make
everything lowercase. The "sort()" block then compares the
lowercased values to determine in which order to put the
keys.
my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
Note: if the computation is expensive or the hash has many
elements, you may want to look at the Schwartzian Transform
to cache the computation results.
If we want to sort by the hash value instead, we use the
hash key to look it up. We still get out a list of keys, but
this time they are ordered by their value.
my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
From there we can get more complex. If the hash values are
the same, we can provide a secondary sort on the hash key.
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my @keys = sort {
$hash{$a} <=> $hash{$b}
or
"\L$a" cmp "\L$b"
} keys %hash;
How can I always keep my hash sorted?
You can look into using the DB_File module and tie() using
the $DB_BTREE hash bindings as documented in "In Memory
Databases" in DB_File. The Tie::IxHash module from CPAN
might also be instructive.
What's the difference between "delete" and "undef" with
hashes?
Hashes contain pairs of scalars: the first is the key, the
second is the value. The key will be coerced to a string,
although the value can be any kind of scalar: string,
number, or reference. If a key $key is present in %hash,
"exists($hash{$key})" will return true. The value for a
given key can be "undef", in which case $hash{$key} will be
"undef" while "exists $hash{$key}" will return true. This
corresponds to ($key, "undef") being in the hash.
Pictures help... here's the %hash table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
And these conditions hold
$hash{'a'} is true
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is true
exists $hash{'a'} is true (Perl5 only)
grep ($_ eq 'a', keys %hash) is true
If you now say
undef $hash{'a'}
your table now reads:
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keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl5 only)
grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined
key!
Now, consider this:
delete $hash{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl5 only)
grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
Why don't my tied hashes make the defined/exists distinc-
tion?
This depends on the tied hash's implementation of EXISTS().
For example, there isn't the concept of undef with hashes
that are tied to DBM* files. It also means that exists() and
defined() do the same thing with a DBM* file, and what they
end up doing is not what they do with ordinary hashes.
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How do I reset an each() operation part-way through?
Using "keys %hash" in scalar context returns the number of
keys in the hash and resets the iterator associated with the
hash. You may need to do this if you use "last" to exit a
loop early so that when you re-enter it, the hash iterator
has been reset.
How can I get the unique keys from two hashes?
First you extract the keys from the hashes into lists, then
solve the "removing duplicates" problem described above.
For example:
%seen = ();
for $element (keys(%foo), keys(%bar)) {
$seen{$element}++;
}
@uniq = keys %seen;
Or more succinctly:
@uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
%seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
@uniq = keys %seen;
How can I store a multidimensional array in a DBM file?
Either stringify the structure yourself (no fun), or else
get the MLDBM (which uses Data::Dumper) module from CPAN and
layer it on top of either DB_File or GDBM_File.
How can I make my hash remember the order I put elements
into it?
Use the Tie::IxHash from CPAN.
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use Tie::IxHash;
tie my %myhash, 'Tie::IxHash';
for (my $i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
my @keys = keys %myhash;
# @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a hash
create it?
If you say something like:
somefunc($hash{"nonesuch key here"});
Then that element "autovivifies"; that is, it springs into
existence whether you store something there or not. That's
because functions get scalars passed in by reference. If
somefunc() modifies $_[0], it has to be ready to write it
back into the caller's version.
This has been fixed as of Perl5.004.
Normally, merely accessing a key's value for a nonexistent
key does not cause that key to be forever there. This is
different than awk's behavior.
How can I make the Perl equivalent of a C structure/C++
class/hash or array of hashes or arrays?
Usually a hash ref, perhaps like this:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};
References are documented in perlref and the upcoming
perlreftut. Examples of complex data structures are given in
perldsc and perllol. Examples of structures and object-
oriented classes are in perltoot.
How can I use a reference as a hash key?
(contributed by brian d foy)
Hash keys are strings, so you can't really use a reference
as the key. When you try to do that, perl turns the
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reference into its stringified form (for instance,
"HASH(0xDEADBEEF)"). From there you can't get back the
reference from the stringified form, at least without doing
some extra work on your own. Also remember that hash keys
must be unique, but two different variables can store the
same reference (and those variables can change later).
The Tie::RefHash module, which is distributed with perl,
might be what you want. It handles that extra work.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so this shouldn't be a problem. For
example, this works fine (assuming the files are found):
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On less elegant (read: Byzantine) systems, however, you have
to play tedious games with "text" versus "binary" files.
See "binmode" in perlfunc or perlopentut.
If you're concerned about 8-bit ASCII data, then see perllo-
cale.
If you want to deal with multibyte characters, however,
there are some gotchas. See the section on Regular Expres-
sions.
How do I determine whether a scalar is a
number/whole/integer/float?
Assuming that you don't care about IEEE notations like "NaN"
or "Infinity", you probably just want to use a regular
expression.
if (/\D/) { print "has nondigits\n" }
if (/^\d+$/) { print "is a whole number\n" }
if (/^-?\d+$/) { print "is an integer\n" }
if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
{ print "a C float\n" }
There are also some commonly used modules for the task.
Scalar::Util (distributed with 5.8) provides access to
perl's internal function "looks_like_number" for determining
whether a variable looks like a number. Data::Types exports
functions that validate data types using both the above and
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other regular expressions. Thirdly, there is
"Regexp::Common" which has regular expressions to match
various types of numbers. Those three modules are available
from the CPAN.
If you're on a POSIX system, Perl supports the
"POSIX::strtod" function. Its semantics are somewhat
cumbersome, so here's a "getnum" wrapper function for more
convenient access. This function takes a string and returns
the number it found, or "undef" for input that isn't a C
float. The "is_numeric" function is a front end to "getnum"
if you just want to say, "Is this a float?"
sub getnum {
use POSIX qw(strtod);
my $str = shift;
$str =~ s/^\s+//;
$str =~ s/\s+$//;
$! = 0;
my($num, $unparsed) = strtod($str);
if (($str eq '') || ($unparsed != 0) || $!) {
return undef;
} else {
return $num;
}
}
sub is_numeric { defined getnum($_[0]) }
Or you could check out the String::Scanf module on the CPAN
instead. The POSIX module (part of the standard Perl distri-
bution) provides the "strtod" and "strtol" for converting
strings to double and longs, respectively.
How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM
modules. See AnyDBM_File. More generically, you should con-
sult the FreezeThaw or Storable modules from CPAN. Starting
from Perl 5.8 Storable is part of the standard distribution.
Here's one example using Storable's "store" and "retrieve"
functions:
use Storable;
store(\%hash, "filename");
# later on...
$href = retrieve("filename"); # by ref
%hash = %{ retrieve("filename") }; # direct to hash
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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 Stor-
able module on CPAN (or the 5.8 release of Perl), provides a
function called "dclone" that recursively copies its argu-
ment.
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 punc-
tuation if you had a hash of arrays that you wanted to copy.
%newhash = %{ dclone(\%oldhash) };
How do I define methods for every class/object?
Use the UNIVERSAL class (see UNIVERSAL).
How do I verify a credit card checksum?
Get the Business::CreditCard module from CPAN.
How do I pack arrays of doubles or floats for XS code?
The kgbpack.c code in the PGPLOT module on CPAN does just
this. If you're doing a lot of float or double processing,
consider using the PDL module from CPAN instead--it makes
number-crunching easy.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington,
and other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this
file are hereby placed into the public domain. You are per-
mitted and encouraged to use this code in your own programs
for fun or for profit as you see fit. A simple comment in
the code giving credit would be courteous but is not
required.
perl v5.8.8 2006-06-30 44
