perlfaq4 - Data Manipulation
This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf).
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 shows 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
Floating Point Arithmetic in perlop for more details.
Your int() is most probably working just fine. It's the numbers that
aren't quite what you think.
First, see the answer to "Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?".
For example, this
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.
(contributed by brian d foy)
You're probably trying to convert a string to a number, which Perl only converts as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10:
This problem usually involves one of the Perl built-ins that has the
same name a Unix command that uses octal numbers as arguments on the
command line. In this example, chmod on the command line knows that
its first argument is octal because that's what it does:
- %prompt> chmod 644 file
If you want to use the same literal digits (644) in Perl, you have to tell
Perl to treat them as octal numbers either by prefixing the digits with
a 0
or using oct:
The problem comes in when you take your numbers from something that Perl
thinks is a string, such as a command line argument in @ARGV
:
You can always check the value you're using by printing it in octal notation to ensure it matches what you think it should be. Print it in octal and decimal format:
- printf "0%o %d", $number, $number;
Remember that int() merely truncates toward 0. For rounding to a
certain number of digits, sprintf() or printf() is usually the
easiest route.
- printf("%.3f", 3.1415926535); # prints 3.142
The POSIX
module (part of the standard Perl distribution)
implements ceil()
, floor()
, and a number of other mathematical
and trigonometric functions.
- use POSIX;
- $ceil = ceil(3.5); # 4
- $floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
module. With 5.004, the Math::Trig
module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the Math::Complex
module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
2.
Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system of rounding is being used by Perl, but instead to implement the rounding function you need yourself.
To see why, notice how you'll still have an issue on half-way-point alternation:
- for ($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.
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 later in perlfaq4 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.
Using perl's built in conversion of 0x
notation:
- $dec = 0xDEADBEEF;
Using the hex function:
- $dec = hex("DEADBEEF");
Using pack:
Using the CPAN module Bit::Vector
:
- use Bit::Vector;
- $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
- $dec = $vec->to_Dec();
Using sprintf:
Using unpack:
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();
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
:
Using sprintf:
- $oct = sprintf("%o", 3735928559);
Using Bit::Vector
:
Perl 5.6 lets you write binary numbers directly with
the 0b notation:
- $number = 0b10110110;
Using oct:
Using pack and unpack for larger strings:
Using Bit::Vector
:
- $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
- $dec = $vec->to_Dec();
Using sprintf (perl 5.6+):
- $bin = sprintf("%b", 3735928559);
Using unpack:
Using Bit::Vector
:
- use Bit::Vector;
- $vec = Bit::Vector->new_Dec(32, -559038737);
- $bin = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader.
The behavior of binary arithmetic operators depends on whether they're
used on numbers or strings. The operators treat a string as a series
of bits and work with that (the string "3"
is the bit pattern
00110011
). The operators work with the binary form of a number
(the number 3
is treated as the bit pattern 00000011
).
So, saying 11 & 3
performs the "and" operation on numbers (yielding
3
). Saying "11" & "3"
performs the "and" operation on strings
(yielding "1"
).
Most problems with &
and | arise because the programmer thinks
they have a number but really it's a string or vice versa. To avoid this,
stringify the arguments explicitly (using ""
or qq()) or convert them
to numbers explicitly (using 0+$arg
). 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]/) {
- # ...
- }
Use the Math::Matrix
or Math::MatrixReal
modules (available from CPAN)
or the PDL
extension (also available from CPAN).
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($_) } (5 .. 25);
but you should be aware that the ..
operator creates a list of
all integers in the range. This can take a lot of memory for large
ranges. Instead use:
- @results = ();
- for ($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.
will not create a list of 500,000 integers.
Get the http://www.cpan.org/modules/by-module/Roman module.
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 $] < 5.004 }
5.004 and later automatically call srand at the beginning. Don't
call srand more than once--you make your numbers less random,
rather than more.
Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). The random article in the "Far More Than You Ever Wanted To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz, courtesy of Tom Phoenix, talks more about this. John von Neumann said, "Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin."
If you want numbers that are more random than rand with srand
provides, you should also check out the Math::TrulyRandom
module from
CPAN. It uses the imperfections in your system's timer to generate
random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
"Numerical Recipes in C" at http://www.nr.com/.
To get a random number between two values, you can use the rand()
built-in to get a random number between 0 and 1. From there, you shift
that into the range that you want.
rand($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.
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_between(50,120)
.
The day of the year is in the list returned
by the localtime function. Without an
argument localtime uses the current time.
The POSIX
module can also format a date as the day of the year or
week of the year.
To get the day of year for any date, use POSIX
's mktime
to get
a time in epoch seconds for the argument to localtime.
You can also use Time::Piece
, which comes with Perl and provides a
localtime that returns an object:
The Date::Calc
module provides two functions to calculate these, too:
Use the following simple functions:
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 determine reliably the current century or
millennium.
(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.
The Time::Piece
module, which comes with Perl, replaces localtime
with a version that returns an object. It also overloads the comparison
operators so you can compare them directly:
You can also get differences with a subtraction, which returns a
Time::Seconds
object:
If you want to work with formatted dates, the Date::Manip
,
Date::Calc
, or DateTime
modules can help you.
If it's a regular enough string that it always has the same format,
you can split it up and pass the parts to timelocal
in the standard
Time::Local
module. Otherwise, you should look into the Date::Calc
,
Date::Parse
, and Date::Manip
modules from CPAN.
(contributed by brian d foy and Dave Cross)
You can use the Time::Piece
module, part of the Standard Library,
which can convert a date/time to a Julian Day:
- $ perl -MTime::Piece -le 'print localtime->julian_day'
- 2455607.7959375
Or the modified Julian Day:
- $ perl -MTime::Piece -le 'print localtime->mjd'
- 55607.2961226851
Or even the day of the year (which is what some people think of as a Julian day):
- $ perl -MTime::Piece -le 'print localtime->yday'
- 45
You can also do the same things with the DateTime
module:
- $ perl -MDateTime -le'print DateTime->today->jd'
- 2453401.5
- $ perl -MDateTime -le'print DateTime->today->mjd'
- 53401
- $ perl -MDateTime -le'print DateTime->today->doy'
- 31
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):
- $ perl -MTime::JulianDay -le 'print local_julian_day( time )'
- 55608
(contributed by brian d foy)
To do it correctly, you can use one of the Date
modules since they
work with calendars instead of times. The DateTime
module makes it
simple, and give you the same time of day, only the day before,
despite daylight saving time changes:
You can also use the Date::Calc
module using its Today_and_Now
function.
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. For example, the rest of the suggestions will be wrong sometimes:
Starting with Perl 5.10, Time::Piece
and Time::Seconds
are part
of the standard distribution, so you might think that you could do
something like this:
The Time::Piece
module exports a new localtime that returns an
object, and Time::Seconds
exports the ONE_DAY
constant that is a
set number of seconds. This means that it always gives the time 24
hours ago, which is not always yesterday. This can cause problems
around the end of daylight saving time when there's one day that is 25
hours long.
You have the same problem with Time::Local
, which will give the wrong
answer for those same special cases:
(contributed by brian d foy)
Perl itself never had a Y2K problem, although that never stopped people
from creating Y2K problems on their own. See the documentation for
localtime for its proper use.
Starting with Perl 5.12, localtime and gmtime can handle dates past
03:14:08 January 19, 2038, when a 32-bit based time would overflow. You
still might get a warning on a 32-bit perl
:
- % perl5.12 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
- Integer overflow in hexadecimal number at -e line 1.
- Wed Nov 1 19:42:39 5576711
On a 64-bit perl
, you can get even larger dates for those really long
running projects:
- % perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
- Thu Nov 2 00:42:39 5576711
You're still out of luck if you need to keep track of decaying protons though.
(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
.
It depends just what you mean by "escape". URL escapes are dealt
with in perlfaq9. Shell escapes with the backslash (\
)
character are removed with
- s/\\(.)/$1/g;
This won't expand "\n"
or "\t"
or any other special escapes.
(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 \g1
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/(.)\g1/$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
(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 examples, we call the function inside the braces used to dereference a reference. If we have 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. Note that
the use of parens creates a list context, so we need scalar to
force the scalar context on the function:
If your function already returns a reference, you don't need to create the reference yourself.
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.
In most cases, it is probably easier to simply use string concatenation, which also forces scalar context.
This isn't something that can be done in one regular expression, no
matter how complicated. To find something between two single
characters, a pattern like /x([^x]*)x/
will get the intervening
bits in $1. For multiple ones, then something more like
/alpha(.*?)omega/
would be needed. But none of these deals with
nested patterns. For balanced expressions using (, {, [ or
<
as delimiters, use the CPAN module Regexp::Common, or see
(??{ code }) in perlre. For other cases, you'll have to write a
parser.
If you are serious about writing a parser, there are a number of
modules or oddities that will make your life a lot easier. There are
the CPAN modules Parse::RecDescent
, Parse::Yapp
, and
Text::Balanced
; and the byacc
program. Starting from perl 5.8
the Text::Balanced
is part of the standard distribution.
One simple destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time:
- while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
- # do something with $1
- }
A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work:
Use reverse() in scalar context, as documented in
reverse.
- $reversed = reverse $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);
Use Text::Wrap
(part of the standard Perl distribution):
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.
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";
You have to keep track of N yourself. For example, let's say you want
to change the fifth occurrence of "whoever"
or "whomever"
into
"whosoever"
or "whomsoever"
, case insensitively. These
all assume that $_ contains the string to be altered.
- $count = 0;
- s{((whom?)ever)}{
- ++$count == 5 # is it the 5th?
- ? "${2}soever" # yes, swap
- : $1 # renege and leave it there
- }ige;
In the more general case, you can use the /g modifier in a while
loop, keeping count of matches.
- $WANT = 3;
- $count = 0;
- $_ = "One fish two fish red fish blue fish";
- while (/(\w+)\s+fish\b/gi) {
- if (++$count == $WANT) {
- print "The third fish is a $1 one.\n";
- }
- }
That prints out: "The third fish is a red one."
You can also use a
repetition count and repeated pattern like this:
- /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
There are a number of ways, with varying efficiency. If you want a
count of a certain single character (X) within a string, you can use the
tr/// function like so:
- $string = "ThisXlineXhasXsomeXx'sXinXit";
- $count = ($string =~ tr/X//);
- print "There are $count X characters in the string";
This is fine if you are just looking for a single character. However,
if you are trying to count multiple character substrings within a
larger string, tr/// won't work. What you can do is wrap a while()
loop around a global pattern match. For example, let's count negative
integers:
- $string = "-9 55 48 -2 23 -76 4 14 -44";
- while ($string =~ /-\d+/g) { $count++ }
- print "There are $count negative numbers in the string";
Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches.
- $count = () = $string =~ /-\d+/g;
(contributed by brian d foy)
Damian Conway's Text::Autoformat handles all of the thinking for you.
How do you want to capitalize those words?
- FRED AND BARNEY'S LODGE # all uppercase
- Fred And Barney's Lodge # title case
- Fred and Barney's Lodge # highlight case
It's not as easy a problem as it looks. How many words do you think
are in there? Wait for it... wait for it.... If you answered 5
you're right. Perl words are groups of \w+
, but that's not what
you want to capitalize. How is Perl supposed to know not to capitalize
that s after the apostrophe? You could try a regular expression:
- $string =~ s/ (
- (^\w) #at the beginning of the line
- | # or
- (\s\w) #preceded by whitespace
- )
- /\U$1/xg;
- $string =~ s/([\w']+)/\u\L$1/g;
Now, what if you don't want to capitalize that "and"? Just use Text::Autoformat and get on with the next problem. :)
Several modules can handle this sort of parsing--Text::Balanced
,
Text::CSV
, Text::CSV_XS
, and Text::ParseWords
, among others.
Take the example case of trying to split a string that is
comma-separated into its different fields. You can't use split(/,/)
because you shouldn't split if the comma is inside quotes. For
example, take a data line like this:
- SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex
problem. Thankfully, we have Jeffrey Friedl, author of
Mastering Regular Expressions, to handle these for us. He
suggests (assuming your string is contained in $text
):
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, $text);
(contributed by brian d foy)
A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions:
- s/^\s+//;
- s/\s+$//;
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
absolute 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 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 doesn't remove it. This pattern 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 you 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;
In the following examples, $pad_len
is the length to which you wish
to pad the string, $text
or $num
contains the string to be padded,
and $pad_char
contains the padding character. You can use a single
character string constant instead of the $pad_char
variable if you
know what it is in advance. And in the same way you can use an integer in
place of $pad_len
if you know the pad length in advance.
The simplest method uses the sprintf function. It can pad on the left
or right with blanks and on the left with zeroes and it will not
truncate the result. The pack function can only pad strings on the
right with blanks and it will truncate the result to a maximum length of
$pad_len
.
- # Left padding a string with blanks (no truncation):
- $padded = sprintf("%${pad_len}s", $text);
- $padded = sprintf("%*s", $pad_len, $text); # same thing
- # Right padding a string with blanks (no truncation):
- $padded = sprintf("%-${pad_len}s", $text);
- $padded = sprintf("%-*s", $pad_len, $text); # same thing
- # Left padding a number with 0 (no truncation):
- $padded = sprintf("%0${pad_len}d", $num);
- $padded = sprintf("%0*d", $pad_len, $num); # same thing
- # Right padding a string with blanks using pack (will truncate):
- $padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you can use
one of the following methods. They all generate a pad string with the
x
operator and combine that with $text
. These methods do
not truncate $text
.
Left and right padding with any character, creating a new string:
Left and right padding with any character, modifying $text
directly:
(contributed by brian d foy)
If you know the columns that contain the data, you can
use substr to extract a single column.
You can use split if the columns are separated by whitespace or
some other delimiter, as long as whitespace or the delimiter cannot
appear as part of the data.
If you want to work with comma-separated values, don't do this since
that format is a bit more complicated. Use one of the modules that
handle that format, such as Text::CSV
, Text::CSV_XS
, or
Text::CSV_PP
.
If you want to break apart an entire line of fixed columns, you can use
unpack with the A (ASCII) format. By using a number after the format
specifier, you can denote the column width. See the pack and unpack
entries in perlfunc for more details.
Note that spaces in the format argument to unpack do not denote literal
spaces. If you have space separated data, you may want split instead.
(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.
(contributed by brian d foy)
If you can avoid it, don't, or if you can use a templating system,
such as Text::Template
or Template
Toolkit, do that instead. You
might even be able to get the job done with sprintf or printf:
However, for the one-off simple case where I don't want to pull out a
full templating system, I'll use a string that has two Perl scalar
variables in it. In this example, I want to expand $foo
and $bar
to their variable's values:
One way I can do this involves the substitution operator and a double
/e flag. The first /e evaluates $1
on the replacement side and
turns it into $foo
. The second /e starts with $foo
and replaces
it with its value. $foo
, then, turns into 'Fred', and that's finally
what's left in the string:
- $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
The /e will also silently ignore violations of strict, replacing
undefined variable names with the empty string. Since I'm using the
/e flag (twice even!), I have all of the same security problems I
have with eval in its string form. If there's something odd in
$foo
, perhaps something like @{[ system "rm -rf /" ]}
, then
I could get myself in trouble.
To get around the security problem, I could also pull the values from
a hash instead of evaluating variable names. Using a single /e, I
can check the hash to ensure the value exists, and if it doesn't, I
can replace the missing value with a marker, in this case ??? to
signal that I missed something:
The problem is that those double-quotes force stringification--coercing numbers and references into strings--even when you don't want them to be strings. Think of it this way: double-quote expansion is used to produce new strings. If you already have a string, why do you need more?
If you get used to writing odd things like these:
- print "$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:
You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical ++
autoincrement operator or the
syscall() function.
Stringification also destroys arrays.
Here documents are found in perlop. Check for these three things:
If you want to indent the text in the here document, you can do this:
- # all in one
- ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
- your text
- goes here
- HERE_TARGET
But the HERE_TARGET must still be flush against the margin. If you want that indented also, you'll have to quote in the indentation.
- ($quote = <<' FINIS') =~ s/^\s+//gm;
- ...we will have peace, when you and all your works have
- perished--and the works of your dark master to whom you
- would deliver us. You are a liar, Saruman, and a corrupter
- of men's hearts. --Theoden in /usr/src/perl/taint.c
- FINIS
- $quote =~ s/\s+--/\n--/;
A nice general-purpose fixer-upper function for indented here documents follows. It expects to be called with a here document as its argument. It looks to see whether each line begins with a common substring, and if so, strips that substring off. Otherwise, it takes the amount of leading whitespace found on the first line and removes that much off each subsequent line.
This works with leading special strings, dynamically determined:
- $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
- @@@ int
- @@@ runops() {
- @@@ SAVEI32(runlevel);
- @@@ runlevel++;
- @@@ while ( op = (*op->op_ppaddr)() );
- @@@ TAINT_NOT;
- @@@ return 0;
- @@@ }
- MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved:
- $poem = fix<<EVER_ON_AND_ON;
- Now far ahead the Road has gone,
- And I must follow, if I can,
- Pursuing it with eager feet,
- Until it joins some larger way
- Where many paths and errands meet.
- And whither then? I cannot say.
- --Bilbo in /usr/src/perl/pp_ctl.c
- EVER_ON_AND_ON
(contributed by brian d foy)
A list is a fixed collection of scalars. An array is a variable that holds a variable collection of scalars. An array can supply its collection for list operations, so list operations also work on arrays:
- # slices
- ( 'dog', 'cat', 'bird' )[2,3];
- @animals[2,3];
- # iteration
- foreach ( qw( dog cat bird ) ) { ... }
- foreach ( @animals ) { ... }
- my @three = grep { length == 3 } qw( dog cat bird );
- my @three = grep { length == 3 } @animals;
- # supply an argument list
- wash_animals( qw( dog cat bird ) );
- wash_animals( @animals );
Array operations, which change the scalars, rearranges them, or adds
or subtracts some scalars, only work on arrays. These can't work on a
list, which is fixed. Array operations include shift, unshift,
push, pop, and splice.
An array can also change its length:
- $#animals = 1; # truncate to two elements
- $#animals = 10000; # pre-extend to 10,001 elements
You can change an array element, but you can't change a list element:
- $animals[0] = 'Rottweiler';
- qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!
- foreach ( @animals ) {
- s/^d/fr/; # works fine
- }
- foreach ( qw( dog cat bird ) ) {
- s/^d/fr/; # Error! Modification of read only value!
- }
However, if the list element is itself a variable, it appears that you can change a list element. However, the list element is the variable, not the data. You're not changing the list element, but something the list element refers to. The list element itself doesn't change: it's still the same variable.
You also have to be careful about context. You can assign an array to a scalar to get the number of elements in the array. This only works for arrays, though:
- my $count = @animals; # only works with arrays
If you try to do the same thing with what you think is a list, you get a quite different result. Although it looks like you have a list on the righthand side, Perl actually sees a bunch of scalars separated by a comma:
- my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets bird
Since you're assigning to a scalar, the righthand side is in scalar
context. The comma operator (yes, it's an operator!) in scalar
context evaluates its lefthand side, throws away the result, and
evaluates it's righthand side and returns the result. In effect,
that list-lookalike assigns to $scalar
it's rightmost value. Many
people mess this up because they choose a list-lookalike whose
last element is also the count they expect:
- my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentally
(contributed by brian d foy)
The difference is the sigil, that special character in front of the
array name. The $
sigil means "exactly one item", while the @
sigil means "zero or more items". The $
gets you a single scalar,
while the @
gets you a list.
The confusion arises because people incorrectly assume that the sigil denotes the variable type.
The $array[1]
is a single-element access to the array. It's going
to return the item in index 1 (or undef if there is no item there).
If you intend to get exactly one element from the array, this is the
form you should use.
The @array[1]
is an array slice, although it has only one index.
You can pull out multiple elements simultaneously by specifying
additional indices as a list, like @array[1,4,3,0]
.
Using a slice on the lefthand side of the assignment supplies list context to the righthand side. This can lead to unexpected results. For instance, if you want to read a single line from a filehandle, assigning to a scalar value is fine:
- $array[1] = <STDIN>;
However, in list context, the line input operator returns all of the
lines as a list. The first line goes into @array[1]
and the rest
of the lines mysteriously disappear:
- @array[1] = <STDIN>; # most likely not what you want
Either the use warnings
pragma or the -w flag will warn you when
you use an array slice with a single index.
(contributed by brian d foy)
Use a hash. When you think the words "unique" or "duplicated", 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 important how you
create that hash: just that you use keys to get the unique
elements.
If you want to use a module, try the uniq
function from
List::MoreUtils
. In list context it returns the unique elements,
preserving their order in the list. In scalar context, it returns the
number of unique elements.
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.
You can write this more briefly using a grep, which does the same thing.
(portions of this answer contributed by Anno Siegel and brian d foy)
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. In Perl 5.10 and later, you can use the smart match operator to check that an item is contained in an array or a hash:
With earlier versions of Perl, you have to do a bit more work. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values:
- @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
- %is_blue = ();
- for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}
. It might have
been a good idea to keep the blues all in a hash in the first place.
If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space:
- @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
- @is_tiny_prime = ();
- for (@primes) { $is_tiny_prime[$_] = 1 }
- # or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead:
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 equivalent
looks like this subroutine:
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.
If you want to actually extract the matching elements, simply use grep in list context.
Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array:
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.
With Perl 5.10 and later, the smart match operator can give you the answer with the least amount of work:
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.
For multilevel structures, you may wish to use an approach more
like this one. It uses the CPAN module FreezeThaw
:
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.
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".
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.
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.
(contributed by brian d foy)
Perl's arrays do not have a fixed size, so you don't need linked lists
if you just want to add or remove items. You can use array operations
such as push, pop, shift, unshift, or splice to do
that.
Sometimes, however, linked lists can be useful in situations where you want to "shard" an array so you have have many small arrays instead of a single big array. You can keep arrays longer than Perl's largest array index, lock smaller arrays separately in threaded programs, reallocate less memory, or quickly insert elements in the middle of the chain.
Steve Lembark goes through the details in his YAPC::NA 2009 talk "Perly
Linked Lists" ( http://www.slideshare.net/lembark/perly-linked-lists ),
although you can just use his LinkedList::Single
module.
(contributed by brian d foy)
If you want to cycle through an array endlessly, you can increment the index modulo the number of elements in the array:
You can also use Tie::Cycle
to use a scalar that always has the
next element of the circular array:
The Array::Iterator::Circular
creates an iterator object for
circular arrays:
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; # $deck is a reference to an array
- return unless @$deck; # must not be empty!
- 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
This is bad because splice is already O(N), and since you do it N times, you just invented a quadratic algorithm; that is, O(N**2). This does not scale, although Perl is so efficient that you probably won't notice this until you have rather largish arrays.
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:
- 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.
Use the rand() function (see rand):
- $index = rand @array;
- $element = $array[$index];
Or, simply:
Use the List::Permutor
module on CPAN. If the list is actually an
array, try the Algorithm::Permute
module (also on CPAN). It's
written in XS code and is very efficient:
For even faster execution, you could do:
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-Krause 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;
The Algorithm::Loops
module also provides the NextPermute
and
NextPermuteNum
functions which efficiently find all unique permutations
of an array, even if it contains duplicate values, modifying it in-place:
if its elements are in reverse-sorted order then the array is reversed,
making it sorted, and it returns false; otherwise the next
permutation is returned.
NextPermute
uses string order and NextPermuteNum
numeric order, so
you can enumerate all the permutations of 0..9
like this:
Supply a comparison function to sort() (described in sort):
- @list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would
sort (1, 2, 10)
into (1, 10, 2)
. <=>
, used above, is
the numerical comparison operator.
If you have a complicated function needed to pull out the part you want to sort on, then don't do it inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the same element. Here's an example of how to pull out the first word after the first number on each item, and then sort those words case-insensitively.
which could also be written this way, using a trick that's come to be known as the Schwartzian Transform:
If you need to sort on several fields, the following paradigm is useful.
- @sorted = sort {
- field1($a) <=> field1($b) ||
- field2($a) cmp field2($b) ||
- field3($a) cmp field3($b)
- } @data;
This can be conveniently combined with precalculation of keys as given above.
See the sort article in the "Far More Than You Ever Wanted To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for more about this approach.
See also the question later in perlfaq4 on sorting hashes.
Use pack() and unpack(), or else vec() and the bitwise
operations.
For example, you don't have to store individual bits in an array
(which would mean that you're wasting a lot of space). To convert an
array of bits to a string, use vec() to set the right bits. This
sets $vec
to have bit N set only if $ints[N]
was set:
- @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
- $vec = '';
- foreach( 0 .. $#ints ) {
- vec($vec,$_,1) = 1 if $ints[$_];
- }
The string $vec
only takes up as many bits as it needs. For
instance, if you had 16 entries in @ints
, $vec
only needs two
bytes to store them (not counting the scalar variable overhead).
Here's how, given a vector in $vec
, you can get those bits into
your @ints
array:
- sub bitvec_to_list {
- my $vec = shift;
- my @ints;
- # Find null-byte density then select best algorithm
- if ($vec =~ tr/\0// / length $vec > 0.95) {
- use integer;
- my $i;
- # This method is faster with mostly null-bytes
- while($vec =~ /[^\0]/g ) {
- $i = -9 + 8 * pos $vec;
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- push @ints, $i if vec($vec, ++$i, 1);
- }
- }
- else {
- # This method is a fast general algorithm
- use integer;
- my $bits = unpack "b*", $vec;
- push @ints, 0 if $bits =~ s/^(\d)// && $1;
- push @ints, pos $bits while($bits =~ /1/g);
- }
- return \@ints;
- }
This method gets faster the more sparse the bit vector is. (Courtesy of Tim Bunce and Winfried Koenig.)
You can make the while loop a lot shorter with this suggestion from Benjamin Goldberg:
Or use the CPAN module Bit::Vector
:
- $vector = Bit::Vector->new($num_of_bits);
- $vector->Index_List_Store(@ints);
- @ints = $vector->Index_List_Read();
Bit::Vector
provides efficient methods for bit vector, sets of
small integers and "big int" math.
Here's a more extensive illustration using vec():
- # vec demo
- $vector = "\xff\x0f\xef\xfe";
- print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
- unpack("N", $vector), "\n";
- $is_set = vec($vector, 23, 1);
- print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
- pvec($vector);
- set_vec(1,1,1);
- set_vec(3,1,1);
- set_vec(23,1,1);
- set_vec(3,1,3);
- set_vec(3,2,3);
- set_vec(3,4,3);
- set_vec(3,4,7);
- set_vec(3,8,3);
- set_vec(3,8,7);
- set_vec(0,32,17);
- set_vec(1,32,17);
- sub set_vec {
- my ($offset, $width, $value) = @_;
- my $vector = '';
- vec($vector, $offset, $width) = $value;
- print "offset=$offset width=$width value=$value\n";
- pvec($vector);
- }
- sub pvec {
- my $vector = shift;
- my $bits = unpack("b*", $vector);
- my $i = 0;
- my $BASE = 8;
- print "vector length in bytes: ", length($vector), "\n";
- @bytes = unpack("A8" x length($vector), $bits);
- print "bits are: @bytes\n\n";
- }
The short story is that you should probably only use defined on scalars or functions, not on aggregates (arrays and hashes). See defined in the 5.004 release or later of Perl for more detail.
(contributed by brian d foy)
There are a couple of ways that you can process an entire hash. You can get a list of keys, then go through each key, or grab a one key-value pair at a time.
To go through all of the keys, use the keys function. This extracts
all of the keys of the hash and gives them back to you as a list. You
can then get the value through the particular key you're processing:
Once you have the list of keys, you can process that list before you process the hash elements. For instance, you can sort the keys so you can process them in lexical order:
Or, you might want to only process some of the items. If you only want
to deal with the keys that start with text:
, you can select just
those using grep:
If the hash is very large, you might not want to create a long list of
keys. To save some memory, you can grab one key-value pair at a time using
each(), which returns a pair you haven't seen yet:
The each operator returns the pairs in apparently random order, so if
ordering matters to you, you'll have to stick with the keys method.
The each() operator can be a bit tricky though. You can't add or
delete keys of the hash while you're using it without possibly
skipping or re-processing some pairs after Perl internally rehashes
all of the elements. Additionally, a hash has only one iterator, so if
you use keys, values, or each on the same hash, you can reset
the iterator and mess up your processing. See the each entry in
perlfunc for more details.
(contributed by brian d foy)
Before you decide to merge two hashes, you have to decide what to do if both hashes contain keys that are the same and if you want to leave the original hashes as they were.
If you want to preserve the original hashes, copy one hash (%hash1
)
to a new hash (%new_hash
), then add the keys from the other hash
(%hash2
to the new hash. Checking that the key already exists in
%new_hash
gives you a chance to decide what to do with the
duplicates:
If you don't want to create a new hash, you can still use this looping
technique; just change the %new_hash
to %hash1
.
If you don't care that one hash overwrites keys and values from the other, you
could just use a hash slice to add one hash to another. In this case, values
from %hash2
replace values from %hash1
when they have keys in common:
(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.
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:
(contributed by brian d foy)
This is very similar to "How do I process an entire hash?", also in perlfaq4, but a bit simpler in the common cases.
You can use the keys() built-in function in scalar context to find out
have many entries you have in a hash:
If you want to find out how many entries have a defined value, that's
a bit different. You have to check each value. A grep is handy:
You can use that same structure to count the entries any way that you like. If you want the count of the keys with vowels in them, you just test for that instead:
The grep in scalar context returns the count. If you want the list
of matching items, just use it in list context instead:
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().
(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 compares them ASCIIbetically (which might be affected by your locale settings). The output list has the keys in ASCIIbetical order. Once we have the keys, we can go through them to create a report which lists the keys in ASCIIbetical order.
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.
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.
From there we can get more complex. If the hash values are the same, we can provide a secondary sort on the hash key.
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. Although this does keep your hash sorted, you might not
like the slowdown you suffer from the tie interface. Are you sure you
need to do this? :)
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 $hash{'a'} is true (Perl 5 only)
- grep ($_ eq 'a', keys %hash) is true
If you now say
- undef $hash{'a'}
your table now reads:
- keys values
- +------+------+
- | a | undef|
- | x | 7 |
- | d | 0 |
- | e | 2 |
- +------+------+
and these conditions now hold; changes in caps:
- $hash{'a'} is FALSE
- $hash{'d'} is false
- defined $hash{'d'} is true
- defined $hash{'a'} is FALSE
- exists $hash{'a'} is true (Perl 5 only)
- grep ($_ 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 $hash{'a'} is FALSE (Perl 5 only)
- grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
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.
(contributed by brian d foy)
You can use the keys or values functions to reset each. To
simply reset the iterator used by each without doing anything else,
use one of them in void context:
See the documentation for each in perlfunc.
First you extract the keys from the hashes into lists, then solve the "removing duplicates" problem described above. For example:
Or more succinctly:
- @uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
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. You might also try DBM::Deep, but it can be a bit slow.
Use the Tie::IxHash
from CPAN.
(contributed by brian d foy)
Are you using a really old version of Perl?
Normally, accessing a hash key's value for a nonexistent key will not create the key.
Passing $hash{ 'foo' }
to a subroutine used to be a special case, though.
Since you could assign directly to $_[0]
, Perl had to be ready to
make that assignment so it created the hash key ahead of time:
Since Perl 5.004, however, this situation is a special case and Perl creates the hash key only when you make the assignment:
However, if you want the old behavior (and think carefully about that because it's a weird side effect), you can pass a hash slice instead. Perl 5.004 didn't make this a special case:
- my_sub( @hash{ qw/foo/ } );
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 perlreftut. Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in perltoot.
(contributed by brian d foy and Ben Morrow)
Hash keys are strings, so you can't really use a reference as the key.
When you try to do that, perl turns the 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.
Remember that the entry in the hash will still be there even if the referenced variable goes out of scope, and that it is entirely possible for Perl to subsequently allocate a different variable at the same address. This will mean a new variable might accidentally be associated with the value for an old.
If you have Perl 5.10 or later, and you just want to store a value against the reference for lookup later, you can use the core Hash::Util::Fieldhash module. This will also handle renaming the keys if you use multiple threads (which causes all variables to be reallocated at new addresses, changing their stringification), and garbage-collecting the entries when the referenced variable goes out of scope.
If you actually need to be able to get a real reference back from each hash entry, you can use the Tie::RefHash module, which does the required work for you.
(contributed by brian d foy)
The trick to this problem is avoiding accidental autovivification. If you want to check three keys deep, you might naïvely try this:
Even though you started with a completely empty hash, after that call to
exists you've created the structure you needed to check for key3
:
- %hash = (
- 'key1' => {
- 'key2' => {}
- }
- );
That's autovivification. You can get around this in a few ways. The
easiest way is to just turn it off. The lexical autovivification
pragma is available on CPAN. Now you don't add to the hash:
The Data::Diver
module on CPAN can do it for you too. Its Dive
subroutine can tell you not only if the keys exist but also get the
value:
You can easily do this yourself too by checking each level of the hash
before you move onto the next level. This is essentially what
Data::Diver
does for you:
Perl is binary-clean, so it can handle binary data just fine.
On Windows or DOS, however, you have to use binmode for binary
files to avoid conversions for line endings. In general, you should
use binmode any time you want to work with binary data.
Also see binmode or perlopentut.
If you're concerned about 8-bit textual data then see perllocale. If you want to deal with multibyte characters, however, there are some gotchas. See the section on Regular Expressions.
Assuming that you don't care about IEEE notations like "NaN" or "Infinity", you probably just want to use a regular expression:
- use 5.010;
- given( $number ) {
- when( /\D/ )
- { say "\thas nondigits"; continue }
- when( /^\d+\z/ )
- { say "\tis a whole number"; continue }
- when( /^-?\d+\z/ )
- { say "\tis an integer"; continue }
- when( /^[+-]?\d+\z/ )
- { say "\tis a +/- integer"; continue }
- when( /^-?(?:\d+\.?|\.\d)\d*\z/ )
- { say "\tis a real number"; continue }
- when( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i)
- { say "\tis a C float" }
- }
There are also some commonly used modules for the task.
Scalar::Util (distributed with 5.8) provides access to perl's
internal function looks_like_number
for determining whether a
variable looks like a number. Data::Types exports functions that
validate data types using both the above and other regular
expressions. Thirdly, there is Regexp::Common
which has regular
expressions to match various types of numbers. Those three modules are
available from the CPAN.
If you're on a POSIX system, Perl supports the POSIX::strtod
function for converting strings to doubles (and also POSIX::strtol
for longs). 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?"
Or you could check out the String::Scanf module on the CPAN instead.
For some specific applications, you can use one of the DBM modules.
See AnyDBM_File. More generically, you should consult the FreezeThaw
or Storable
modules from CPAN. Starting from Perl 5.8 Storable
is part
of the standard distribution. Here's one example using Storable
's store
and retrieve
functions:
- use Storable;
- store(\%hash, "filename");
- # later on...
- $href = retrieve("filename"); # by ref
- %hash = %{ retrieve("filename") }; # direct to hash
The Data::Dumper
module on CPAN (or the 5.005 release of Perl) is great
for printing out data structures. The Storable
module on CPAN (or the
5.8 release of Perl), provides a function called dclone
that recursively
copies its argument.
- use Storable qw(dclone);
- $r2 = dclone($r1);
Where $r1
can be a reference to any kind of data structure you'd like.
It will be deeply copied. Because dclone
takes and returns references,
you'd have to add extra punctuation if you had a hash of arrays that
you wanted to copy.
- %newhash = %{ dclone(\%oldhash) };
(contributed by Ben Morrow)
You can use the UNIVERSAL
class (see UNIVERSAL). However, please
be very careful to consider the consequences of doing this: adding
methods to every object is very likely to have unintended
consequences. If possible, it would be better to have all your object
inherit from some common base class, or to use an object system like
Moose that supports roles.
Get the Business::CreditCard
module from CPAN.
The arrays.h/arrays.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.
See http://search.cpan.org/dist/PGPLOT for the code.
Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.