=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.2 $, $Date: 2001/09/26 15:42:12 $)
+perlfaq4 - Data Manipulation ($Revision: 1.24 $, $Date: 2002/05/20 16:50:08 $)
=head1 DESCRIPTION
-The section of the FAQ answers questions related to the manipulation
-of data as numbers, dates, strings, arrays, hashes, and miscellaneous
-data issues.
+This section of the FAQ answers questions related to manipulating
+numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
=head1 Data: Numbers
machines) will work pretty much like mathematical integers. Other numbers
are not guaranteed.
-=head2 How do I convert bits into ints?
+=head2 How do I convert between numeric representations?
-To turn a string of 1s and 0s like C<10110110> into a scalar containing
-its binary value, use the pack() and unpack() functions (documented in
-L<perlfunc/"pack"> and L<perlfunc/"unpack">):
+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.
- $decimal = unpack('c', pack('B8', '10110110'));
+Some of the examples below use the Bit::Vector module from CPAN.
+The reason you might choose Bit::Vector over the perl built in
+functions is that it works with numbers of ANY size, that it is
+optimized for speed on some operations, and for at least some
+programmers the notation might be familiar.
-This packs the string C<10110110> into an eight bit binary structure.
-This is then unpacked as a character, which returns its ordinal value.
+=over 4
+
+=item How do I convert hexadecimal into decimal
+
+Using perl's built in conversion of 0x notation:
+
+ $int = 0xDEADBEEF;
+ $dec = sprintf("%d", $int);
+
+Using the hex function:
+
+ $int = hex("DEADBEEF");
+ $dec = sprintf("%d", $int);
+
+Using pack:
+
+ $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
+ $dec = sprintf("%d", $int);
+
+Using the CPAN module Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
+ $dec = $vec->to_Dec();
+
+=item How do I convert from decimal to hexadecimal
+
+Using sprint:
+
+ $hex = sprintf("%X", 3735928559);
+
+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();
+
+=item How do I convert from octal to decimal
+
+Using Perl's built in conversion of numbers with leading zeros:
+
+ $int = 033653337357; # note the leading 0!
+ $dec = sprintf("%d", $int);
+
+Using the oct function:
+
+ $int = oct("33653337357");
+ $dec = sprintf("%d", $int);
+
+Using Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new(32);
+ $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
+ $dec = $vec->to_Dec();
+
+=item How do I convert from decimal to octal
+
+Using sprintf:
+
+ $oct = sprintf("%o", 3735928559);
+
+Using Bit::Vector
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $oct = reverse join('', $vec->Chunk_List_Read(3));
-This does the same thing:
+=item How do I convert from binary to decimal
+
+Perl 5.6 lets you write binary numbers directly with
+the 0b notation:
+
+ $number = 0b10110110;
+
+Using pack and ord
$decimal = ord(pack('B8', '10110110'));
-Here's an example of going the other way:
+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();
+
+=item How do I convert from decimal to binary
- $binary_string = unpack('B*', "\x29");
+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.
+
+=back
=head2 Why doesn't & work the way I want it to?
=head2 How can I output Roman numerals?
-Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
+Get the http://www.cpan.org/modules/by-module/Roman module.
=head2 Why aren't my random numbers random?
than more.
Computers are good at being predictable and bad at being random
-(despite appearances caused by bugs in your programs :-).
-http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom
-Phoenix, talks more about this. John von Neumann said, ``Anyone who
-attempts to generate random numbers by deterministic means is, of
+(despite appearances caused by bugs in your programs :-). see the
+F<random> artitcle in the "Far More Than You Ever Wanted To Know"
+collection in http://www.cpan.org/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 C<rand> with C<srand>
pseudorandom generator than comes with your operating system, look at
``Numerical Recipes in C'' at http://www.nr.com/ .
+=head2 How do I get a random number between X and Y?
+
+Use the following simple function. It selects a random integer between
+(and possibly including!) the two given integers, e.g.,
+C<random_int_in(50,120)>
+
+ sub random_int_in ($$) {
+ my($min, $max) = @_;
+ # Assumes that the two arguments are integers themselves!
+ return $min if $min == $max;
+ ($min, $max) = ($max, $min) if $min > $max;
+ return $min + int rand(1 + $max - $min);
+ }
+
=head1 Data: Dates
=head2 How do I find the week-of-the-year/day-of-the-year?
@( = ('(','');
@) = (')','');
($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
- @$ = (eval{/$re/},$@!~/unmatched/);
+ @$ = (eval{/$re/},$@!~/unmatched/i);
print join("\n",@$[0..$#$]) if( $$[-1] );
=head2 How do I reverse a string?
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}' $*"
+
+See the documentation for Text::Autoformat to appreciate its many
+capabilities.
+
=head2 How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
while ($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;
+
=head2 How do I capitalize all the words on one line?
To make the first letter of each word upper case:
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--/;
+ $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.
That being said, there are several ways to approach this. If you
are going to make this query many times over arbitrary string values,
-the fastest way is probably to invert the original array and keep an
-associative array lying about whose keys are the first array's values.
+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/;
- undef %is_blue;
+ %is_blue = ();
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might have been a
array. This kind of an array will take up less space:
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
- undef @is_tiny_prime;
+ @is_tiny_prime = ();
for (@primes) { $is_tiny_prime[$_] = 1 }
# or simply @istiny_prime[@primes] = (1) x @primes;
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';
+ use List::Util 'shuffle';
@shuffled = shuffle(@list);
-If not, you can use this:
+If not, you can use a Fisher-Yates shuffle.
- # fisher_yates_shuffle( \@array ) :
- # generate a random permutation of @array in place
sub fisher_yates_shuffle {
- my $array = shift;
- my $i = @$array;
- while (--$i) {
+ my $deck = shift; # $deck is a reference to an array
+ my $i = @$deck;
+ while ($i--) {
my $j = int rand ($i+1);
- @$array[$i,$j] = @$array[$j,$i];
+ @$deck[$i,$j] = @$deck[$j,$i];
}
}
- fisher_yates_shuffle( \@array ); # permutes @array in place
+ # 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
$_ *= (4/3) * 3.14159; # this will be constant folded
}
-If you want to do the same thing to modify the values of the hash,
-you may not use the C<values> function, oddly enough. You need a slice:
+If you want to do the same thing to modify the values of the
+hash, you can use the C<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 ( @orbits{keys %orbits} ) {
+ for $orbit ( values %orbits ) {
($orbit **= 3) *= (4/3) * 3.14159;
}
+Prior to perl 5.6 C<values> returned copies of the values,
+so older perl code often contains constructions such as
+C<@orbits{keys %orbits}> instead of C<values %orbits> where
+the hash is to be modified.
+
=head2 How do I select a random element from an array?
Use the rand() function (see L<perlfunc/rand>):
This can be conveniently combined with precalculation of keys as given
above.
-See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
-this approach.
+See the F<sort> artitcle article in the "Far More Than You Ever Wanted
+To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz for
+more about this approach.
See also the question below on sorting hashes.
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
-And here's how, given a vector in $vec, you can
+Here's how, given a vector in $vec, you can
get those bits into your @ints array:
sub bitvec_to_list {
This method gets faster the more sparse the bit vector is.
(Courtesy of Tim Bunce and Winfried Koenig.)
-Here's a demo on how to use vec():
+You can make the while loop a lot shorter with this suggestion
+from Benjamin Goldberg:
+
+ while($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():
# vec demo
$vector = "\xff\x0f\xef\xfe";
=head2 How can I know how many entries are in a hash?
If you mean how many keys, then all you have to do is
-take the scalar sense of the keys() function:
+use the keys() function in a scalar context:
- $num_keys = scalar keys %hash;
+ $num_keys = keys %hash;
-The keys() function also resets the iterator, which in void context is
-faster for tied hashes than would be iterating through the whole
-hash, one key-value pair at a time.
+The keys() function also resets the iterator, which means that you may
+see strange results if you use this between uses of other hash operators
+such as each().
=head2 How do I sort a hash (optionally by value instead of key)?
=head2 What's the difference between "delete" and "undef" with hashes?
-Hashes are pairs of scalars: the first is the key, the second is the
-value. The key will be coerced to a string, although the value can be
-any kind of scalar: string, number, or reference. If a key C<$key> is
-present in the array, C<exists($key)> will return true. The value for
-a given key can be C<undef>, in which case C<$array{$key}> will be
-C<undef> while C<$exists{$key}> will return true. This corresponds to
-(C<$key>, C<undef>) being in the hash.
+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 $key is present in
+%hash, C<exists($hash{$key})> will return true. The value
+for a given key can be C<undef>, in which case
+C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
+will return true. This corresponds to (C<$key>, C<undef>)
+being in the hash.
-Pictures help... here's the C<%ary> table:
+Pictures help... here's the %hash table:
keys values
+------+------+
And these conditions hold
- $ary{'a'} is true
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is true
- exists $ary{'a'} is true (Perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $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 $ary{'a'}
+ undef $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is FALSE
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is FALSE
- exists $ary{'a'} is true (Perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $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 $ary{'a'}
+ delete $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is false
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is false
- exists $ary{'a'} is FALSE (Perl5 only)
- grep ($_ eq 'a', keys %ary) is FALSE
+ $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!
=head2 Why don't my tied hashes make the defined/exists distinction?
-They may or may not implement the EXISTS() and DEFINED() methods
-differently. For example, there isn't the concept of undef with hashes
-that are tied to DBM* files. This means the true/false tables above
-will give different results when used on such a hash. It also means
-that exists and defined do the same thing with a DBM* file, and what
-they end up doing is not what they do with ordinary hashes.
+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.
=head2 How do I reset an each() operation part-way through?
=head2 How can I use a reference as a hash key?
-You can't do this directly, but you could use the standard Tie::Refhash
+You can't do this directly, but you could use the standard Tie::RefHash
module distributed with Perl.
=head1 Data: Misc
if (/^-?\d+$/) { print "is an integer\n" }
if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
- if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
+ if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
- { print "a C float" }
+ { print "a C float\n" }
+
+You can also use the L<Data::Types|Data::Types> module on
+the CPAN, which exports functions that validate data types
+using these and other regular expressions.
If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
function. Its semantics are somewhat cumbersome, so here's a C<getnum>
sub is_numeric { defined getnum($_[0]) }
-Or you could check out the String::Scanf module on CPAN instead. The
-POSIX module (part of the standard Perl distribution) provides the
-C<strtod> and C<strtol> for converting strings to double and longs,
+Or you could check out the L<String::Scanf|String::Scanf> module on the CPAN
+instead. The POSIX module (part of the standard Perl distribution) provides
+the C<strtod> and C<strtol> for converting strings to double and longs,
respectively.
=head2 How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM modules.
-See L<AnyDBM_File>. More generically, you should consult the FreezeThaw,
-Storable, or Class::Eroot modules from CPAN. Starting from Perl 5.8
-Storable is part of the standard distribution. Here's one example using
-Storable's C<store> and C<retrieve> functions:
+See L<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 C<store>
+and C<retrieve> functions:
use Storable;
store(\%hash, "filename");
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
This documentation is free; you can redistribute it and/or modify it
projects / p5sagit/p5-mst-13.2.git / blobdiff