X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq4.pod;h=18d709169b3fdfe90ca9271405df93d609f798b5;hb=14aaa8fc27b8350048cdee657c0128eb979d0b2a;hp=08f23f0b7ccea8a36d5512c8528b492cff15103c;hpb=2a2bf5f4414cf2a1984ea82a90bfbb2c3384d4e1;p=p5sagit%2Fp5-mst-13.2.git

diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod
index 08f23f0..18d7091 100644
--- a/pod/perlfaq4.pod
+++ b/pod/perlfaq4.pod
@@ -1,12 +1,11 @@
 =head1 NAME
 
-perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $)
+perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 2002/05/30 07:04:25 $)
 
 =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
 
@@ -123,24 +122,138 @@ 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.
 
-=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));
+
+=item How do I convert from binary to decimal
+
+Perl 5.6 lets you write binary numbers directly with
+the 0b notation:
+
+	$number = 0b10110110;
 
-This does the same thing:
+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
 
-    $binary_string = unpack('B*', "\x29");
+    $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
+
+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?
 
@@ -216,7 +329,7 @@ will not create a list of 500,000 integers.
 
 =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?
 
@@ -227,10 +340,11 @@ call C<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 :-).
-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>
@@ -240,6 +354,20 @@ 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/ .
 
+=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?
@@ -475,7 +603,7 @@ really does work:
     @( = ('(','');
     @) = (')','');
     ($re=$_)=~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?
@@ -507,6 +635,15 @@ 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.
+
 =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
@@ -575,6 +712,11 @@ integers:
     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:
@@ -583,7 +725,7 @@ To make the first letter of each word upper case:
 
 This has the strange effect of turning "C<don't do it>" into "C<Don'T
 Do It>".  Sometimes you might want this.  Other times you might need a
-more thorough solution (Suggested by brian d.  foy):
+more thorough solution (Suggested by brian d foy):
 
     $string =~ s/ (
                  (^\w)    #at the beginning of the line
@@ -860,7 +1002,7 @@ in the indentation.
             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.
@@ -1010,11 +1152,11 @@ designed to answer this question quickly and efficiently.  Arrays aren't.
 
 That being said, there are several ways to approach this.  If you
 are going to make this query many times over arbitrary string values,
-the fastest way is probably to invert the original array and 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
@@ -1024,7 +1166,7 @@ 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);
-    undef @is_tiny_prime;
+    @is_tiny_prime = ();
     for (@primes) { $is_tiny_prime[$_] = 1 }
     # or simply  @istiny_prime[@primes] = (1) x @primes;
 
@@ -1198,20 +1340,33 @@ lists, or you could just do something like this with an array:
 
 =head2 How do I shuffle an array randomly?
 
-Use this:
+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.
 
-    # fisher_yates_shuffle( \@array ) : 
-    # generate a random permutation of @array in place
     sub fisher_yates_shuffle {
-        my $array = shift;
-        my $i;
-        for ($i = @$array; --$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
+a new shuffled list.
 
 You've probably seen shuffling algorithms that work using splice,
 randomly picking another element to swap the current element with
@@ -1244,13 +1399,20 @@ Here's another; let's compute spherical volumes:
 	$_ *= (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>):
@@ -1291,6 +1453,12 @@ in the permute() function should work on any list:
 	}
     }
 
+Unfortunately, this algorithm is very inefficient. The Algorithm::Permute
+module from CPAN runs at least an order of magnitude faster. If you don't
+have a C compiler (or a binary distribution of Algorithm::Permute), then
+you can use List::Permutor which is written in pure Perl, and is still
+several times faster than the algorithm above.
+
 =head2 How do I sort an array by (anything)?
 
 Supply a comparison function to sort() (described in L<perlfunc/sort>):
@@ -1332,8 +1500,9 @@ If you need to sort on several fields, the following paradigm is useful.
 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.
 
@@ -1346,7 +1515,7 @@ For example, this sets $vec to have bit N set if $ints[N] was set:
     $vec = '';
     foreach(@ints) { vec($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 {
@@ -1381,7 +1550,23 @@ get those bits into your @ints array:
 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";
@@ -1485,13 +1670,13 @@ worry you, you can always reverse the hash into a hash of arrays instead:
 =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)?
 
@@ -1525,15 +1710,17 @@ The Tie::IxHash module from CPAN might also be instructive.
 
 =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
 	+------+------+
@@ -1545,16 +1732,16 @@ Pictures help...  here's the C<%ary> table:
 
 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:
 
@@ -1569,18 +1756,18 @@ 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:
 
@@ -1593,23 +1780,22 @@ 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?
 
@@ -1699,7 +1885,7 @@ in L<perltoot>.
 
 =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
@@ -1734,9 +1920,13 @@ Assuming that you don't care about IEEE notations like "NaN" or
    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>
@@ -1761,18 +1951,18 @@ if you just want to say, ``Is this a float?''
 
     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");
@@ -1813,7 +2003,7 @@ the PDL module from CPAN instead--it makes number-crunching easy.
 
 =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