X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq4.pod;h=18d709169b3fdfe90ca9271405df93d609f798b5;hb=14aaa8fc27b8350048cdee657c0128eb979d0b2a;hp=b5305164318b2745c1c2037523445e7f6d3cdaf8;hpb=881bdbd4cff2623b5a5979fcc7b7c3078938e0fd;p=p5sagit%2Fp5-mst-13.2.git

diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod
index b530516..18d7091 100644
--- a/pod/perlfaq4.pod
+++ b/pod/perlfaq4.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
-perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date: 2002/03/11 22:15:19 $)
+perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 2002/05/30 07:04:25 $)
 
 =head1 DESCRIPTION
 
@@ -135,7 +135,9 @@ 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.
 
-=item B<How do I convert Hexadecimal into decimal:>
+=over 4
+
+=item How do I convert hexadecimal into decimal
 
 Using perl's built in conversion of 0x notation:
 
@@ -158,7 +160,7 @@ Using the CPAN module Bit::Vector:
     $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
     $dec = $vec->to_Dec();
 
-=item B<How do I convert from decimal to hexadecimal:>
+=item How do I convert from decimal to hexadecimal
 
 Using sprint:
 
@@ -181,7 +183,7 @@ And Bit::Vector supports odd bit counts:
     $vec->Resize(32); # suppress leading 0 if unwanted
     $hex = $vec->to_Hex();
 
-=item B<How do I convert from octal to decimal:>
+=item How do I convert from octal to decimal
 
 Using Perl's built in conversion of numbers with leading zeros:
 
@@ -200,7 +202,7 @@ Using Bit::Vector:
     $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
     $dec = $vec->to_Dec();
 
-=item B<How do I convert from decimal to octal:>
+=item How do I convert from decimal to octal
 
 Using sprintf:
 
@@ -212,7 +214,12 @@ Using Bit::Vector
     $vec = Bit::Vector->new_Dec(32, -559038737);
     $oct = reverse join('', $vec->Chunk_List_Read(3));
 
-=item B<How do I convert from binary to decimal:>
+=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
 
@@ -231,7 +238,7 @@ Using Bit::Vector:
     $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
     $dec = $vec->to_Dec();
 
-=item B<How do I convert from decimal to binary:>
+=item How do I convert from decimal to binary
 
 Using unpack;
 
@@ -246,6 +253,7 @@ Using Bit::Vector:
 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?
 
@@ -1335,28 +1343,21 @@ lists, or you could just do something like this with an array:
 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
-    # generate a random permutation of an array in place
-    # As in shuffling a deck of cards
-    #
     sub fisher_yates_shuffle {
         my $deck = shift;  # $deck is a reference to an array
         my $i = @$deck;
-        while (--$i) {
+        while ($i--) {
             my $j = int rand ($i+1);
             @$deck[$i,$j] = @$deck[$j,$i];
         }
     }
 
-And here is an example of using it:
-
-    #
     # shuffle my mpeg collection
     #
     my @mpeg = <audio/*/*.mp3>;
@@ -1398,13 +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>):
@@ -1542,6 +1550,13 @@ 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.)
 
+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);
@@ -1695,15 +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
 	+------+------+
@@ -1715,16 +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:
 
@@ -1739,18 +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:
 
@@ -1763,23 +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?
 
@@ -1908,6 +1924,10 @@ Assuming that you don't care about IEEE notations like "NaN" or
    if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
 			{ 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>
 wrapper function for more convenient access.  This function takes
@@ -1931,9 +1951,9 @@ 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?