Prototypes in perl5 have gotten a bit of a bad reputation,
some of which is deserved. If people ask me why we should
use them at all, however, I do have an example that always
respond with map2, which is like the built-in
map except that it accepts two arrays as
arguments (instead of a single list).
=item my @result = map2 BLOCK ARRAY, ARRAY;
Similar to c<map>, but operate on two lists at once. Inside the
BLOCK, the value are passed in as C<$a> and C<$b> (as in C<sort>).
my @a = ( 1, 2, 5, 7, );
my @b = ( 3, 4, 6, 8 );
my @sums = map2 { $a+$b } @a, @b;
my @prods = map2 { $a*$b } @a, @b;
One use is to initialize hashes from two arrays. You can do this in
straight perl with:
my %hash; @hash{@hash_keys} = @values;
With this subroutine, you could also do:
my %hash = map2 { ( $a => $b ) } @hash_keys, @values;
If the arrays are not the same length, the shorter array is used to
limit how far we go in each array.
BLOCK is evaluated in list context.
=cut
sub map2 ( & \@ \@ )
{
my ( $code_ref, $a_ref, $b_ref ) = @_;
my $rv_len = ( @$a_ref < @$b_ref ) ? @$a_ref : @$b_ref;
my @rv;
for ( my $i = 0; $i < $rv_len; ++$i )
{
local ( $a, $b ) = ( $a_ref->[$i], $b_ref->[$i] );
push @rv, $code_ref->();
}
return @rv;
}
I know I saw something like this from Abigail (or is
that Abigail-II?) in the late '90s, but I have since
reimplemented it a few times. As I said, it serves as
a handy example to demonstrate how prototypes make some
things possible that would otherwise be difficult to
impossible to code.
Update (2004-05-02 00:15 -0700):
Incorporated some suggestions from Zaxo to fix
some misleading variable names. Thanks!
