On 8/24/07, Dan Sopher <[EMAIL PROTECTED]> wrote:
> Aside from the syntax, is there a difference in the way 'map' and
> 'foreach' process?
snip
There are many differences, in addition to what has already been said
map can be more or less cpu efficient than depending on the task*. If
you can use map to avoid a temporary array, then it can save you some
time, but if you need the intermediate result, then for is faster.
Personally, I prefer map (and its cousin grep) for things that can be
done in one line:
my @files = map { $_->{filename} } $sftp->ls();
rather than
my @files;
push @files, $_->{filename} for $sftp->ls();
But if the code being passed to map becomes to complex, I prefer a for loop:
my @headers;
for my $filename (@filenames) {
open my $file, "<", $filename
or die "could not open $filename:$!";
push @headers, scalar <$file>;
}
rather than
my @headers = map { open my $f, "<", $_ or die "could not open $_:$!";
scalar <$_>; } @filenames;
* Here are the benchmarks for a two different scenarios
Map vs for vs map with temp variable
Rate map_with_var for map
map_with_var 3.51/s -- -11% -57%
for 3.92/s 12% -- -52%
map 8.11/s 131% 107% --
#!/usr/bin/perl
use strict;
use warnings;
use Benchmark;
my @a = 1 .. 1_000_000;
my %subs = (
for => sub {
my @b;
push @b, ($_ + 4) for @a;
return @b;
},
map => sub {
return map { $_ + 4 } @a;
},
map_with_var => sub {
my @b = map { $_ + 4 } @a;
return @b;
}
);
Benchmark::cmpthese(-1, \%subs);
Map of Map vs Two for loops vs Map of Map with temp variable vs map of
map with two temp variables:
Rate map_with_2var for map_with_var map
map_with_2var 1793/s -- -8% -16% -45%
for 1950/s 9% -- -8% -40%
map_with_var 2124/s 18% 9% -- -35%
map 3258/s 82% 67% 53% --
#!/usr/bin/perl
use strict;
use warnings;
use Benchmark;
my @a = 1 .. 1_000;
my %subs = (
for => sub {
my @b;
push @b, ($_ + 4) for @a;
my @c;
push @c, ($_ + 4) for @b;
return @b;
},
map => sub {
return map { $_ + 4 } map { $_ + 4 } @a;
},
map_with_var => sub {
my @b = map { $_ + 4 } map { $_ + 4 } @a;
return @b;
},
map_with_2var => sub {
my @b = map { $_ + 4 } @a;
my @c = map { $_ + 4 } @b;
return @c;
}
);
Benchmark::cmpthese(-1, \%subs);
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