in reply to When should I use map, for?

So, I might be mistaken here, but:

sub { my @e = map { $_ if (s/0/./g || 1) } @elem }

modifies global @elem in place. The s///g acts on $_, which is an alias to the list item. After the first iteration of mapc, you've altered the sample data. Not sure that matters, as this is a /g it has to inspect all the characters regardless.

As has been mentioned already, the map versions both have two needless logical branches, which taints the results.

A better comparison might be something that doesn't act in-place. Like an addition: 

my @b = map { $_ + 2 } @elem;
# vs
my @b;
for ( @elem ) { push @b, $_ + 2; }

# -> results:
#             Rate       map       for
# map       60.3/s        --      -23%
# for       78.2/s       30%        --

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Re^2: When should I use map, for?
by radiantmatrix (Parson) on May 19, 2005 at 21:06 UTC
    That particular map does not modify @elem in-place. Rather, it copies one element at a time, leading to the same result. The needless logical branch isn't, because map would otherwise return a list of the results of the s///.

    I'm aware that the extra logical branch adds a small amount of complexity; However, the || short-circuts, so if a substitute is made, the '1' isn't evalated. Also, the '1' doesn't take a lot of time to evaluate. It's also needed to accomplish the substitution in that way. Even in your results the for is notably faster -- so your point seems moot anyhow.

    When is map more appropriate (faster) than for, and vice-versa?

    The Eightfold Path: 'use warnings;', 'use strict;', 'use diagnostics;', perltidy, CGI or CGI::Simple, try the CPAN first, big modules and small scripts, test first.

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      Have you tried it? It copies -and- modifies -both-. 

      my @elem = ( 1..20 );
my @e = map { $_ if (s/0/./g || 1) } @elem;
print join( " ", @elem ), "\n";
__END__
1 2 3 4 5 6 7 8 9 1. 11 12 13 14 15 16 17 18 19 2.

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      I understand -why- you put the logical path there. Just saying s/0/./g; $_; makes more sense than the if and or.

      But you are quite right. It is moot. map is still slower, though arguably clearer in the $_ + 2 case. I don't know that map {} is ever an optimization.

      I would use grep to filter records, or map to preprocess going into a sort. I find that makes sense to me coding-wise. I doubt that it is better speed-wise.

      addendum:

      Okay, here is a case where I think map is cleaner and (slightly) faster: 

      my @elem = map { int rand 1000 } 1..10000;

sub map_s {
   my @mod_sorted = sort { $a <=> $b } map { $_ % 2**6 } @elem;
}

sub for_s {
   my @a;
   for ( @elem )
   {
      push @a, $_ % 2**6;
   }
   my @mod_sorted = sort { $a <=> $b } @a;
}
__END__
      Rate for map
for 15.8/s  -- -4%
map 16.5/s  4%  --

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        In your for_s() test, the creation of the extra array may taint the results. What happens to that test when you eliminate the sort, and just create an array of the mod values?
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