map efficiency

snowcrash has asked for the wisdom of the Perl Monks concerning the following question:

hi fellow monks!

please explain the enormous difference in computation time for the following two statements:

my @data = ( 0 .. 50000);
my %hash = () ;

### version 1
map { $hash{$_}++ } @data;

### version 2
%hash = map { $_ => 1 } @data;

### version 3
### as fast as version 1
%hash = map { $_ => } @data
[download]

version 1 is about 100 times faster on my machine... i'm running 5.005_03 built for i386-linux.

thanks,
snowcrash //////

Comment on map efficiency Download Code

Replies are listed 'Best First'.
(tye)Re: map efficiency by tye (Sage) on Jan 31, 2001 at 00:17 UTC
Turn on warnings and you'll probably realize that trailing commas are silently ignored and that `=>` is a "fat comma". With warnings, your last example complains "Odd number of elements in hash assignment" and doesn't do what you thought. It is a "bug" that map is too slow when more than one item is output per input item. This bug will be fixed when perl 5.6.1 or perl 5.8 becomes available. - tye (but my friends call me "Tye")	[reply] [d/l]
Re: map efficiency by japhy (Canon) on Jan 31, 2001 at 00:17 UTC
Because the second two are making a rather large list and copying it over to a hash. That sounds like it would take a bit of work. `japhy` -- Perl and Regex Hacker	[reply]
A reply falls below the community's threshold of quality. You may see it by logging in.