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Re^5: better array to hash conversion

by tobyink (Canon)
on Dec 11, 2012 at 16:24 UTC ( [id://1008335]=note: print w/replies, xml ) Need Help??

in reply to Re^4: better array to hash conversion
in thread better array to hash conversion

The following map variant works out significantly faster than the previously posted map variants, though not as fast as for-loop based variants:

sub variation3ep { my $idx = -1; my %hash = map(($_ => ++$idx), @array); }

While the block form of map is often favoured for its clarity, the expression form of map is faster because it avoids the overhead of creating a lexical pad.

This is actually another reason why BrowserUk's variation4 is fast - he uses the for statement modifier rather than a for loop with a block. Compare:

#!/usr/bin/env perl use strict; use warnings; use Benchmark qw(:all); my @array='aa' .. 'zz'; sub variation4 { my $idx = 0; my %hash; $hash{ $_ } = $idx++ for @array; } sub variation5 { my $idx = 0; my %hash; for (@array) { $hash{ $_ } = $idx++; } } cmpthese(-3, { 'variation4' => \&variation4, 'variation5' => \&variation5, }); __END__ Rate variation5 variation4 variation5 740/s -- -4% variation4 773/s 5% --

Yes, it's a small difference, but it's pretty consistently observable.

The absolute fastest I've been able to achieve is a small variation on BrowserUk's variation4 using the prefix increment rather than postfix increment:

sub variation6 { my $idx = -1; my %hash; $hash{ $_ } = ++$idx for @array; }

It seems to give you about a 6% speed up.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

Replies are listed 'Best First'.
Re^6: better array to hash conversion (wrong metric)
by tye (Sage) on Dec 11, 2012 at 16:44 UTC

    In the unlikely event of having hundreds of thousands of entries, the run-time will be a fraction of a second faster (and nobody will notice or care). In the very unlikely event of this particular operation being even as much as 10% of the total run-time, the difference between the worst and the best shown so far will result in a 5% reduction in total run-time (which nobody will notice or care about).

    - tye        

      Thank you very much and best regards, Karl

      «The Crux of the Biscuit is the Apostrophe»

Re^6: better array to hash conversion
by karlgoethebier (Abbot) on Dec 12, 2012 at 07:31 UTC

    Thank you very much and best regards, Karl

    «The Crux of the Biscuit is the Apostrophe»

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