From a different post today i used the example of list assignment and scalar assignment as being less and more efficient respectively. 
use Benchmark 'cmpthese';

my ($x,$y)=(1,2);

cmpthese(-2,
         {list  =>sub {
                       ($x,$y)=($y,$x) #list assign, slow
                      },
          scalar=>sub{
                      #three lines. fast.
                      my $tmp=$x;
                      $x=$y;
                      $y=$tmp
                     }
         }
        );

__END__

Benchmark: running list, scalar, each for at least 2 CPU seconds...
      list:  ( 2.34 usr +  0.00 sys =  2.34 CPU) @ 550148.10/s (n=1288
+997)
    scalar:  ( 2.11 usr +  0.00 sys =  2.11 CPU) @ 878828.44/s (n=1854
+328)
           Rate   list scalar
list   550148/s     --   -37%
scalar 878828/s    60%     --

[download]
At bare minimum what you are doing with a hash slice is using list context. As the above shows list assignment is slower than the equivelent scalar assignments. The reason, once you think about how list assignment works is easy to understand. The right side of the assignment is evaluated and the results are placed into a temporary array/list. Then these results are poured into the variables on the left. This means that three temp variables (the list, and the two values) are created. In the scalar version only one temp variable is created. Thus the speed difference.

It wouldn't suprise me that there isn't really a good way to make list assignment as fast as scalar assignment is, irrelevent of the implementation language used, because of the semantic implications of the operation and the wide number of circumstances it needs to work within.

Yves
--
You are not ready to use symrefs unless you already know why they are bad. -- tadmc (CLPM)

In reply to Re: Considering hash slices, past, present and future by demerphq
in thread Considering hash slices, past, present and future by grinder

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