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Re: references best practice

by hipowls (Curate)
on Apr 25, 2008 at 11:33 UTC ( #682790=note: print w/ replies, xml ) Need Help??


in reply to references best practice

The first form returns a list, each item of which is copied. The other two forms copy a single scalar, a reference to an array. I would expect that either of these two to have similar performance.

Suspicion, however is no substitute for benchmarking so I ran the following script.

use warnings; use strict; use Benchmark qw(cmpthese); sub prepData1 { my @data = ( map { log $_ } 1 .. 100 ); return @data; } sub runPrep1 { my @result = prepData1(); } sub prepData2 { my $refData = shift; $refData = [ map { log $_ } 1 .. 100 ]; } sub runPrep2 { my @result; prepData2( \@result ); } sub prepData3 { my @data = ( map { log $_ } 1 .. 100 ); return \@data; } sub runPrep3 { my $result = prepData3(); } sub prepData4 { my $data = [ map { log $_ } 1 .. 100 ]; return $data; } sub runPrep4 { my $result = prepData4(); } cmpthese( -5, # Run each function for at least 5 seconds { array_out => \&runPrep1, array_ref_in => \&runPrep2, array_ref_out1 => \&runPrep3, array_ref_out2 => \&runPrep4, } ); __END__ Rate array_out array_ref_out1 array_ref_in arra +y_ref_out2 array_out 11270/s -- -30% -43% + -44% array_ref_out1 16137/s 43% -- -19% + -20% array_ref_in 19873/s 76% 23% -- + -1% array_ref_out2 20101/s 78% 25% 1% + --
I added a fourth style since that is the form I prefer where the data is directly put in an array reference. Once again benchmarking proves me wrong;-) That's why I always do it rather than guess.

Update 1: I prefer my idiom because I create a single container of related values but I can't pass that back since the function returns a list of values. To retain the relationship between them I use an array reference so that the caller of the function gets back a group of related values. If, for some reason, I need to extend the function to return a second group of data I can pass back two references thereby maintaining the logical grouping of data.

Update 2: As Haarg kindly pointed out I got the second case wrong, I was creating a new anonymous array reference and assigning it to the array ref that was passed int. Updating the code & benchmarks I get

sub prepData2 { my $refData = shift; @$refData = ( map { log $_ } 1 .. 100 ); } __END__ Rate array_out array_ref_out1 array_ref_in arra +y_ref_out2 array_out 11801/s -- -28% -28% + -39% array_ref_out1 16406/s 39% -- -1% + -15% array_ref_in 16496/s 40% 1% -- + -14% array_ref_out2 19238/s 63% 17% 17% + --
Looks my original guess was correct which just shows if benchmarking gives an unexpected result you should check both your assumptions and your benchmarking.


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Re^2: references best practice
by amarquis (Curate) on Apr 25, 2008 at 18:47 UTC

    Benchmarking doesn't prove you wrong, it proves that something might be slower in a given case.

    If I had to choose between a marginal speed increase an an idiom I am comfortable with, I'd say the latter is correct. (Until I have proof that speed is a problem and profiling tells me this is the bottleneck).

Re^2: references best practice
by Haarg (Chaplain) on Apr 25, 2008 at 22:51 UTC
    Your second example isn't doing what you intend. The array outside of the sub isn't modified. The first line of prepData2 sets $refData to the incoming reference. The second replaces that reference with a different one, leaving the contents of the first reference unchanged. You'd want something more like:
    sub prepData2 { my $refData = shift; @$refData = map { log $_ } 1 .. 100; }
    With that change, the benchmark changes somewhat, making it almost identical to array_ref_out1 in my tests.

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