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in reply to Re^4: Abusing Map (Corrected second code block)
in thread Abusing Map

Ignore this - it's the same error as the GP, sorry. I've now modified the script in the previous post to include this as the fourth comparison. It does look to be the most efficient as you suspected.

I've just tried that but it errors out with Modification of non-creatable array value attempted, subscript -2 at (eval 11) line 1.. Here's the code:

#!/usr/bin/env perl 
use strict;
use warnings;
use Benchmark 'cmpthese';

for my $arrsize (1e4, 1e6, 1e8) {
    print "Source array has $arrsize elements\n";
    my @x;
    push @x, rand for 1..$arrsize;

    cmpthese (1e7, {
        'BUK' => ' my @y; $y[ $_-1 ] = $x[ $_-1 ] - $x[ $_ ] for 1 .. 
+$#x; ',
        'ike1' => ' my @y; $y[ $_ ] = $x[ $_ ] - $x[ $_+1 ] for 0 .. $
+#x-1; ',
        'ike2' => ' my @y = @x; $y[ $_ ] -= $y[ $_+1 ] for 0 .. $#y-1;
+ pop @y; ',
        'while' => ' my @y; my $i = $#x; $y[ $i ] = $x[ $i ] + $x[ --$
+i ] while $i; ',
    });
    print '-' x 80 . "\n";
}

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I'm not sure how it manages to get down to -2. Have I mis-typed your "while" version?

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Re^6: Abusing Map (Corrected second code block)
by BrowserUk (Patriarch) on May 18, 2018 at 15:38 UTC
    It does look to be the most efficient as you suspected.

    The really fastest version -- which is only applicable if you no longer need the original array and can modify it in place, which is often the case in my work -- is: 

    my $i = 0; $x[ $i ] -= $x[ ++$i ] while $i < $#x; --$#x;

    [download]

    But the saving comes from not allocating more memory; and all the algorithms would benefit from that.

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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