Abigail,
I like your logic and agree it should be more efficient with large step sizes, but it doesn't appear to be faster. I've tried the following step sizes on an array of 1000000 elements: 5, 10, 100, 100, etc, 100000, 200000, 300000 & 500000. I bencmarked the results. The output I have included is with the step size of 500000 but is a fair summary of the results I saw:

Benchmark: running Step 1, Step 2, Step 3 for at least 10 CPU seconds.
+..
    Step 1: 10 wallclock secs (10.47 usr +  0.02 sys = 10.49 CPU) @ 18
+5755.48/s (n=1948575)
    Step 2: 11 wallclock secs (10.48 usr +  0.02 sys = 10.50 CPU) @ 18
+7398.10/s (n=1967680)
    Step 3: 10 wallclock secs (10.01 usr +  0.00 sys = 10.01 CPU) @ 31
+5978.92/s (n=3162949)

[download]

Here's the code:

#!/usr/bin/perl -w

use strict;
use Benchmark;

sub step1 {
  my $step = shift;
  @_[map {$_ * $step} 0..($#_ / $step)]; # curious what effect the bac
+kets would have
}

sub step2 {
  my $step = shift;
  @_[map {$_ * $step} 0..$#_ / $step];
}

sub step3 {
  my $step = shift;
  return map { $_[$_] } grep { $_ % $step == 0 } 0..$#_;
}

my @array = (0..1000000);

timethese(-10, {
  'Step 1' => 'step1(500000, @array)',
  'Step 2' => 'step2(500000, @array)',
  'Step 3' => 'step3(500000, @array)'
});

[download]

The only reason I post this is I'm curoius as to why what would appear to be a quicker and more effient piece of code is consistently the slowest (although not by much).

In reply to Re: Re: Stepping through an array by Anonymous Monk
in thread Stepping through an array by rob_au

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.