In the worst case, index will crawl the entirety of $str_a looking for the character you extracted from $str_b so it can't be the fastest solution.

You want to make sure you only make a single one-character comparison. And because one of the strings is invariant during the loop, you want to move its first-character extraction out of the loop. That gives us the canonical solution of

my $str_a = 'Foo';

my $chr_a = substr $str_a, 0, 1;

for my $str_b ( @list ) {
    next if $chr_a eq substr($str_b, 0, 1);
    expensive_function($str_a , $str_b);
}

[download]

Update: beaten to the punch by kvale.

Makeshifts last the longest.

In reply to Re: Matching First Character of Strings Efficiently by Aristotle
in thread Matching First Character of Strings Efficiently by Limbic~Region

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.