This is fine if you have just a few keys. However, this approach is O(n) (i.e. execution time increases linearly with the number of items in the set), whereas a hash has approximately constant lookup time — or O(1).

Even for only 10 items, a hash is considerably faster: 

#!/usr/bin/perl -w

my $nkeys = 10;

my @keys;
our $key; # (keep last key for lookup below)
for (1..$nkeys) {
    # generate random keys consisting of 3..10 chars
    $key = join "", map chr(97+int(rand(26))), (1..3+int(rand(8)));
    push @keys, $key;
}

# setup 'if'-code
my $code = "sub isMember_if {\n"
           . join("", map "return 1 if \$_[0] eq '$_';\n", @keys)
           . "return 0;}";
#print "$code\n";
eval $code;

# setup hash
my %hash = map { $_ => 1 } @keys;
sub isMember_hash { return $hash{$_[0]} };


use Benchmark 'cmpthese';

cmpthese( -1, {
    'if'   => 'isMember_if($key)',
    'hash' => 'isMember_hash($key)'
});

[download]

On my machine, this prints (results vary slightly, depending on random keys being used) 

          Rate   if hash
if    628278/s   -- -68%
hash 1946613/s 210%   --

[download]

Update: Actually, Perl's hashes are blazingly fast: even for just one key/comparison ($nkeys = 1), the if-approach still doesn't outperform the hash; and with key lengths of more than about 3 chars, the hash lookup is even noticeably faster (which seems to indicate that comparing two identical strings is slower than computing the hashed value of that string...).

In reply to Re^2: Fastest way to test membership in a constant set by almut
in thread Fastest way to test membership in a constant set by Anonymous Monk

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.