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Making a match

by Baz (Friar)
on Apr 12, 2003 at 12:36 UTC ( #250049=perlquestion: print w/replies, xml ) Need Help??

Baz has asked for the wisdom of the Perl Monks concerning the following question:

I have a list of numbers -

10,11,12,13,14,15,17,18,22,23,24,25,26,27,28,29,30,31,32,33,34,35
and a varible $x which stores an arbitary value
I want an if loop which is true if $x matches a value in my list. In other words, if $x is 10 then the if loop is executed, if its 5 then it is not.
whats the simplest way of doing this?

Replies are listed 'Best First'.
The simplest way
by TheDamian (Priest) on Apr 13, 2003 at 00:25 UTC
    Previous respondents have suggested efficient ways for various circumstances. As for the simplest way:
    use Quantum::Superpositions; if ($x == any(@list)) { ... }

    Note that the latest release of Q::S (just uploaded to the CPAN last week) is iterator-based and therefore much more efficient that previous versions. We have Steven Lembark to thank for that.

    Note too that, whilst the performance of the grep- and hash-based solutions may improve slightly over time, the any solution will continue to improve significantly with future releases of Quantum::Superpositions, then with the new Perl6::Junctions module, and ultimately with the availability of any as a built-in feature of Perl 6.

Re: Making a match
by DrManhattan (Chaplain) on Apr 12, 2003 at 13:30 UTC
    If you're checking $x aginst your list multiple times, it might be more efficient to use a hash:
    my @list = (10,11,12,13,14,15,17,18,22,23,24,25,26,27,28,29,30,31,32,3 +3,34,35 ); my %hash; @hash{@list} = (1) x @list; # This solution only makes sense if you're # making the comparison multiple times foreach my $x (1..1000) { if ($hash{$x}) { print "$x is in the list\n"; } }
    You spend some initial setup time creating the hash, but then all your comparisons are O(1).

    -Matt

      Hashes certainly are the best solution, if you can amortize the set-up costs (and only slightly worse than grep even if you have to set up repeatedly -- i.e. if the list keeps changing).

      But if ultimate efficiency is the goal, then there is an even more efficient way to use hash lookup:

      my %hash; @hash{@list} = (); # Don't bother filling in values foreach my $x (1..1000) { if (exists $hash{$x}) # Just test for existence of key { print "$x is in the list\n"; } }
        ++ to you and perlplexer. I hadn't realized you could do it that way. Benchmark.pm shows defining the hash is over twice as fast as assigning values to it. The benchmark results on the comparison techniques are more interesting:
        if ($hash{$x} == 1) { # always slower than if($hash{$x}) # or if(defined($hash{$x})) } if ($hash{$x}) { # slower than if(defined($hash{$x})) # when $hash{$x} is undefined # faster than if(defined($hash{$x})) # when $hash{$x} is defined }
        Here's the benchmark code I used:
        #!/usr/bin/perl use strict; use Benchmark; my @list = (10,11,12,13,14,15,17,18,22,23,24,25,26,27,28,29,30,31,32,3 +3,34,35 ); my %hash; @hash{@list} = (1) x @list; timethese(1000000, { 'Hash Defined Miss' => sub { if (defined($hash{1})) { #nop } }, 'Hash Defined Hit' => sub { if (defined($hash{10})) { #nop } }, 'Hash Assigned Miss Truth' => sub { if ($hash{1}) { #nop } }, 'Hash Assigned Miss Equality' => sub { if ($hash{1} == 1) { #nop } }, 'Hash Assigned Hit Truth' => sub { if ($hash{10}) { #nop } }, 'Hash Assigned Hit Equality' => sub { if ($hash{10} == 1) { #nop } } });
        Benchmark: timing 1000000 iterations of Hash Assigned Hit Equality, Ha +sh Assigned Hit Truth, Hash Assigned Miss Equality, Hash Assigned Miss Truth, Hash Define +d Hit, Hash Defined Miss... Hash Assigned Hit Equality: 5 wallclock secs ( 1.52 usr + 0.00 sys = + 1.52 CPU) @ 657894.74/s (n=1000000) Hash Assigned Hit Truth: 3 wallclock secs ( 1.29 usr + -0.01 sys = 1 +.28 CPU) @ 781250.00/s (n=1000000) Hash Assigned Miss Equality: 3 wallclock secs ( 1.30 usr + 0.00 sys += 1.30 CPU) @ 769230.77/s (n=1000000) Hash Assigned Miss Truth: 1 wallclock secs ( 1.15 usr + 0.01 sys = +1.16 CPU) @ 862068.97/s (n=1000000) Hash Defined Hit: 1 wallclock secs ( 1.41 usr + 0.00 sys = 1.41 CPU +) @ 709219.86/s (n=1000000) Hash Defined Miss: 2 wallclock secs ( 0.95 usr + 0.00 sys = 0.95 CP +U) @ 1052631.58/s (n=1000000)

        -Matt

Re: Making a match
by perlplexer (Hermit) on Apr 12, 2003 at 13:35 UTC
    How often does this need to be executed?
    If you only need to perform the check once, use grep(), as the Anonymous Monk already suggested.
    If, however, the check needs to be performed multiple times, say, in a loop, you'll be better off putting those numbers in a hash and then using exists(); e.g.,
    my %numbers = (); @numbers{10..15,17,18,22..35} = (); if (exists $numbers{$x}){ # do stuff }
    --perlplexer
Re: Making a match
by Anonymous Monk on Apr 12, 2003 at 12:52 UTC
    my @list = (10,11,12,13,14,15,17,18,22,23,24,25,26,27,28,29,30,31,32,33,34,35);
    
    if (grep $_ == $x, @list) {
        print "Found $x\n";
    }
    
Re: Making a match
by Anonymous Monk on Apr 13, 2003 at 12:01 UTC
    #!/usr/bin/perl my $x = 14; my @ary=(10,11,12,13,14); map{print "$x matches\n" if /^$x$/} @ary; is nice to look at :)

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