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Checking a string's presence within an array.

by antirice (Priest)
on May 16, 2003 at 17:56 UTC ( #258712=perlquestion: print w/ replies, xml ) Need Help??
antirice has asked for the wisdom of the Perl Monks concerning the following question:

Ok, I don't want to beat a dead horse but I've been wondering about this for quite some time now. Suppose you want to see whether or not 'c' is in (a .. z). Of course, I want general case (where the array may contain words instead of letters). What method would you employ to have the program say, "yes, c is a letter that would exist within that array you just created there." Some of the methods I have seen are included in the readmore.

Create a hash and just check for existence (I usually use this):
my @array_I_just_created_here = (a .. z); my @hash_I_just_created_here{@array_I_just_created_here} = (); print "yes, c is a letter that would exist within that array you just +created there.$/" if exists $hash_I_just_created_here{"c"};

Linear search:
my $found = 0; my @array_I_just_created_here = (a .. z); $found |= ("c" eq $array_I_just_created_here[$_]) foreach (0..$#array_ +I_just_created_here); print "yes, c is a letter that would exist within that array you just +created there.$/" if $found;

Binary search (not complex enough to justify requiring Search::Binary but very similar parameters passed, just in case):
my $find_item = "c"; my @array_I_just_created_here = (a .. z); my $pos = bin_search(\@array_I_just_created_here, $find_item, 0, $#arr +ay_I_just_created_here); print "yes, c is a letter that would exist within that array you just +created there.$/" if ($a[$pos] eq $find_item); sub bin_search { my ($array, $val, $min, $max) = @_; my $pos = int($min + $max / 2); my $check = $val cmp $array->[$pos]; if ($check == -1) { $pos--; return $pos if ($pos <= $min); return bin_search($array, $val, $min, $pos); } elsif ($check == 0) { return $pos; } else { $pos++; return $pos if ($pos >= $max); return bin_search($array, $val, $pos, $max); } }

Regex hard-coded (seen this used extensively twice before):
print "yes, c is a letter that would exist within that array you hadn' +t created there.$/" if "c" =~ m/^(?:a|b|c|d|e|f|g|h|i|j|k|l|m|n|o|p|q +|r|s|t|u|v|w|x|y|z)$/;

Introduce quantum theory?
use Quantum::Superpositions; my @array_I_just_created_here = (a .. z); print "yes, c is a letter that would exist within that array you just +created there.$/" if ("c" eq any(@array_I_just_created_here);
I am just curious which methods people use for this very common problem and which is the most popular. If there are particular cases where you use one method over another, please cite them and explain why...if you don't mind of course :-)

antirice    
The first rule of Perl club is - use Perl
The
ith rule of Perl club is - follow rule i - 1 for i > 1

Comment on Checking a string's presence within an array.
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Re: Checking a string's presence within an array.
by hardburn (Abbot) on May 16, 2003 at 18:11 UTC

    Usually, I when I expect to need to check for the presence of something in an array, I code it with a hash from the start. Hard to beat O(1) efficency.

    ----
    I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
    -- Schemer

    Note: All code is untested, unless otherwise stated

Re: Checking a string's presence within an array.
by pzbagel (Chaplain) on May 16, 2003 at 18:20 UTC

    How about grep. A hash is probably best, but if you are stuck with an array, grep is the simplest way to check.

    $x=grep /^c$/,@array_I_just_created_here;

    Assigning the results of grep in a scalar context returns number of matches. $x == 0 means it's not in the array.

    HTH

Re: Checking a string's presence within an array.
by broquaint (Abbot) on May 16, 2003 at 18:47 UTC
    I am just curious which methods people use for this very common problem and which is the most popular
    Generally I'll just use a grep and double negation e.g
    my @array = qw/ a list of words /; print "yep, it's there" if !!grep { $_ eq "of" } @array; __output__ yep, it's there
    Nice and efficient in newer versions of perl (5.6.1+ ?) as it won't build a list IRC. Even quicker would be a simple regex
    my @array = qw/ a list of words /; print "yep, it's there" if "of" =~ /\b (?: ${\join('|', @array) }) \b/x; __output__ yep, it's there
    It's not quicker on it's own (list size etc will effect performance) but if you prebuild the word list it should be quicker than grepping every time.
    HTH

    _________
    broquaint

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