Pathologically Eclectic Rubbish Lister PerlMonks

### Checking a string's presence within an array.

by antirice (Priest)
 on May 16, 2003 at 17:56 UTC 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

Replies are listed 'Best First'.
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

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

Create A New User
Node Status?
node history
Node Type: perlquestion [id://258712]
Approved by Enlil
help
Chatterbox?
 [Corion]: Hmm - my Italian isn't that great - I assume this is more an accident and less an art project, right? [Discipulus]: yes we are falling down [Corion]: Discipulus: As is great tradition, just build another level on it ;)

How do I use this? | Other CB clients
Other Users?
Others wandering the Monastery: (9)
As of 2017-02-28 10:38 GMT
Sections?
Information?
Find Nodes?
Leftovers?
Voting Booth?
Before electricity was invented, what was the Electric Eel called?

Results (399 votes). Check out past polls.