Hi all.

Two of the most common search algorithms are the:

linear search and the binary search

Each has it's advantages as well as it's 'faults'. In a linear search, each element of the data structure is examined. Needless to say, in a small array ( for example ) this isn't a problem since there are only a relatively small number of elements to analyze. If the array is large, a binary search would be most effective. In a binary search, the array needs to be sorted before a search is initiated. Once that has been accomplished, there exists a tremendous gain in performance ( please see below ). By establishing a 'mid-point' in the array, we are able to eliminate 1/2 of the array ( the lower or upper portion ) when comparing our key ( the value we want to find ) to it's position in the array and it's relation to the mid-point. With each successive comparison, we divide the array in 1/2 thereby establishing a new mid-point. Then it's a simple matter of establishing whether or not our key is higher or lower than it.

An array with 2_048 elements could be successfully searched using ( at most ) only 11 comparisons ( 2**11 == 2048 ). When compared to the time it would probably take to search the same array using a linear search, the benefits should be readily apparent. The formula for determining the maximum number of comparisons needed is the first power of 2 greater than the number of elements in the array. 

#!/usr/bin/perl -w

use strict;

my @array;
my $low = 0;
my $mid;
my $found_key = 0;
my $num;
my $key;
my $index;

@array = ( 5, 10, 30, 4, -3, 18, 101, 2001, 46, 23 );
@array = sort { $a <=> $b } @array;

my $high = $#array;

print "Please enter key to search for: ";
chomp( $key = <STDIN> );

while( ( $low <= $high ) && !$found_key ) {
  $mid = ( $low + $high ) / 2;
  if( $key == $array[$mid] ) {
    $found_key = 1;
    $index = int( $mid );
  }

  elsif( $key < $array[$mid] ) {
    $high = $mid - 1;
  }

  else {
    $low = $mid + 1;
  }
}

if( $found_key ) {
  print "$key is at position: $index\n";
}

else {
  print "Sorry. I could not find: $key"; 
}

[download]


This is my first attempt at implementing a binary search in perl. If you could help me improve the code, I would be most appreciative.

Thanks,
-Katie.

In reply to Binary searching with Perl. by DigitalKitty

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