#!perl

use strict;

my ($list_size, $max_num, @numbers, $start_time);

#Generate a large number of random integers

$list_size = 100000;

$max_num = 1000;

@numbers = ();

for(1..$list_size) {

push(@numbers, int(rand($max_num)));

}

#Print out the number array, then sort it, then print it out again in sorted order

#print "@numbers\n";

$start_time = time;

quicksort(0, $#numbers, \@numbers);

#print "@numbers\n";

print "QUICKSORT sorted $list_size random integers between 1 and $max_num in " . (time - $start_time) . " seconds.\n";

#Quicksort works by dividing the list into two parts around an arbitrary pivot number

#We arbitrarily choose the first number in the list to be the pivot

#Then we simply recurse over the bottom and top parts of the list

#The recursion stops when the list of the length ("right minus left") is 1

sub quicksort {

my( $left, $right, $quicknums ) = @_;

if( $right > $left ) {

my $pivotNewIndex = partition($left, $right, $left, $quicknums);

quicksort($left, $pivotNewIndex -1, $quicknums);

quicksort($pivotNewIndex + 1, $right, $quicknums);

}

}

#To partition a list around a pivot index, we put each element above or below the pivot

#The result is a list which is partially sorted, in that the list as a whole has a high and low part

#We do this by first moving the pivot value safely to the end of the list

#Then we compare each item with the list to the pivot, moving lower values to the left in the array

#Finally we put the pivot back into place between the two parts of the list and return its new index

sub partition {

my( $left, $right, $pivotIndex, $partnums ) = @_;

my ($pivotValue, $storeIndex);

$pivotValue = $partnums->[$pivotIndex];

swap($pivotIndex, $right, $partnums);

$storeIndex = $left;

for( my $i = $left; $i <= $right; $i++ ) {

if( $partnums->[$i] < $pivotValue ) {

swap($i, $storeIndex, $partnums);

$storeIndex++;

}

}

swap($storeIndex, $right, $partnums);

return $storeIndex;

}

#A swap operation takes a list reference and exchanges the values at the indicated indices

#The swap operation allows quicksort to work in one list sub swap {

my( $index1, $index2, $anyarray ) = @_;

$_ = $anyarray->[$index1];

$anyarray->[$index1] = $anyarray->[$index2];

$anyarray->[$index2] = $_;

}

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## Friday, September 11, 2009

### Quicksort in perl

This is quicksort implemented in perl. This implementation run on a Macbook Pro 2.4GHz Dual Core machine sorts 100,000 random integers between 1 and 1000 in about three seconds. That is reasonably quick, but compares approximately to mergesort, and performance degrades considerably if the integers have lower cardinality (a smaller overall numerical range).

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