Quick sort is a highly efficient sorting algorithm and is based on partitioning of array of data into smaller arrays. A large array is partitioned into two arrays one of which holds values smaller than the specified value, say pivot, based on which the partition is made and another array holds values greater than the pivot value.
Quick sort partitions an array and then calls itself recursively twice to sort the two resulting subarrays. This algorithm is quite efficient for large-sized data sets as its average and worst case complexity are of Ο(n2), where n is the number of items.
Following animated representation explains how to find the pivot value in an array.
The pivot value divides the list into two parts. And recursively, we find the pivot for each sub-lists until all lists contains only one element.
Based on our understanding of partitioning in quick sort, we will now try to write an algorithm for it, which is as follows.
Step 1 − Choose the highest index value has pivot Step 2 − Take two variables to point left and right of the list excluding pivot Step 3 − left points to the low index Step 4 − right points to the high Step 5 − while value at left is less than pivot move right Step 6 − while value at right is greater than pivot move left Step 7 − if both step 5 and step 6 does not match swap left and right Step 8 − if left ≥ right, the point where they met is new pivot
The pseudocode for the above algorithm can be derived as −
function partitionFunc(left, right, pivot) leftPointer = left rightPointer = right - 1 while True do while A[++leftPointer] < pivot do //do-nothing end while while rightPointer > 0 && A[--rightPointer] > pivot do //do-nothing end while if leftPointer >= rightPointer break else swap leftPointer,rightPointer end if end while swap leftPointer,right return leftPointer end function
Using pivot algorithm recursively, we end up with smaller possible partitions. Each partition is then processed for quick sort. We define recursive algorithm for quicksort as follows −
Step 1 − Make the right-most index value pivot Step 2 − partition the array using pivot value Step 3 − quicksort left partition recursively Step 4 − quicksort right partition recursively
To get more into it, let see the pseudocode for quick sort algorithm −
procedure quickSort(left, right) if right-left <= 0 return else pivot = A[right] partition = partitionFunc(left, right, pivot) quickSort(left,partition-1) quickSort(partition+1,right) end if end procedure
To know about quick sort implementation in C programming language, please click here.