Like Merge Sort, QuickSort is a Divide and Conquer algorithm. It picks an element as pivot and partitions the given array around the picked pivot. There are many different versions of quickSort that pick pivot in different ways.
// simple C program for Quick Sort
# include <stdio.h>
// to swap two numbers
void swap(int* a, int* b)
{
int t = *a;
*a = *b;
*b = t;
}
/*
a[] is the array, p is starting index, that is 0,
and r is the last index of array.
*/
void quicksort(int a[], int p, int r)
{
if(p < r)
{
int q;
q = partition(a, p, r);
quicksort(a, p, q);
quicksort(a, q+1, r);
}
}
int partition (int a[], int low, int high)
{
int pivot = arr[high]; // selecting last element as pivot
int i = (low - 1); // index of smaller element
for (int j = low; j <= high- 1; j++)
{
// If current element is smaller than or equal to pivot
if (arr[j] <= pivot)
{
i++; // increment index of smaller element
swap(&arr[i], &arr[j]);
}
}
swap(&arr[i + 1], &arr[high]);
return (i + 1);
}
// function to print the array
void printArray(int a[], int size)
{
int i;
for (i=0; i < size; i++)
{
printf("%d ", a[i]);
}
printf("\n");
}
int main()
{
int arr[] = {9, 7, 5, 11, 12, 2, 14, 3, 10, 6};
int n = sizeof(arr)/sizeof(arr[0]);
// call quickSort function
quickSort(arr, 0, n-1);
printf("Sorted array: n");
printArray(arr, n);
return 0;
}
Sorted List of elements
Following are the steps involved in quick sort algorithm:
Let's consider an array with values {9, 7, 5, 11, 12, 2, 14, 3, 10, 6}
Below, we have a pictorial representation of how quick sort will sort the given array.
In step 1, we select the last element as the pivot, which is 6 in this case, and call for partitioning, hence re-arranging the array in such a way that 6 will be placed in its final position and to its left will be all the elements less than it and to its right, we will have all the elements greater than it.
Then we pick the subarray on the left and the subarray on the right and select a pivot for them, in the above diagram, we chose 3 as pivot for the left subarray and 11 as pivot for the right subarray.
And we again call for partitioning.
First understand the algorithm carefully. Then study the program line-by-line and compare it with the output. Finally, review the explanation section to strengthen your logic and programming understanding.
Rewrite the program without looking at the code. Modify values, conditions or logic and run it again. This helps improve confidence and strengthens coding skills much faster.