TheAlgorithms-C/sorting/QuickSort.c

#include <stdio.h>

/*Displays the array, passed to this method*/

void display(int arr[], int n){

	int i;
	for(i = 0; i < n; i++){
		printf("%d ", arr[i]);
	}

	printf("\n");

}

/*Swap function to swap two values*/
void swap(int *first, int *second){

	int temp = *first;
	*first = *second;
	*second = temp;
	
}


/*Partition method which selects a pivot
  and places each element which is less than the pivot value to its left
  and the elements greater than the pivot value to its right

  arr[] --- array to be partitioned
  lower --- lower index 
  upper --- upper index
*/
int partition(int arr[], int lower, int upper){

	int i = (lower  - 1);

	int pivot = arr[upper];	// Selects last element as the pivot value

	int j ;	
	for(j = lower; j < upper ; j++){

		if(arr[j] <= pivot){ // if current element is smaller than the pivot

			i++; // increment the index of smaller element
			swap(&arr[i], &arr[j]);
		}
	}

	swap(&arr[i + 1] , &arr[upper]); // places the last element i.e, the pivot to its correct position
   
	return (i + 1); 
}


/*This is where the sorting of the array takes place
	arr[] --- Array to be sorted
	lower --- Starting index
	upper --- Ending index
*/
void quickSort(int arr[], int lower,  int upper){

	if(upper > lower){

		// partitioning index is returned by the partition method , partition element is at its correct poition

		int partitionIndex = partition(arr, lower, upper); 


		// Sorting elements before and after the partition index
		quickSort(arr, lower, partitionIndex - 1);
		quickSort(arr, partitionIndex + 1, upper);
	}
}


int main(){


	int n;
	printf("Enter size of array:\n");
	scanf("%d", &n); // E.g. 8

	printf("Enter the elements of the array\n");
	int i;
	int arr[n];
	for(i = 0; i < n; i++){
		scanf("%d", &arr[i] ); 
	}

	printf("Original array: "); 
	display(arr, n);       			// Original array : 10 11 9 8 4 7 3 8

	quickSort(arr, 0, n-1);

	printf("Sorted array: "); 
	display(arr, n);				// Sorted array : 3 4 7 8 8 9 10 11

	return 0;
}