Quick Sort in C

February 7, 2024

How Quick Sort works in C

Quick sort is an algorithm of the divide and conquer type. That is,the problem of sorting a set is reduced of the problem of sorting two smaller sets.

quickly
Time Complexity O(n log n)
Best Case O(n log n)
Worst Case O(n2)
Space Complexity O(1)
Auxiliary Space Complexity O(log n)
In Place Yes
Stable No

How it works?

Quick sort works in the following way – 

  • Choose an item from array called as pivot
  • Move all the elements smaller than pivot to left partition
  • Move all the elements greater than pivot to right partition.

Choose new pivot item in each partition and keep doing the same process again until partition of one element each aren’t formed.

How to choose Pivot?

These are the following ways you can choose pivot –

  • First element as pivot.
  • Last element as pivot (We use this in our examples & Code)
  • Random element as pivot.
  • Median element as pivot.
Quick Sort in C Example – 1

Execution of quick sort

Quicksort works recursively, Partitioning moves all smaller elements to the left of the pivot and greater to the right side of the pivot, thus each time two sub-array partitions are formed. Thus again, we do partitioning in each of these sub-arrays individually.

Following code below gives a perfect example for this –

QuickSort Algorithm

quickSort(arr[], low, high)
{
    if (low < high)
    {
        /* indexPI is partitioning index, partition() function will 
        return index of partition */
        indexPI = partition(arr, low, high);

        quickSort(arr, low, indexPI - 1);  // left partition
        quickSort(arr, indexPI + 1, high); // right partition
    }
}

Partition Algorithm

partition (arr[], low, high)
{
    // pivot element selected as right most element in array each time
    pivot = arr[high];  

    swapIndex = (low - 1)  //swapping index

    for (j = low; j <= high- 1; j++)
    {
        // Check if current element is smaller than pivot element
        if (arr[j] < pivot)
        {
            i++;    // increment swapping index
            swap arr[j] and arr[swapIndex]
        }
    }
    swap arr[swapIndex + 1] and arr[high])
    return (swapIndex + 1)
}
Quick Sort in C Example – 2
Quick Sort in C Example – 3 PrepInsta

Code of Quick Sort in C

Run
#include<stdio.h>
  
// A utility function to swap two elements 
void swap(int* x, int* y) 
{ 
    int temp = *x; 
    *x = *y; 
    *y = temp; 
} 
  
/* Partition function to do Partition
elements on the left side of pivot elements would be smaller than pivot
elements on the right side of pivot would be greater than the pivot
*/
int partition (int array[], int low, int high) 
{ 
    // pivot element selected as right most element in array each time
    int pivot = array[high];    
    int swapIndex  = (low - 1);   //swapping index
  
    for (int j = low; j <= high- 1; j++) 
    { 
        // Check if current element is smaller than pivot element
        if (array[j] < pivot) 
        { 
            swapIndex ++;    // increment swapping index
            swap(&array[swapIndex], &array[j]); 
        } 
    } 
    swap(&array[swapIndex + 1], &array[high]); 
    return (swapIndex + 1); 
} 
  
//Recursive function to apply quickSort
void quickSort(int array[], int low, int high) 
{ 
    if (low < high) 
    { 
       /* indexPI is partitioning index, partition() function will 
        return index of partition */
        int indexPI  = partition(array, low, high); 
  
        quickSort(array, low, indexPI  - 1);  // left partition
        quickSort(array, indexPI  + 1, high); // right partition
    } 
} 
  
/* Function to display the array */
void display(int array[], int size) 
{ 
    int i; 
    for (i=0; i < size; i++) 
        printf("%d ", array[i]); 
} 
  
//Main function to run the program
int main() 
{ 
    int array[] = {70, 90, 10, 30, 50, 20, 60}; 
    int size = sizeof(array)/sizeof(array[0]); 
    quickSort(array, 0, size-1); 
    printf("Array after Quick Sorting: "); 
    display(array, size); 
    return 0; 
}

Output

Array after Quick Sorting: 
10 20 30 50 60 70 90

Facts about Quick Sort

  • Quicksort is an in-place algorithm. In-place sorting means, it does not use additional storage space to perform sorting.
  • The algorithm is efficient for large-sized data sets.
  • The average or best-case complexity of quicksort is O(n log n).
  • Worst-case time complexity is O(n^2)
  • Cache Friendly: Quick Sort is also a cache-friendly sorting algorithm as it has a good locality of reference when used for arrays
Quick sort Meme C++

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