# DSA using C - Quick Sort

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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 specified value say pivot based on which the partition is made and another array holds values greater than pivot value.

The quick sort partitions an array and then calls itself recursively twice to sort the resulting two subarray. This algorithm is quite efficient for large sized data sets as its average and worst case complexity are of O(nlogn) where n are no. of items.

## Pseudocode

```A : array of items

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

function partitionFunc(left, right, pivot)
leftPointer = left -1
rightPointer = right

while True do
while A[++leftPointer] < pivot do
//donothing
end while
while rightPointer > 0 && A[--rightPointer] > pivot do
//donothing
end while
if leftPointer >= rightPointer
break
else
swap leftPointer,rightPointer
end if
end while
swap leftPointer,right
return leftPointer
end function

procedure swap (num1, num2)
temp = A[num1]
A[num1] = A[num2]
A[num2] = temp;
end procedure
```

## Example

```#include <stdio.h>
#include <stdbool.h>
#define MAX 7

int intArray[MAX] = {4,6,3,2,1,9,7};

void printline(int count){
int i;
for(i=0;i <count-1;i++){
printf("=");
}
printf("=\n");
}
void display(){
int i;
printf("[");

// navigate through all items
for(i=0;i<MAX;i++){
printf("%d ",intArray[i]);
}
printf("]\n");
}
void swap(int num1, int num2){
int temp = intArray[num1];
intArray[num1] = intArray[num2];
intArray[num2] = temp;
}
int partition(int left, int right, int pivot){
int leftPointer = left -1;
int rightPointer = right;

while(true){
while(intArray[++leftPointer] < pivot){
//do nothing
}
while(rightPointer > 0 && intArray[--rightPointer] > pivot){
//do nothing
}
if(leftPointer >= rightPointer){
break;
} else {
printf(" item swapped :%d,%d\n",
intArray[leftPointer],intArray[rightPointer]);
swap(leftPointer,rightPointer);
}
}
printf(" pivot swapped :%d,%d\n", intArray[leftPointer],intArray[right]);
swap(leftPointer,right);
printf("Updated Array: ");
display();
return leftPointer;
}
void quickSort(int left, int right){
if(right-left <= 0){
return;
} else {
int pivot = intArray[right];
int partitionPoint = partition(left, right, pivot);
quickSort(left,partitionPoint-1);
quickSort(partitionPoint+1,right);
}
}
main(){
printf("Input Array: ");
display();
printline(50);
quickSort(0,MAX-1);
printf("Output Array: ");
display();
printline(50);
}
```

## Output

If we compile and run the above program then it would produce following output −

```Input Array: [4 6 3 2 1 9 7 ]
==================================================
pivot swapped :9,7
Updated Array: [4 6 3 2 1 7 9 ]
pivot swapped :4,1
Updated Array: [1 6 3 2 4 7 9 ]
item swapped :6,2
pivot swapped :6,4
Updated Array: [1 2 3 4 6 7 9 ]
pivot swapped :3,3
Updated Array: [1 2 3 4 6 7 9 ]
Output Array: [1 2 3 4 6 7 9 ]
==================================================
```
dsa_using_c_sorting_techniques.htm