C Exercises: Sort an array of 0s, 1s and 2s

Last update on March 18 2025 13:23:34 (UTC/GMT +8 hours)

54. Sort Array of 0s, 1s, and 2s

Write a program in C to sort an array of 0s, 1s and 2s.

Expected Output :
The given array is : 0 1 2 2 1 0 0 2 0 1 1 0
After sortig the elements in the array are:
0 0 0 0 0 1 1 1 1 2 2 2

To sort an array containing only 0s, 1s, and 2s, the program uses a three-way partitioning approach, such as the Dutch National Flag algorithm. This method efficiently sorts the array in a single pass by maintaining three pointers to separate the elements into three groups. The result is a sorted array with all 0s, followed by all 1s, and then all 2s.

Visual Presentation:

Sample Solution:

C Code:

#include <stdio.h>

// Function to swap elements at given indices i and j in the array
void changePlace(int arr1[], int i, int j) {
    int tmp = arr1[i];
    arr1[i] = arr1[j];
    arr1[j] = tmp;
}

// Function to sort elements in the array based on a pivot value (in this case, 1)
int sortElements(int arr1[], int end) {
    int start = 0, mid = 0;
    int pivot = 1;

    while (mid <= end) {
        if (arr1[mid] < pivot) {
            changePlace(arr1, start, mid);
            ++start, ++mid;
        } else if (arr1[mid] > pivot) {
            changePlace(arr1, mid, end);
            --end;
        } else {
            ++mid;
        }
    }
}

int main() {
    int arr1[] = { 0, 1, 2, 2, 1, 0, 0, 2, 0, 1, 1, 0 };
    int n = sizeof(arr1) / sizeof(arr1[0]);
    int i;

    //------------- print original array ------------------
    printf("The given array is: ");
    for (i = 0; i < n; i++) {
        printf("%d ", arr1[i]);
    }
    printf("\n");
    //------------------------------------------------------

    printf("After sorting the elements in the array are:\n");
    sortElements(arr1, n - 1);
    for (int i = 0 ; i < n; i++) {
        printf("%d ", arr1[i]);
    }
    return 0;
}

Output:

The given array is :  0  1  2  2  1  0  0  2  0  1  1  0  
After sortig the elements in the array are: 
0 0 0 0 0 1 1 1 1 2 2 2 

Flowchart:/p>

For more Practice: Solve these Related Problems:

• Write a C program to sort an array containing only 0s, 1s, and 2s using the Dutch National Flag algorithm.
• Write a C program to sort an array of 0s, 1s, and 2s without using extra space by counting frequencies first.
• Write a C program to sort an array of 0s, 1s, and 2s using recursion.
• Write a C program to sort an array of 0s, 1s, and 2s and then verify the sort by outputting counts of each element.

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