Move All Zeros to Start and Ones to End in C++

In this tutorial, we are going to write a program that moves all zeroes to front and ones to end of the array.

Given an array with zeroes and ones along with random integers. We have to move all the zeroes to start and ones to the end of the array. Let's see an example.

Input

arr = [4, 5, 1, 1, 0, 0, 2, 0, 3, 1, 0, 1]

Output

0 0 0 0 4 5 2 3 1 1 1 1

Algorithm

• Initialise the array.

• Initialise an index to 1.

• Iterate over the given array.

  • If the current element is not zero, then update the value at the index with the current element.

  • Increment the index.

• Write a loop that iterates from the above index to n

  • Update all the elements to 1.

• Similarly, do for 0. Instead of increasing the index, decrease it to move all zeroes to front of the array.

Implementation

Following is the implementation of the above algorithm in C++

#include <bits/stdc++.h>
using namespace std;
void update1And0Positions(int arr[], int n) {
   int index = 0;
   for (int i = 0; i < n; i++) {
      if (arr[i] != 1) {
         arr[index++] = arr[i];
      }
   }
   while (index < n) {
      arr[index++] = 1;
   }
   index = 0;
   for (int i = n - 1; i >= 0; i--) {
      if (arr[i] == 1) {
         continue;
      }
      if (!index) {
         index = i;
      }
      if (arr[i] != 0) {
         arr[index--] = arr[i];
      }
   }
   while (index >= 0) {
      arr[index--] = 0;
   }
}
int main() {
   int arr[] = { 4, 5, 1, 1, 0, 0, 2, 0, 3, 1, 0, 1 };
   int n = 12;
   update1And0Positions(arr, n);
   for (int i = 0; i < n; i++) {
      cout << arr[i] << " ";
   }
   cout << endl;
   return 0;
}

Output

If you run the above code, then you will get the following result.

0 0 0 0 4 5 2 3 1 1 1 1