Rearrange an array such that every odd indexed element is greater than its previous in C++

We are given a positive integer type array, let's say, arr[] of any given size. The task is to rearrange an array in such a manner that all the elements present at an odd index should value greater than the element presents at an even index and print the result.

Let us see various input output scenarios for this −

Input − int arr[] = {2, 1, 5, 4, 3, 7, 8}

Output − Array before Arrangement: 2 1 5 4 3 7 8 Rearrangement of an array such that every odd indexed element is greater than it previous is: 1 4 2 5 3 8 7

Explanation − we are given an integer array of size 7. Now, we will swap the elements at even index with the elements at odd index if even indexed elements are greater i.e.

Arr[0] > arr[1] = call swap = {1, 2, 5, 4, 3, 7, 8}
Arr[2] > arr[3] = call swap = {1, 2, 4, 5, 3, 7, 8}
Arr[6] > arr[5] = call swap = {1, 2, 4, 5, 3, 8, 7}
Arr[2] > arr[1] = call swap = {1, 4, 2, 5, 3, 8, 7}

Input − int arr[] = {3, 2, 6, 9}

Output − Array before Arrangement: 3 2 6 9 Rearrangement of an array such that every odd indexed element is greater than it previous is: 2 3 6 9

Explanation − we are given an integer array of size 4. Now, we will swap the elements at even index with the elements at odd index if even indexed elements are greater i.e. Arr[0] > arr[1] = call swap = {2, 3, 6, 9}. No need to further call the swap method as all the elements at positions satisfies the conditions

Approach used in the below program is as follows

• Input an array of integer type elements and calculate the size of an array.

• Print the array before arrangement and call the function Rearrangement(arr, size)

• Inside the function Rearrangement(arr, size)

  • Create a variable of integer type let’s say, ptr and set it with size-1.

  • Start loop FOR, from i to 0 till i less than ptr and i = i + 1. Inside the loop, check if arr[i] is greater than arr[i+1] then call swap(arr[i], arr[i+1]).

  • Check IF size & 1 then start loop FOR from i to ptr till i greater than 0 and i = i - 2. Inside the loop, check IF arr[i] greater than arr[i - 1] then call swap(arr[i], arr[i-1])

• Print the array after the rearrangement of values of an array.

Example

#include <iostream>
using namespace std;
void Rearrangement(int arr[], int size){
   int ptr = size - 1;
   for(int i = 0; i < ptr; i = i+2){
      if(arr[i] > arr[i+1]){
         swap(arr[i], arr[i+1]);
      }
   }
   if(size & 1){
      for(int i = ptr; i > 0; i = i-2){
         if(arr[i] > arr[i-1]){
            swap(arr[i], arr[i-1]);
         }
      }
   }
}
int main(){
   //input an array
   int arr[] = {2, 1, 5, 4, 3, 7, 8};
   int size = sizeof(arr) / sizeof(arr[0]);
   //print the original Array
   cout<<"Array before Arrangement: ";
   for (int i = 0; i < size; i++){
      cout << arr[i] << " ";
   }
   //calling the function to rearrange the array
   Rearrangement(arr, size);
   //print the array after rearranging the values
   cout<<"\nRearrangement of an array such that every odd indexed element is greater than it previous is: ";
   for(int i = 0; i < size; i++){
      cout<< arr[i] << " ";
   }
   return 0;
}

Output

If we run the above code it will generate the following Output

Array before Arrangement: 2 1 5 4 3 7 8
Rearrangement of an array such that every odd indexed element is greater than it previous is: 1 4 2 5 3 8 7