Rearrange an array so that arr = i, using C++

We have an array of positive integer type, assuming it is arr[], its size can be given arbitrarily, and the elements in the array should be greater than 0 but less than the size of the array. The task is to rearrange the array such that if arr[i] is equal to 'i', then 'i' exists in the array, otherwise set the arr[i] element to -1 and print the final result.

Let us take a look at various input and output scenarios for this problem:

Input − int arr[] = {0, 8, 1, 5, 4, 3 , 2, 9 }

Output − The rearranged array is: 0 1 2 3 4 5 -1 -1

Explanation − We have an integer array of size 8, all elements in the array are less than 8. Now, we will rearrange the array i.e.

arr[0] = 0(present in an array)
arr[1] = 1(present in an array)
arr[2] = 2(present in an array)
arr[3] = 3(present in an array)
arr[4] = 4(present in an array)
arr[5] = 5(present in an array)
arr[6] = -1(NOT present in an array)
arr[7] = -1(NOT present in an array)

input− int arr[] = {1, 2, 6, 9, 10}

output − Rearrange the array so that arr[i] = i is: -1 1 2 -1 -1

Explanation− We get an integer array of size 5, and All elements in the array have values ​​less than or greater than 5. Now, we will rearrange the array i.e.

arr[0] = -1(NOT present in an array)
arr[1] = 1(present in an array)
arr[2] = 2(present in an array)
arr[3] = -1(NOT present in an array)
arr[4] = -1(NOT present in an array)

The method used in the program below is as follows:

• Enter an array of integer type and calculate the size of the array.

• Print the array before sorting and call the function Rearranging(arr, size)

• Inside the function Rearranging(arr, size)

  • Declare an integer type variable, assuming it is ptr

  • Start looping from i to 0 until i is less than size. Inside the loop, another loop starts from j to 0 until j is less than size.

  • Inside the loop, check if arr[j] = i, then set ptr = arr[j], arr[j] = arr[i], arr[i] = ptr And break out of the loop.

  • Start looping from i to size. Inside the loop, check if arr[i]! = i, then set arr[i] to -1.

• #Print an array after rearranging the array values.

Example

#include <iostream>
using namespace std;
void Rearranging(int arr[], int size){
   int ptr;
   for(int i = 0; i < size; i++){
      for(int j = 0; j < size; j++){
         if(arr[j] == i){
            ptr = arr[j];
            arr[j] = arr[i];
            arr[i] = ptr;
            break;
         }
      }
   }
   for(int i = 0; i < size; i++){
      if(arr[i] != i){
         arr[i] = -1;
      }
   }
}
int main(){
   int arr[] = {0, 8, 1, 5, 4, 3, 2, 9 };
   int size = sizeof(arr) / sizeof(arr[0]);
   //calling the function to rearrange an array such that arr[i] = i
   Rearranging(arr, size);
   //Printing the array
   cout<<"Rearrangement of an array such that arr[i] = i is: ";
   for(int i = 0; i < size; i++){
      cout << arr[i] << " ";
   }
}

Output

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

Rearrangement of an array such that arr[i] = i is: 0 1 2 3 4 5 -1 -1

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