Description

To rearrange an array such that arr[i] = i, first check if any element is 
equal to index or not, if yes swap the element.

At last, check if any element not equal to its index, replace it with -1.

C/C++

/* C Program to Rearrange an array such that arr[i] = i */
//Save it as RearrangeArray.c

#include<stdio.h>
int main(){

    int n, i, j, temp;

    printf("Enter the size of array : ");
    scanf("%d",&n);

    int arr[n];

    printf("Enter the array elements : ");
    for(i=0;i<n;i++) {
        scanf("%d",&arr[i]);
    }

    printf("Entered the array elements are : ");
    for(i=0;i<n;i++) {
        printf("%d ", arr[i]);
    }

    //Checking in array
    for(i=0;i<n;i++) {
        for(j=0;j<n;j++) {
            if(arr[j] == i) {
                temp = arr[j];
                arr[j] = arr[i];
                arr[i] = temp;
                break;
            }
        }
    }

    //if arr[i] is not equal to i then assign it to -1
    for(i=0;i<n;i++)  {
        if(arr[i] != i) {
            arr[i] = -1;
        }
    }

    printf("\n");
    printf("Elements after rearrangement : ");
    for(i=0;i<n;i++) {
        printf("%d ",arr[i]);
    }
}

Input:
Enter the size of array : 
5
Enter the array elements : 
3
5
1
4
6

Output:
Entered the array elements are : 
3 5 1 4 6 
Elements after rearrangement : 
-1 1 -1 3 4

Java

/* Java Program to Rearrange an array such that arr[i] = i */
//Save it as RearrangeArray.java

import java.io.*;
import java.util.Scanner;

public class RearrangeArray {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        
        System.out.println("Enter the size of array : ");
        int size = scanner.nextInt();
        
        int arr[] = new int[size]; 
        int i, j, temp;
        
        System.out.println("Enter the array elements : ");
        for(i=0;i<size;i++) {
            arr[i] = scanner.nextInt();
        }
        
        System.out.println("Entered the array elements are : ");
        for(i=0;i<size;i++) {
            System.out.print(arr[i] + " ");
        }
        
        //Checking in array
        for(i=0;i<size;i++) {
            for(j=0;j<size;j++) {
                if(arr[j] == i) {
                    temp = arr[j];
                    arr[j] = arr[i];
                    arr[i] = temp;
                    break;
                }
            }
        }
        
        //if arr[i] is not equal to i then assign it to -1
        for(i=0;i<size;i++)  { 
            if(arr[i] != i) {
                arr[i] = -1; 
            } 
        }
        
        System.out.println();
        System.out.println("Elements after rearrangement : ");
        for(i=0;i<size;i++) {
            System.out.print(arr[i] + " ");
        }
    }
}

Input:
Enter the size of array : 
5
Enter the array elements : 
3
5
1
4
6

Output:
Entered the array elements are : 
3 5 1 4 6 
Elements after rearrangement : 
-1 1 -1 3 4

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