C Exercises: Rearrange an array such that arr[i]=i

Last update on March 18 2025 13:20:52 (UTC/GMT +8 hours)

101. Rearrange Array so that arr[i] = i

Write a program in C to rearrange an array such that arr[i]=i.

N.B.: Given array contains N elements, from 0 to N – 1. All elements within the range may not be present in the array. There will be -1 if an element within the range is not present in the array.

Expected Output:
The given array is:
2 5 -1 6 -1 8 7 -1 9 1
The new array is: -1 1 2 -1 -1 5 6 7 8 9

The task is to write a C program that rearranges an array such that each element at index i equals i. If an element within the range [0, N-1] is not present in the array, it should be represented by -1. The program should iterate through the array, reposition elements to satisfy the condition arr[i] = i , and display the modified array.

Sample Solution:

C Code:

#include<stdio.h>

// Function to rearrange the array
int arrayRearrange(int arr1[], int l) 
{ 
    for (int i = 0; i < l; i++)  
    { 
        if (arr1[i] != -1 && arr1[i] != i)  
        { 
            int x = arr1[i]; 
            // Swap elements until an element is -1 or the same as its index
            while (arr1[x] != -1 && arr1[x] != x) 
            { 
                int y = arr1[x]; 
                arr1[x] = x; 
                x = y; 
            } 
            arr1[x] = x; 
            if (arr1[i] != i)  
            { 
                arr1[i] = -1; // Set the element to -1 if it doesn't match its index
            } 
        } 
    } 
} 

int main() 
{ 
    int arr1[] = { 2, 5, -1, 6, -1, 8, 7, -1, 9, 1 }; 
    int n = sizeof(arr1) / sizeof(arr1[0]);     
    int i = 0; 

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

    // Rearrange the array and print the new array
    printf("The new array is: ");
    arrayRearrange(arr1, n); 
    for (int i = 0; i < n; i++) 
        printf("%d  ",arr1[i]); 
}

Output:

The given array is:  
2  5  -1  6  -1  8  7  -1  9  1  
The new array is: -1  1  2  -1  -1  5  6  7  8  9  

Visual Presentation:

Flowchart:/p>

For more Practice: Solve these Related Problems:

• Write a C program to rearrange an array so that each element equals its index, filling missing positions with -1.
• Write a C program to update an array based on the mapping arr[i] = arr[arr[i]] and then fix missing indices.
• Write a C program to rearrange the array using in-place swaps such that arr[i] becomes i, using minimal extra space.
• Write a C program to rearrange an array so that each index is filled with the correct value or -1 if absent, using recursion.

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