Reverse an Array in C: Program to Reverse Array Elements

Introduction

In this tutorial, we will learn how to write a C program to reverse an array.

Reversing an array means changing the order of elements so that the first element becomes the last, the second becomes the second last, and so on. It is used in many real-world applications, such as:

• Reversing a string (which is an array of characters)
• Processing data in reverse order
• Undoing operations in data structures
• Palindrome checking algorithms

C Program to Reverse an Array (Method 1: Using Another Array)

#include <stdio.h>

int main() {
    int n, i;
    
    // Ask user for number of elements
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    // Declare arrays
    int arr[n];
    int reversed[n];
    
    // Read elements into the array
    printf("Enter %d elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    // Reverse the array by copying in reverse order
    for(i = 0; i < n; i++) {
        reversed[i] = arr[n - 1 - i];
    }
    
    // Display original array
    printf("\nOriginal array: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    
    // Display reversed array
    printf("\nReversed array: ");
    for(i = 0; i < n; i++) {
        printf("%d ", reversed[i]);
    }
    printf("\n");
    
    return 0;
}

Sample Output

Enter the number of elements: 5
Enter 5 elements:
10 20 30 40 50

Original array: 10 20 30 40 50
Reversed array: 50 40 30 20 10

Another Example:

Enter the number of elements: 4
Enter 4 elements:
5 15 25 35

Original array: 5 15 25 35
Reversed array: 35 25 15 5

Program Explanation

Let's break down the code step by step:

1. Include Header File: #include <stdio.h> includes the standard input/output library.
2. Declare Variables:
   • int n; — stores the number of elements
   • int i; — loop counter
3. Declare Arrays:
   • int arr[n]; — the original array
   • int reversed[n]; — the reversed array
4. Get User Input: Prompts the user to enter the number of elements and then reads them into the array.
5. Reverse the Array: The for loop copies elements from arr to reversed in reverse order:
   • reversed[i] = arr[n - 1 - i];
   • When i = 0, copies the last element to the first position
   • When i = 1, copies the second last element to the second position
   • And so on...
6. Display Results: Prints both the original and reversed arrays.
7. Return: return 0; indicates successful program execution.

Method 2: In-Place Reversal (More Efficient)

The in-place reversal method does not use extra space. It swaps elements from both ends of the array until the middle is reached.

#include <stdio.h>

int main() {
    int n, i, temp;
    
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    int arr[n];
    
    printf("Enter %d elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    // In-place reversal using two pointers
    for(i = 0; i < n/2; i++) {
        // Swap arr[i] with arr[n-1-i]
        temp = arr[i];
        arr[i] = arr[n - 1 - i];
        arr[n - 1 - i] = temp;
    }
    
    // Display reversed array
    printf("\nReversed array: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    
    return 0;
}

Sample Output:

Enter the number of elements: 6
Enter 6 elements:
10 20 30 40 50 60

Reversed array: 60 50 40 30 20 10

How In-Place Reversal Works

The algorithm uses two pointers:

• Pointer 1: Starts from the beginning of the array (i)
• Pointer 2: Starts from the end of the array (n - 1 - i)
• Swap the elements at these two positions
• Move pointer 1 forward and pointer 2 backward
• Stop when the pointers meet or cross (i < n/2)

Algorithm for In-Place Reversal

1. Start
2. Read the number of elements (n)
3. Read n elements into the array arr
4. For i = 0 to n/2 - 1:
   • Set temp = arr[i]
   • Set arr[i] = arr[n - 1 - i]
   • Set arr[n - 1 - i] = temp
5. Print the reversed array
6. End

#include <stdio.h>

void reverseArray(int arr[], int start, int end) {
    // Base case: if start is greater than or equal to end
    if(start >= end) {
        return;
    }
    
    // Swap elements at start and end
    int temp = arr[start];
    arr[start] = arr[end];
    arr[end] = temp;
    
    // Recursively reverse the remaining array
    reverseArray(arr, start + 1, end - 1);
}

int main() {
    int n, i;
    
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    int arr[n];
    
    printf("Enter %d elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    // Call recursive function to reverse
    reverseArray(arr, 0, n - 1);
    
    printf("\nReversed array: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    
    return 0;
}

Frequently Asked Questions

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