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Loop Programs in C

Print reverse of a negative number

Introduction

Reversing a number with possible negative input means the sign is preserved after reversing the digits.

Example:

  • Input: 12345 → Output: 54321
  • Input: -123 → Output: -321

We’ll handle the sign separately and then reverse the absolute value.

 

C Program: Print Reverse of a Negative Number

NOTE: The Program is written using for, while and do..while loop.
Method 1 is for loop, Method 2 is while loop and Method 3 is do..while loop

Method 1: Using for loop

C 
#include <stdio.h>


 


int main() {


    int num, original, digit, reversed = 0;


    int isNegative = 0;


 


    // Input


    printf("Enter an integer: ");


    scanf("%d", &num);


 


    original = num; // store original number


 


    // Check if the number is negative


    if (num < 0) {


        isNegative = 1;


        num = -num; // make it positive for reversal


    }


 


    // Reverse the number using for loop


    for (; num != 0; num /= 10) {


        digit = num % 10;


        reversed = reversed * 10 + digit;


    }


 


    // Restore negative sign if needed


    if (isNegative) {


        reversed = -reversed;


    }


 


    // Display result


    printf("Reverse of %d = %d\n", original, reversed);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter an integer: 12345
Reverse of 12345 = 54321

OUTPUT 2 :
Enter an integer: -9876
Reverse of -9876 = -6789

OUTPUT 3 :
Enter an integer: 400
Reverse of 400 = 4

Explanation

  1. Read the number num and store it in original.
  2. Check if num is negative:
    • Set isNegative = 1 and convert num to positive.
  3. Use a for loop to reverse the digits of num:
    • Extract last digit using digit = num % 10.
    • Append it to reversed using reversed = reversed * 10 + digit.
    • Remove the last digit using num /= 10.
  4. If the number was negative, restore the negative sign.
  5. Print the reversed number.

 

C Program: Print Reverse of a Negative Number

Method 2: Using while loop

C 
#include <stdio.h>


 


int main() {


    int num, original, digit, reversed = 0;


    int isNegative = 0;


 


    // Input


    printf("Enter an integer: ");


    scanf("%d", &num);


 


    original = num; // store original number


 


    // Check if the number is negative


    if (num < 0) {


        isNegative = 1;


        num = -num; // make it positive


    }


 


    // Reverse the number using while loop


    while (num != 0) {


        digit = num % 10;             // extract last digit


        reversed = reversed * 10 + digit; // append digit


        num = num / 10;               // remove last digit


    }


 


    // Restore negative sign if needed


    if (isNegative) {


        reversed = -reversed;


    }


 


    // Display result


    printf("Reverse of %d = %d\n", original, reversed);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter an integer: 12345
Reverse of 12345 = 54321


OUTPUT 2 :
Enter an integer: -9876
Reverse of -9876 = -6789


OUTPUT 3 :
Enter an integer: 400
Reverse of 400 = 4

Explanation

  1. Read the number num and store it in original.
  2. Check if num is negative:
    • Set isNegative = 1
    • Convert num to positive for reversal
  3. Use a while loop:
    • Extract the last digit with digit = num % 10
    • Append it to reversed using reversed = reversed * 10 + digit
    • Remove the last digit using num = num / 10
  4. Restore negative sign if the original number was negative.
  5. Print the reversed number.

 

C Program: Print Reverse of a Negative Number

Method 3: Using do..while loop

C 
#include <stdio.h>


 


int main() {


    int num, original, digit, reversed = 0;


    int isNegative = 0;


 


    // Input


    printf("Enter an integer: ");


    scanf("%d", &num);


 


    original = num; // store original number


 


    // Check if number is negative


    if (num < 0) {


        isNegative = 1;


        num = -num; // make positive


    }


 


    // Reverse the number using do-while loop


    if (num == 0) {


        reversed = 0;


    } else {


        do {


            digit = num % 10;               // get last digit


            reversed = reversed * 10 + digit; // append digit


            num = num / 10;                 // remove last digit


        } while (num != 0);


    }


 


    // Restore negative sign if needed


    if (isNegative) {


        reversed = -reversed;


    }


 


    // Display result


    printf("Reverse of %d = %d\n", original, reversed);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter an integer: 12345
Reverse of 12345 = 54321


OUTPUT 2 :
Enter an integer: -9876
Reverse of -9876 = -6789


OUTPUT 3 :
Enter an integer: 0
Reverse of 0 = 0

Explanation

  1. Store the original number in original.
  2. Check if the number is negative:
    • Set isNegative = 1
    • Convert num to positive
  3. Use a ..while loop to reverse digits:
    • Extract last digit using digit = num % 10
    • Append to reversed using reversed = reversed * 10 + digit
    • Remove last digit using num / 10
  4. Restore negative sign if needed.
  5. Print the reversed number.

 

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