Python Program to Reverse a Number Using While Loop

You can reverse a number in Python using a while loop by extracting the last digit with num % 10, adding it to a reversed number multiplied by 10, and then removing the last digit from the original number with num //= 10 until the number becomes zero.

Examples

Input1234

Output4321

Input1000

Output1

Input7

Output7

How to Think About It

To reverse a number, think of peeling off its last digit repeatedly and building a new number from these digits in reverse order. Use a while loop to keep doing this until the original number is zero. Each time, get the last digit using num % 10, add it to the reversed number after shifting the reversed number left by one digit (multiply by 10), and then remove the last digit from the original number using num //= 10.

Algorithm

1

Get the input number.

2

Initialize reversed number as 0.

3

While the input number is greater than 0:

4

Extract the last digit using modulus 10.

5

Multiply reversed number by 10 and add the last digit.

6

Remove the last digit from the input number by integer division by 10.

7

Return the reversed number.

Code

python

num = int(input("Enter a number: "))
reversed_num = 0
while num > 0:
    digit = num % 10
    reversed_num = reversed_num * 10 + digit
    num //= 10
print("Reversed number:", reversed_num)

Output

Enter a number: 1234
Reversed number: 4321

Dry Run

Let's trace the number 1234 through the code to see how it reverses.

1

Initial values

num = 1234, reversed_num = 0

2

First loop iteration

digit = 1234 % 10 = 4; reversed_num = 0 * 10 + 4 = 4; num = 1234 // 10 = 123

3

Second loop iteration

digit = 123 % 10 = 3; reversed_num = 4 * 10 + 3 = 43; num = 123 // 10 = 12

4

Third loop iteration

digit = 12 % 10 = 2; reversed_num = 43 * 10 + 2 = 432; num = 12 // 10 = 1

5

Fourth loop iteration

digit = 1 % 10 = 1; reversed_num = 432 * 10 + 1 = 4321; num = 1 // 10 = 0

6

Loop ends

num is now 0, exit loop

num	digit	reversed_num
1234	4	4
123	3	43
12	2	432
1	1	4321

Why This Works

Step 1: Extract last digit

Using num % 10 gets the last digit of the number, which we add to the reversed number.

Step 2: Build reversed number

Multiply the current reversed number by 10 to shift digits left, then add the extracted digit to append it.

Step 3: Remove last digit

Use integer division num //= 10 to remove the last digit from the original number and continue the loop.

Alternative Approaches

String conversion

python

num = input("Enter a number: ")
reversed_num = int(num[::-1])
print("Reversed number:", reversed_num)

This method is simpler and uses string slicing but does not use a while loop as requested.

Recursive function

python

def reverse_num(num, rev=0):
    if num == 0:
        return rev
    return reverse_num(num // 10, rev * 10 + num % 10)

num = int(input("Enter a number: "))
print("Reversed number:", reverse_num(num))

Uses recursion instead of a loop; elegant but less intuitive for beginners.

Complexity: O(d) time, O(1) space

Time Complexity

The loop runs once for each digit in the number, so the time complexity is O(d), where d is the number of digits.

Space Complexity

Only a few variables are used regardless of input size, so space complexity is O(1).

Which Approach is Fastest?

The while loop method is efficient and uses constant space. String conversion is simpler but uses extra space for the string. Recursion uses more stack space and is less efficient.

Approach	Time	Space	Best For
While loop	O(d)	O(1)	Memory-efficient, beginner-friendly
String conversion	O(d)	O(d)	Quick and simple for small inputs
Recursion	O(d)	O(d)	Elegant but uses more memory