Python Tutorial



Python Tutorial

Reverse a Number in Python


In Python programming, manipulating and transforming data is a fundamental task in which one important task is the need to reverse a number – a task where the digits of a numeric value are reordered in the opposite sequence. This is an important requirement when working on numerical algorithms, cryptographic operations, or simply problem-solving.

This article will discuss various methods, techniques, and Python constructs that allow you to reverse numeric values of different magnitudes. This will also help you to develop a clear understanding of how to reverse a number in Python using function.


Let’s explore a diverse range of techniques for reversing numbers in Python, explore fundamental concepts like loops, and progress to advanced methods like recursion and stack-based operations. Each method is explained with practical code examples. 

  • Enhance your understanding of core concepts and gain proficiency in applying them to real Python projects. 

  • Suitable for both beginners looking for foundational knowledge and experienced developers seeking to broaden their skill set.

  • Provide a comprehensive guide catering to a wide spectrum of skill levels and applications.

Reverse a Number Using Python While Loop

To Reverse a number in Python using while loop is a fundamental concept in programming, and Python provides an elegant and straightforward way to achieve this task. In this section, we will understand how to reverse a number step by step using a while loop in Python.

The idea behind reversing a number with a while loop is to extract its digits from right to left and build the reversed number by adding digits progressively. Here are the steps in the process:

  • Initialize two variables: one to store the original number (num) and another to store the reversed number (reversed_num) initially set to 0.

  • In each iteration of the while loop, extract the last digit of the num using the modulo operator (%). This digit represents the rightmost digit of the original number.

  • Add the extracted digit to the reversed_num, multiplying the existing reversed_num by 10 to shift its digits one place to the left.

  • Remove the last digit from the original num using integer division (//). This operation effectively removes the rightmost digit.

  • Repeat steps 2-4 until the original number becomes 0, meaning all its digits have been processed.

  • Finally, the reversed_num will contain the reversed version of the original number.

Let's illustrate this process with an example:

Example: Reverse a Number using a While Loop

def reverse_number(num):
    reversed_num = 0
    while num > 0:
        digit = num % 10
        reversed_num = reversed_num * 10 + digit
        num = num // 10
    return reversed_num

# Example Usage
num = int(input("Enter a number: "))
reversed_num = reverse_number(num)
print(f"The reversed number is: {reversed_num}")

In this example, the `reverse_number` function takes an integer `num` as input and uses a while loop to reverse it. Inside the loop, it extracts the last digit of `num` using the modulo operator `%`, and appends it to `reversed_num`. Then, it updates `num` to exclude the last digit using floor division `//`.

For example, if you input `12345`, the function will return `54321` as the reversed number.

In this example,

  • To reverse a number using a while loop, we begin with the original number, such as 12345.

  • A while loop is employed to iterate through each digit of the number.

  • During each iteration, the last digit is extracted, added to the reversed_num, and then removed from the original number.

  • This process continues until all digits have been processed.

  • The result of the while loop is the reversed number, which, in this example, becomes 54321.

Reverse a Number using a While Loop is efficient, versatile, and suitable for various programming scenarios.

Reversing a Number Using String-Slicing

Reversing a number using string slicing involves utilizing Python's string manipulation capabilities.

  • The initial step is to convert the number into a string.

  • String slicing is employed with [::-1] to effectively reverse the string, changing the order of its characters.

  • The resulting reversed string can then be converted back into an integer, providing the reversed number.

  • This method is straightforward and useful for reversing numbers, especially when there's no need to preserve the numeric representation.

This method is suitable when you want a reversed representation of the number as a string. The output is:

In the same example,

  • The original number 12345 is converted into a string using the str() function.

  • String slicing with [::-1] is applied to the string, which effectively reverses the order of its characters.

  • The reversed string is then converted back into an integer, yielding the reversed number 54321.

Reverse a Number Using Recursion

Reversing a number using recursion is achieved by dividing the problem into smaller sub-problems.

  • In this approach, you extract the last digit of the number, recursively reverse the remaining part, and combine them to create the reversed number.

  • Recursion offers an elegant solution and enhances understanding of recursive thinking in Python.:

In an example,

  • We define a recursive function reverse_number that takes an integer num as input.

  • The base case checks if number has only one digit (i.e., less than 10). If so, it returns the number as it is.

  • Otherwise, it recursively reverses the remaining part of the number by extracting the last digit (remainder) and reversing the rest (reversed_remainder).

  • Finally, it combines the reversed remainder and the last digit to form the reversed number.

  • The output is 123456789, the reversed version of the original number 987654321.

Reversing a Number Using for loop

Reversing a number using a for loop entails iterating over the digits of the input number from left to right and constructing the reversed number.

  • This method employs a for loop, offering an alternative approach to reversing integers.

  • The Python code provided illustrates how to reverse a number in python using inbuilt functionreverse a number in python using for loop. 

This method provides an effective means of reversing the digits of an input number.

In an example,

  • The function reverse_number takes an integer num as input.

  • Inside the function, reversed_num is initialized to 0.

  • A while loop is used to extract the last digit of num, add it to reversed_num, and update num by removing the last digit (using integer division by 10).

  • This process continues until num becomes 0.

  • The output is 54321, the reversed version of the original number 12345.

This method efficiently reverses the digits of the input number using a while loop.

Reversing a Number Using Reversed Method

Python's reversed() function efficiently reverses elements in an iterable like strings, lists, or sequences.

  • It ensures resource-efficient and optimized processing for the specific task.

  • To reverse a number, you can convert it to a string.

  • Then, you apply reversed() to the string.

  • After reversing the characters, you can convert the result back to an integer.

This method is illustrated in the following Python code example.

In an example,

  • Define the function reverse_number that accepts an integer num as an argument.

  • Convert the integer num to a string using str(num).

  • Use the reversed() method to reverse the characters in the string.

  • Join the reversed characters back together into a single string.

  • Convert the reversed string back to an integer using int(reversed_str).

Reversing a Number Using the List

  • To reverse a number using the list, we first convert the number to a list of its digits.

  • This can be done by converting the number to a string and then using a list comprehension to create a list of its digits.

  • Once we have the list of digits, we can simply reverse it using Python's list slicing with [::-1].

  • After reversing the list, we can reconstruct the reversed number by joining the digits in the list and converting the result back to an integer.

  • This method is straightforward and leverages Python's list manipulation capabilities.

  • It provides an alternative approach to reversing numbers, especially when you want to work with the individual digits of the number.

Reverse a Number using the Stack

Reversing a number using a stack is based on the principles of the stack data structure.

  • In this method, each digit of the original number is pushed onto a stack in the order they appear.

  • Then, the digits are popped from the stack in reverse order, allowing the construction of the reversed number.

  • This approach showcases stack operations in Python and provides an alternative technique for reversing numbers.

In an example,

  • Digit by digit, we push the original number onto the stack.

  • Subsequently, the digits are popped from the stack in reverse order.

  • The reversed number is constructed by efficiently utilizing stack operations in Python.


In conclusion, the process of reversing a number in Python is a multifaceted endeavor that offers a spectrum of techniques and methods, each with its unique advantages and insights into programming paradigms. This skill is essential for programmers and data manipulators working on various domains, including numerical algorithms, cryptography, and general problem-solving.

In the world of programming, the ability to reverse numbers is a key to unlocking the potential of numerical data manipulation. Whether you're tackling intricate algorithms, securing data through cryptography, or simply solving everyday problems, Python equips you with the knowledge and tools to do so with precision and efficiency. 


1. Why would I need to reverse a number in Python?

Reversing a number is a fundamental operation in various programming tasks, including numerical algorithms, cryptography, and data manipulation. It's essential for tasks that involve changing the order of digits.

2. Are there any performance differences between these methods for reversing a number in Python?

In general, using a while loop or list manipulation tends to be more efficient for reversing integers, as they directly manipulate the numeric representation. String manipulation methods like slicing and recursion may involve additional conversions and can be slightly less efficient for very large numbers.

3. Are there any limitations to reversing extremely large numbers?

Reversing large numbers can lead to memory or performance issues, especially with methods like recursion. It's crucial to optimize your code and consider alternative approaches for handling very large numbers.

4. In which scenarios should I use string slicing to reverse a number?

String slicing is suitable when you need the reversed number as a string representation. It's helpful for tasks like formatting numbers for display purposes or generating palindromic sequences.

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