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How to Reverse a Number in Python?

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14th Jun, 2023
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How to Reverse a Number in Python?

Reversing a number means changing the order of an input variable from the back towards the front. If a number is entered and then it is assigned to a variable, reversion will reverse the order of the input variable. The reversal is a kind of logical process. It can be achieved through logic. Therefore any conditional statements forming logic will help in the reversal of a number. Several programming languages offer various methods for the reversal of a number. In the programming language Python, conditional statements are used for the reversal of a number. These conditional statements may be in the form of for loop, if condition, while the statement of conditions, etc.

Reversing a number in Python is a typical job that occurs frequently in programming. Python has several methods to reverse a number, whether it be an integer or a string representation. We will look at many techniques on how to reverse a number in Python in this review post. You will have a thorough understanding of how to reverse number in Python at the end of this review.

Also, lots of methods are pre-defined in the Python programming language that can be applied along with the statements of conditions for the creation of the logic behind the reversing of a number. Reverse functions are such types of functions consisting of pre-defined and conditional statements. The function can be used for any input the user enters for performing the reversal operation. Therefore, in Python, it is possible to reverse a number in several ways. 

In this article, the reversing of a number will be discussed through the methods in Python programming languages. Before going into the details of the reversing of a number, the logic behind the reversal of a number should be understood.

Suppose a given number is of the value 1234, then the Reverse of the number will be 4321. The input of the program will be an integer, and even the Reverse will be an integer.  

A few points are discussed briefly to know the concept behind reversing a number.

  • There are two methods through which the program can read the input numbers. These methods are the input() method or the method raw_input().
  • The program must now check whether the number entered is an integer or not an integer.
  • Now it has to be checked whether the integer read by the program is of value greater than zero, i.e., 0.
  • A variable with the name reverse is to be created and then initialized with the value of 0. 
  • A remainder has to be calculated for the input number. This is calculated through the operator “mod (%).”
  • The reverse variable is to be multiplied by 10, and the value of the remainder that was calculated is added to the reverse variable. 
  • The input variable is to be floor divided by the number 10. 
  • Likewise, the value of the input variable will become zero at some point in time.
  • The above steps, such as steps 5, 6, and 7, must be repeated until and unless the input number turns out to be greater than the value 0. 
  • The last step includes the display of the reverse number.

Reversing a number in Python

 In Python, an integer can be converted in two different ways.

  • At first, the integer can be converted into a string. The string can then be reversed and then converted back into an integer.
  • The integer can be mathematically reversed through the condition statements without converting them into a string. 

Method 1: Using Arithmetic Operations

Using Python’s arithmetic operations is a simple way to reverse a number. One by one, we can remove the digits from the number and create the reversal. Here is the procedure in detail:

  1. Convert the given number to a string using the str() function.
  2. Reverse the string using slicing with a step size of -1.
  3. Convert the reversed string back to an integer using the int() function.

Method 2: Using a While Loop 

Reversing a number can also be accomplished by using a while loop. We can reverse the number using this technique without turning it into a string. Here is how to go about it:

  1. Initialize a variable reversed_num to 0.
  2. Use a while loop to iterate until the given number becomes 0.
  3. In each iteration, extract the last digit of the number using the modulo operator (%).
  4. Multiply the reversed_num by 10 and add the extracted digit.
  5. Divide the given number by 10, discarding the last digit in each iteration.
  6. After the while loop ends, reversed_num will contain the reversed number.

Method 3: Using Recursion 

Python also allows us to use recursion to reverse a number. When using recursion, a function continuously invokes itself until a predetermined condition is satisfied. Here’s an illustration of how to use recursion to reverse a number.

  1. Define a recursive function reverse_number that takes the number as an argument.
  2. Check if the number is less than 10. If true, return the number itself.
  3. Otherwise, calculate the last digit of the number using the modulo operator (%).
  4. Divide the number by 10 to discard the last digit.
  5. Call the reverse_number function recursively with the updated number.
  6. Multiply the result of the recursive call by 10 and add the last digit to obtain the reversed number.

Conversion of an integer into a string and then its reversal

Reversing a number through this method of conversion into a string is easier, and there is no requirement of any logic in the reversal program. The number which the user enters is converted into a string. The string is then reversed, and once the reversal of the string is done, it is converted back into an integer. Python offers methods for reversing a string, and the user can use a suitable method for this task.

  • Python provides several built-in functions for converting an integer into a string and a string into an integer. 
  • The built-in function str() can be used for converting an integer into a string. Any data type is taken by the function and then converted into a string for that data type. Python has one function for the conversion of integers to a string, but it has other methods that the user can use. The keywords “%s”, function .format, or the function f-string can be used for this required conversion. 
  • One function is the int() which is in-built in Python. The function takes the string of any data type and then returns an integer. Another option provided by Python for the conversion of string to integer is the keyword float().
  • Reversal of a string in Python uses several ways. Like converting a string into an integer and vice versa, Python does not have any functions for the reversal of a string. However, a string can be reversed in Python through the use of different functions and methods.

Check out all trending Python tutorial concepts in 2024

Conversion of string mathematically

Through the use of while loop

Before going into the details of the program, the algorithm of the program should be understood. Once the logic is understood, the user will write the program of reversing a number. The user will be able to write programs in Python and any language the user prefers. 

The algorithm of the program is defined below:

  • Initialization of the reverse variable i.e. revs_number = 0 
  • A while loop should be used, where while the number > 0
  • Multiply the reverse variable by the number 10, and then add the value of the remainder of the given number

Division by the number 10

The reverse variable, i.e., revs_number will be equal to the sum of revs_number*10 and number%10;

  • Division by the number 10
  • The reverse variable, i.e., revs_number is returned.

Implementing the algorithm mentioned above into a Python program to reverse a number.

Reverse a Number in Python - image 1

Source

The explanation of the program is:

  • A variable “number” is initialized with the user input number, and a reverse variable “revs_number” is initialized with the value of 0. 
  • After iterating through one round, the following happens in the code.

Reverse a Number in Python - Screenshot 2

Source

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  • In the second iteration, 

Reverse a Number in Python - Screenshot 3

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After the second iteration, the value in the variable number and the variable Reverse has changed into the following values. The value of the variable number has changed to 123, and the value in the revs_number has changed to 54.

After the third iteration 

Reverse a Number in Python - Screenshot 4

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After the fourth iteration 

Reverse a Number in Python - Screenshot 5

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After the fifth iteration

Reverse a Number in Python - Screenshot 6

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 After this iteration, the while loop gets terminated.

Through the use of the Slicing method

The code for the Reverse of a number in Python using the slicing method is shown below.

Reverse a Number in Python - Screenshot 7

Source

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The steps for the execution of the program are as follows:

  • The code in the Python language has to be saved in the user drive. 
  • The command prompt is to be opened, and then the drive where the file is saved is located.
  • The file is then to be executed through the command which is “Python file_name.py”.
  • The programs written in Python are saved with the extension of “.py”.

The output of the above code will be 

Reverse a Number in Python - Screenshot 8

Source 

Apart from these mentioned methods of reversing a string, several other ways can be considered. The user can use a for loop, reversed method, methods like two-digit conversion, three-digit conversion, recursion methods, and non-recursive methods, using functions, list method, stack method, etc. 

Through any of the methods mentioned above, a user can convert the number into its reverse form. 

There are several ways to reverse a number in Python. In this review, we covered three typical strategies: recursion, while loops, and arithmetic operations. These methods make it simple to reverse an integer in Python. Always pick the approach that best satisfies your unique needs and tastes. Now that you know how to do it, you can confidently reverse numbers, including how to reverse an integer in Python, improving your programming abilities and allowing you to quickly find solutions to a variety of coding issues. 

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Pavan Vadapalli

Blog Author
Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.

Frequently Asked Questions (FAQs)

1Is there any built-in function in Python for reversing a string?

No, there are no built-in functions in Python for directly reversing an integer.

2Does the programming language have the function reverse()?

Yes, there is a reverse() function in Python that is used for reversing the objects that are stored in a list. If the function is used with anything other than a list, then it will generate an AttributeError.

3How can an integer be reversed in Python?

An integer can be converted by using condition statements like while loop, for loop, etc., for reversing a given integer. Else, the number can be converted into a string, and then the string is reversed and converted back into the number.

4What does the reversal of a number mean?

The reversal of a number means that the order of the number is changed from back to the front. For example, if an integer is of the value 5436, then the reversal of the number will be 6345.

5What does it mean by a modulus operator in Python?

The modulus operator is used to find the remains of a number that is the dividend from dividing it by a different number, the divisor. The syntax of the modulus operator is %.

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