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# Python Program To Reverse a Number

## Problem Definition

Create a Python program to reverse a number in Python.

## Solution

This article will show multiple solutions for reversing a number in Python.

## Reversing a number mathematically

The algorithm below is used to reverse a number mathematically with time complexity of O(log n) where n is input number.

### Algorithm

Input : `num`

(1) Initialize `rev_num = 0`

(2) Loop while `num > 0`

• (a) Multiply `rev_num` by 10 and add remainder of num divide by 10 to `rev_num`
• (b) Divide num by 10

(3) Return `rev_num`

### Program

``````num = 12345
rev_num = 0
while num != 0:
rev_num = rev_num * 10
rev_num = rev_num + (num%10)
num = num // 10

print(rev_num)``````

### Output

``54321``

## Using reversed() method

Python's built in `reversed()` method returns an iterator that accesses the given sequence in the reverse order.

### Program

``````# input
num = 1234

rev_iterator = reversed(str(num))

rev_num = "".join(rev_iterator)
print(rev_num)``````

### Output

``4321``

Note that the `reversed()` method doesn't accept integer as a parameter therefore data type is converted to a string.

Since `reversed()` returns an iterator we need to join it before printing it out.

## Using slicing

Python string supports slicing to create substring.

``str_object[start_pos:end_pos:step]``

The slicing starts with the `start_pos` index (included) and ends at the `end_pos` index (excluded). The step parameter is used to specify the steps to take from the start to end index.

### Program

``````num = 123

rev_num = str(num)[::-1]
print(rev_num)``````

### Output

``321``

Since slicing work on a string we need to convert the number to a string, then create a slice that starts with the length of the string, and ends at index 0 with a step of -1.

PROGRAMS

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