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.

Suggested read:

• Python program to check if a string is palindrome or not