How to reverse a number in Python

There are several ways to reverse a number in Python. Here is a detailed explanation using three common approaches: converting to a string, mathematical operations, and using recursion.

1. Using String Conversion

The simplest way to reverse a number is by converting it to a string, reversing the string, and then converting it back to a number.

Code Example:

# Input number
num = 12345

# Convert to string, reverse, and convert back to integer
reversed_num = int(str(num)[::-1])

print("Reversed Number:", reversed_num)

Explanation

1. str(num): Converts the number to a string ("12345").
2. [::-1]: Slices the string in reverse order ("54321").
3. int(): Converts the reversed string back to an integer (54321).

2. Using Mathematical Operations

This approach uses arithmetic to reverse the number without converting it to a string.

Code Example:

# Input number
num = 12345
reversed_num = 0

# Loop until the number becomes 0
while num > 0:
    digit = num % 10         # Extract the last digit
    reversed_num = reversed_num * 10 + digit  # Append the digit to the reversed number
    num = num // 10          # Remove the last digit from the original number

print("Reversed Number:", reversed_num)

Explanation:

1. num % 10: Extracts the last digit of the number.
2. reversed_num * 10 + digit: Left shifts the digits of reversed_num and adds a new digit at the right.
3. num // 10: Removes the least-significant digit from num.

3. Using Recursion

This method involves a recursive function to reverse the number.

Code Example:

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

# Input number
num = 12345
reversed_num = reverse_number(num)

print("Reversed Number:", reversed_num)

Explanation:

1. The base case is when num becomes 0, at which point the reversed number (rev) is returned.
2. At each recursive step:
   • Get the last digit by using num % 10.
   • Append the digit to rev by multiplying rev by 10 then adding the digit.
   • Remove the last digit from num by doing num // 10.

Choosing the Right Method

1. String Conversion: Simple and rapid for most application scenarios.
2. Mathematical Operations: Good for problems where you can’t manipulate strings.
3. Recursion: Good for understanding recursion or solving problems programmatically.