# 5 Best Ways to Flip Numbers in Python

π‘ Problem Formulation: Reversing or flipping a number means to invert the order of its digits. For example, if the input is `123`, the output should be `321`. This common programming challenge has multiple solutions in Python. This article explores five distinct ways to flip a number and demonstrates how each method can be implemented with concise Python code snippets.

## Method 1: Using String Conversion and Slicing

String conversion and slicing is a straightforward approach to flipping a number. First, the number is converted to a string. Then, string slicing is used with a step of -1 to reverse the characters. Finally, the reversed string is converted back to an integer.

Here’s an example:

```def flip_number(n):
return int(str(n)[::-1])

print(flip_number(123))
```

Output: `321`

This method first converts the integer to a string with `str(n)`, then reverses the string using slicing `[::-1]`, and finally converts it back to an integer with `int()`. It’s a clean, one-liner solution that leverages Python’s string manipulation capabilities.

## Method 2: Using a Loop and Mathematical Operations

Flipping a number can also be achieved through a loop combined with mathematical operations. This method repeatedly takes the last digit of the number using the modulo operation and adds it to a new number while updating the original number by removing its last digit using integer division.

Here’s an example:

```def flip_number(n):
result = 0
while n:
result = result * 10 + n % 10
n //= 10
return result

print(flip_number(123))
```

Output: `321`

In this example, we create a variable `result` set to zero. As long as `n` is not zero (meaning there are still digits left), we multiply `result` by 10 and add the last digit of `n`. Then we remove the last digit from `n` by integer division with 10. This method avoids string conversion.

## Method 3: Using Recursion

Recursion can also be used to flip a number by calling a function within itself with modified parameters to reach the base case. With each recursive call, we handle a digit and append it to the accumulating result until no digits are left.

Here’s an example:

```def flip_number(n, result=0):
if n == 0:
return result
else:
return flip_number(n // 10, result * 10 + n % 10)

print(flip_number(123))
```

Output: `321`

The function `flip_number` takes two parameters: the number to flip and an accumulator (defaulting to 0). If `n` is 0, it returns the result; otherwise, it calls itself with `n` stripped of the last digit and the result appended with the last digit of `n`. Recursion is a powerful method but is less intuitive than iterative approaches.

## Method 4: Using the `reversed()` Function and Join

Python’s `reversed()` function can reverse any sequence, including strings. By converting the number to a string, we can reverse it with `reversed()` and then join the resulting sequence back into a string to convert it into an integer.

Here’s an example:

```def flip_number(n):
return int(''.join(reversed(str(n))))

print(flip_number(123))
```

Output: `321`

The `flip_number` function converts the input `n` to a string, reverses it with `reversed()`, joins the reversed sequence, and converts it back to an integer with `int()`. It uses Python’s built-in functions to simplify the process.

## Bonus One-Liner Method 5: Using List Comprehension and Join

List comprehension can be used together with `join()` to create a one-liner that flips a number. We convert the number to a list of its digits in reverse order and then join them back into a string to convert into an integer.

Here’s an example:

```flip_number = lambda n: int(''.join([digit for digit in str(n)][::-1]))

print(flip_number(123))
```

Output: `321`

This example uses a lambda function to define `flip_number`. It converts the number to a string, creates a list of digits using list comprehension, reverses the list, joins it into a string, and converts it back to an integer. It’s concise but less readable than other methods.

## Summary/Discussion

• Method 1: String Conversion and Slicing. Simple and concise. The approach may not be optimal for extremely large numbers due to conversion overhead.
• Method 2: Loop with Mathematical Operations. More efficient for long numbers. Less straightforward and requires understanding of arithmetic operations.
• Method 3: Using Recursion. Elegant, but may lead to a stack overflow for very large numbers due to deep recursive calls.
• Method 4: `reversed()` Function and Join. Exploits built-in functions for readability. Involves multiple type conversions which may not be necessary when working with numbers.
• Method 5: List Comprehension and Join. Compact one-liner. Potentially less readable and not the most performant due to list operations.