A **factorial of a number** is the product of all positive integers less than or equal to that number. In mathematics, the **factorial of a non-negative integer n** is represented as n! For example, the factorial of 5 is 5! = 5 x 4 x 3 x 2 x 1 = 120. In this tutorial, we will learn how to **find the factorial of a** **number in Python using recursion.**

### What is Recursion?

**Recursion** is a technique in computer programming where a function calls itself until it reaches a specific stopping condition. This allows us to solve problems by breaking them down into smaller sub-problems and solving each sub-problem individually.

**Python Program to Find Factorial of Number Using Recursion**

# Find Factorial of Number in Python Using Recursion def recur_factorial(n): if n == 1: return n else: return n*recur_factorial(n-1) # we are taking a number from user as input # entered value will be converted to int from string num = int(input("Enter a number: ")) # check if the number is negative if num < 0: print("Sorry, factorial does not exist for negative numbers") elif num == 0: print("The factorial of 0 is 1") else: # number is passed to the recur_factorial() function print("The factorial of", num, "is", recur_factorial(num))

The code given above calculates **the factorial of a number in Python using recursion**. Here’s how the code works:

- recur_factorial(n): This function takes an integer n as an argument and
**calculates the factorial of that number using recursion.** - if n == 1:: If the number entered by the user is 1, the function returns 1 as the factorial of 1 is 1.
- else:: If the number is not 1, the function returns n multiplied by the factorial of n-1. This is where the
**recursion takes place**. The function calls itself with the argument n-1, which is one less than the original number until n becomes 1. - num = int(input(“Enter a number: “)): This line of code takes input from the user and converts it to an integer.
- if num < 0:: If the number entered by the user is negative, an error message is displayed stating that the factorial of a negative number does not exist.
- “elif“ num == 0:: If the number entered by the user is 0, the code displays the message that the factorial of 0 is 1.
- else:: If the number entered by the user is positive, the function recur_factorial(num) is called and the result is displayed as the factorial of the number.

**Output:**

```
Enter a number: 5
The factorial of 5 is 120
```

### Advantages of Using Recursion

- Simplicity: Recursion can make the code much simpler and easier to understand, especially for
**mathematical problems**. - Reusability: Recursive functions can be used again and again for solving similar problems, which
**increases code reusability.** - Abstraction: Recursion allows us to hide the implementation details and work on the abstract level, making the code more abstract and maintainable.

### Limitations of Using Recursion

- Overhead:
**Recursive functions**have overhead due to the function call overhead and the overhead of maintaining the call stack. - Memory usage:
**Recursive functions**use a lot of memory because each call creates a new stack frame, which consumes memory. - Stack overflow: If the
**recursion**goes too deep, it can cause a stack overflow error. This means that the call stack has exceeded its maximum size, and the program terminates.

### Tips for Using Recursion

- Always have a base case: A base case is a condition that stops the recursion. Without a base case, the recursion will go on forever and cause a stack overflow error.
- Use the right data structures: Recursion works well with recursive data structures like trees and links lists.
- Avoid deep recursion:
**Deep recursion**can cause a stack overflow error, so it’s important to keep the recursion depth under control.

### Conclusion

In this tutorial, we learned about **finding the factorial of a number in Python using recursion**. We saw the** advantages and limitations of using recursion**, and also learned **some tips for using recursion** effectively. Recursion is a powerful technique that can simplify code and make it easier to understand, but it’s important to use it correctly to avoid common pitfalls like stack overflow errors.

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