Basic Python 25: Recursion in Python | Python

Python Recursion:
----------------------------
What is recursion?
Recursion is the process of defining something in terms of itself.

real exp:
would be to place two parallel mirrors facing each other. Any object in between them would be reflected recursively.

Python Recursive Function:
we know that a function can call other functions. It is even possible for the function to call itself. These types of construct are termed as recursive functions.
syntax:

def recursion():
-----
----statements
recursion() //call this method
--
--statements

recursion() // calling function
ex:
Factorial of a number

def factorial(x):
"""This is a recursive function
to find the factorial of an integer"""

if x == 1:
return 1
else:
return (x * factorial(x-1))


num = 3
print("The factorial of", num, "is", factorial(num))

how it is run?
--------------
factorial(3) # 1st call with 3
3 * factorial(2) # 2nd call with 2
3 * 2 * factorial(1) # 3rd call with 1
3 * 2 * 1 # return from 3rd call as number=1
3 * 2 # return from 2nd call
6 # return from 1st call

Our recursion ends when the number reduces to 1. This is called the base condition.

The Python interpreter limits the depths of recursion to help avoid infinite recursions, resulting in stack overflows.

By default, the maximum depth of recursion is 1000. If the limit is crossed, it results in RecursionError.
ex:
def recursor():
recursor()
recursor()

result:
Traceback (most recent call last):
File "string", line 3, in module
File "string", line 2, in a
File "string", line 2, in a
File "string", line 2, in a
[Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

Advantages:

- Recursive functions make the code look clean and elegant.
- A complex task can be broken down into simpler sub-problems using recursion.
- Sequence generation is easier with recursion than using some nested iteration.

Disadvantages:

- Sometimes the logic behind recursion is hard to follow through.
- Recursive calls are expensive (inefficient) as they take up a lot of memory and time.
- Recursive functions are hard to debug.
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