Python Recursions

1. Functions
2. Python Recursion

What is recursion and what is recursive function?

Recursion means a technique in which a function calls itself. The function which is programmed in such a way that it can itself for some purpose is known as a recursive function.

The real-world example for a recursive function would be the reflection of two parallel mirrors placed facing each other

Example1

#printing the hello world 10 times using recursion.
def helloworld(n):
    if(n>1):
        print("Helloworld")
        Helloworld(n-1) # self calling functions: recursions
    else:
        return

Helloworld(10)

Output

Helloworld
Helloworld
Helloworld
Helloworld
Helloworld
Helloworld
Helloworld
Helloworld
Helloworld
Helloworld

Example2

#Finding the factorial of a number using python.
def factorial(num):
    if num==1 or num==0:
        return 1
    else:
        return num*factorial(num-1)

print("10! = ",factorial(10))
print("5! = ",factorial(5))

Output

10! = 3628800
5! = 120

Example3

#Finding the Fibonacci series using python recursion.
def fibonacci(num):
    if num==0 or num==1:
        return num
    else:
        return fibonacci(num-1)+fibonacci(num-2)

for number in range(11): #fibonacci series length=10
    print(fibonacci(number))

Output

0
1
1
2
3
5
8
13
21
34
55

Note:

By default, python limits the depth of recursion in order to avoid infinite recursion which may lead to stack overflow. So, in python maximum number of recursion depth is 1000 by default. After the completion of max depth, it will throw an error called RecursionError.

Example

def iamrecursion():
    iamrecursion()
    iamrecursion()

Output

Traceback (most recent call last):
[Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded