DSA. Data structure algorithm dRecursion.pptx

PYTHON RECURSION
What is recursion?
Recursion is the process of defining something in terms of
itself.
A physical world example would be to place two parallel
mirrors facing each other. Any object in between them would
be reflected recursively.

PYTHON RECURSIVE
FUNCTION
In Python, 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.
The following image shows the working of a
recursive function called recurse.

EXAMPLE OF A RECURSIVE
FUNCTION
Following is an example of a recursive
function
to find the factorial of an integer.
Factorial of a number is the product of all
the
integers from 1 to that number.
For example, the factorial of 6 (denoted as
6!) is
1*2*3*4*5*6 = 720.
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))
Output: The factorial of 3 is 6

• When we call this function with a positive integer,
it will recursively call itself by decreasing the number.
• Each function multiplies the number with the factorial of the number below it
until it is equal to one. This recursive call can be explained in the following steps.
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

Let's look at an image that
shows a step-by-step process
of what is going on:

• Our recursion ends when the number reduces to 1. This is called the base condition.
• Every recursive function must have a base condition that stops the recursion or else
the function calls itself infinitely.
• 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.
Let's look at one such condition.
def recursor():
recursor()
recursor()
Output
Traceback (most recent call last):
File "<string>", line 3, in <module>
File "<string>", line 2, in a
[Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

Example 1: A Fibonacci sequence is the integer sequence of 0, 1, 1, 2, 3,
5, 8….
# Program to print the fibonacci series upto n_terms
# Recursive function
def recursive_fibonacci(n):
if n <= 1:
return n
else:
return(recursive_fibonacci(n-1) + recursive_fibonacci(n-2))
n_terms = 10
# check if the number of terms is valid
if n_terms <= 0:
print("Invalid input ! Please input a positive value")
else:
print("Fibonacci series:")
for i in range(n_terms):
print(recursive_fibonacci(i))
Output
Fibonacci
series:
0
1
1
2
3
5
8
13
21
34

ADVANTAGES OF
RECURSION
•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 OF
RECURSION
1.Sometimes the logic behind recursion is hard to follow through.
2.Recursive calls are expensive (inefficient) as they take up a lot of memory
and time.
3.Recursive functions are hard to debug.

DSA. Data structure algorithm dRecursion.pptx

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