Python recursive functions


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When a function call itself is knows as recursion. Recursion works like loop but sometimes it makes more sense to use recursion than loop. You can convert any loop to recursion.

Here is how recursion works. A recursive function calls itself. As you you’d imagine such a process would repeat indefinitely if not stopped by some condition. This condition is known as base condition. A base condition is must in every recursive programs otherwise it will continue to execute forever like an infinite loop.

Overview of how recursive function works

  1. Recursive function is called by some external code.
  2. If the base condition is met then the program do something meaningful and exits.
  3. Otherwise, function does some required processing and then call itself to continue recursion.

Here is an example of recursive function used to calculate factorial.

Factorial is denoted by number followed by ( ! ) sign i.e 4!

For e.g

Here is an example

Expected Output:

Now try to execute the above function like this

You will get

This happens because python stop calling recursive function after 1000 calls by default. To change this behavior you need to amend the code as follows.


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5 thoughts on “Python recursive functions

  1. pal

    Have a question : When I tried above program with sys.setrecursionlimit(2008), it worked fine for print(fact(2000)), it did not work for any limit between 2001 to 2007.
    Can you explain the behavior here ?

    Reply
  2. Mike

    Hi Guru
    Can you explain how this happens. With the following piece of code:

    def recursion(n):
    if n > 0:
    print(“recursion”,n)
    recursion( n-1 )
    print(n)

    recursion( 10 )

    Which outputs the following:

    recursion 10
    recursion 9
    recursion 8
    recursion 7
    recursion 6
    recursion 5
    recursion 4
    recursion 3
    recursion 2
    recursion 1
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10

    How does it output the the print( n ) statement when it is not within the recursion?

    Reply

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