Recursive functions

A recursive function is a function that calls itself in its definition. For example the mathematical function, factorial, defined by factorial(n) = n*(n-1)*(n-2)*...*3*2*1. can be programmed as

def factorial(n):
    #n here should be an integer
    if n == 0:
        return 1
    else:
        return n*factorial(n-1)

the outputs here are:

factorial(0)
#out 1
factorial(1)
#out 1
factorial(2)
#out 2
factorial(3)
#out 6

as expected. Notice that this function is recursive because the second return factorial(n-1), where the function calls itself in its definition.

Some recursive functions can be implemented using lambda, the factorial function using lambda would be something like this:

factorial = lambda n: 1 if n == 0 else n*factorial(n-1)

The function outputs the same as above.