Recursion is particularly useful for divide and conquer problems; however, it can be difficult to understand exactly what is happening, since each recursive call is itself spinning off other recursive calls. At the core of a recursive function are two types of cases: base cases, which tell the recursion when to terminate, and recursive cases that call the function they are in. A simple problem that naturally lends itself to a recursive solution is calculating factorials. The recursive factorial algorithm defines two cases: the base case when n is zero, and the recursive case when n is greater than zero. A typical implementation is the following:
def factorial(n):
#test for a base case
if n==0:
return 1
# make a calculation and a recursive call
f= n*factorial(n-1)
print(f)
return(f)
...
