12th python Chapter-05 Recursion

# Program for Factorial of natural number using recursion

def r_fact(n):
    if n==1:
        return n
    else:
        return n*r_fact(n-1)    


#main programme
n=int(input("enter n"))
if n==0:
    print("factorial is 1")
elif n<0:
    print("Sorry factorial does not exist ")
else:
    print("factorial is:",r_fact(n))

Output
enter n 5
('factorial is:', 120)

enter n -3
Sorry factorial does not exist

enter n 0
factorial is 1



#Program for Fibonacci series using recursion:  video Click here

def fib(n1,n2,n):
    if n==0:
        return
    res=n1+n2
    print(res)
    fib(n2,res,n-1)
a=0
b=1
n=int(input("How many numbers to be printed: "))
print(a)
print(b)
fib(a,b,n-2)

Output
How many numbers to be printed: 5
0
1
1
2
3



# Python Program for recursive binary search. 
 
# Returns index of x in arr if present, else -1
def binarySearch (arr, l, r, x):
 
    # Check base case
    if r >= l:
 
        mid = l + (r - l)/2
 
        # If element is present at the middle itself
        if arr[mid] == x:
            return mid
         
        # If element is smaller than mid, then it can only
        # be present in left subarray
        elif arr[mid] > x:
            return binarySearch(arr, l, mid-1, x)
 
        # Else the element can only be present in right subarray
        else:
            return binarySearch(arr, mid+1, r, x)
  else:
        #Element is not present in the array
        return -1
 
# Test array
arr = [ 2, 3, 4, 10, 40 ]
x = 10
 
# Function call
result = binarySearch(arr, 0, len(arr)-1, x)
 
if result != -1:
    print "Element is present at index %d" % result
else:
    print "Element is not present in array"