Adding double_factorial

maths/double_factorial.py

@@ -0,0 +1,39 @@
+# function to find double factorial of given number.
+
+'''
+Double factorial of a non-negative integer n, 
+is the product of all the integers from 1 to n that have the same parity (odd or even) as n.
+It is also called as semifactorial of a number and is denoted by !!.
+For example, 
+9!! = 9*7*5*3*1 = 945. 
+Note that--> 0!! = 1.
+'''
+
+def double_factorial(input_num): 
+    """
+  Calculate the double factorial of specified number
+
+    >>>double_factorial(5)
+    15
+    >>>double_factorial(8)
+    384
+    >>>double_factorial(3)
+    3
+    >>>double_factorial(0)
+    1
+    >>>-1
+    Traceback (most recent call last):
+      File "<string>", line 11, in <module>
+      File "<string>", line 4, in doublefactorial
+    ValueError: double_factorial() not defined for negative values
+    """
+    if (input_num<0):
+       raise ValueError("double_factorial() not defined for negative values")
+    if (input_num == 0 or input_num == 1): 
+        return 1; 
+    return input_num * double_factorial(input_num - 2)
+
+
+# Driver Code 
+input_num = int(input("Enter the number to calculate its double factorial = ")) 
+print("Double factorial of",input_num,"=", double_factorial(input_num))