Merge pull request #52 from MIT-Emerging-Talent/factorial_36

challenge: factorial_36

solutions/challenge_36/factorial.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""factorial python module."""
+
+
+def factorial(n):
+    """
+    Calculate the factorial of a non-negative integer n using recursion.
+
+    Args:
+        n (int): Non-negative integer whose factorial is to be calculated.
+
+    Returns:
+        int: Factorial of the input number n. The return value must be greater than or equal to 1.
+
+    Raises:
+        ValueError: If n is a negative number.
+        Examples:
+        >>> factorial(0) # 0! = 1
+        1
+        >>> factorial(4) # 4! = 24
+        24
+        >>> factorial(-1)
+        Traceback (most recent call last):
+        ...
+        ValueError: Factorial is not defined for negative numbers.
+    """
+    if n < 0:
+        raise ValueError("Factorial is not defined for negative numbers.")
+    if n == 0:
+        return 1
+    return n * factorial(n - 1)
+
+
+if __name__ == "__main__":
+    import doctest
+
+    print("running doctest...")
+    doctest.testmod(verbose=True)
+    try:
+        num = int(input("Enter a non-negative integer: "))
+        print(f"Factorial of {num} is {factorial(num)}")
+    except ValueError as e:
+        print(f"Error: {e}")

solutions/tests/challenge_36/test_factorial.py

@@ -0,0 +1,21 @@
+"""This unittest tests the python factorial program for positive numbers, negative numbers and zero"""
+
+import unittest
+from solutions.challenge_36.factorial import factorial
+
+
+class TestFactorial(unittest.TestCase):
+    """test the program for positive numbers, negative numbers and zero"""
+
+    def test_factorial_with_positive(self):
+        """test factorial of 5."""
+        self.assertEqual(factorial(5), 120)
+
+    def test_factorial_with_zero(self):
+        """test factorial of 0"""
+        self.assertEqual(factorial(0), 1)
+
+    def test_factorial_with_negative(self):
+        """test factorial of -1"""
+        with self.assertRaises(ValueError):
+            factorial(-1)