Made improvements to combinations.py (#3681)

* Made improvements to combinations.py

* Update maths/combinations.py
Co-authored-by: Du Yuanchao <shellhub.me@gmail.com>

* Function now raises an error when given invalid input

* Update maths/combinations.py
Co-authored-by: Du Yuanchao <shellhub.me@gmail.com>

maths/combinations.py

+"""
+https://en.wikipedia.org/wiki/Combination
+"""
 from math import factorial
 
 
-def combinations(n, k):
+def combinations(n: int, k: int) -> int:
     """
+    Returns the number of different combinations of k length which can
+    be made from n values, where n >= k.
+
+    Examples:
     >>> combinations(10,5)
     252
+
     >>> combinations(6,3)
     20
+
     >>> combinations(20,5)
     15504
+
+    >>> combinations(52, 5)
+    2598960
+
+    >>> combinations(0, 0)
+    1
+
+    >>> combinations(-4, -5)
+    ...
+    Traceback (most recent call last):
+    ValueError: Please enter positive integers for n and k where n >= k
     """
+
+    # If either of the conditions are true, the function is being asked
+    # to calculate a factorial of a negative number, which is not possible
+    if n < k or k < 0:
+        raise ValueError("Please enter positive integers for n and k where n >= k")
     return int(factorial(n) / ((factorial(k)) * (factorial(n - k))))
 
 
 if __name__ == "__main__":
-    from doctest import testmod
 
-    testmod()
+    print(
+        "\nThe number of five-card hands possible from a standard",
+        f"fifty-two card deck is: {combinations(52, 5)}",
+    )
+
+    print(
+        "\nIf a class of 40 students must be arranged into groups of",
+        f"4 for group projects, there are {combinations(40, 4)} ways",
+        "to arrange them.\n",
+    )
+
+    print(
+        "If 10 teams are competing in a Formula One race, there",
+        f"are {combinations(10, 3)} ways that first, second and",
+        "third place can be awarded.\n",
+    )