Fix binomial_distribution overflow at 171 trials

maths/binomial_distribution.py

@@ -1,41 +1,43 @@
-"""For more information about the Binomial Distribution -
-https://en.wikipedia.org/wiki/Binomial_distribution"""
-
-from math import factorial
-
-
-def binomial_distribution(successes: int, trials: int, prob: float) -> float:
-    """
-    Return probability of k successes out of n tries, with p probability for one
-    success
-
-    The function uses the factorial function in order to calculate the binomial
-    coefficient
-
-    >>> binomial_distribution(3, 5, 0.7)
-    0.30870000000000003
-    >>> binomial_distribution (2, 4, 0.5)
-    0.375
-    """
-    if successes > trials:
-        raise ValueError("""successes must be lower or equal to trials""")
-    if trials < 0 or successes < 0:
-        raise ValueError("the function is defined for non-negative integers")
-    if not isinstance(successes, int) or not isinstance(trials, int):
-        raise ValueError("the function is defined for non-negative integers")
-    if not 0 < prob < 1:
-        raise ValueError("prob has to be in range of 1 - 0")
-    probability = (prob**successes) * ((1 - prob) ** (trials - successes))
-    # Calculate the binomial coefficient: n! / k!(n-k)!
-    coefficient = float(factorial(trials))
-    coefficient /= factorial(successes) * factorial(trials - successes)
-    return probability * coefficient
-
-
-if __name__ == "__main__":
-    from doctest import testmod
-
-    testmod()
-    print("Probability of 2 successes out of 4 trials")
-    print("with probability of 0.75 is:", end=" ")
-    print(binomial_distribution(2, 4, 0.75))
+"""For more information about the Binomial Distribution -
+https://en.wikipedia.org/wiki/Binomial_distribution"""
+
+from math import comb
+
+
+def binomial_distribution(successes: int, trials: int, prob: float) -> float:
+    """
+    Return probability of k successes out of n tries, with p probability for one
+    success
+
+    The function uses the factorial function in order to calculate the binomial
+    coefficient
+
+    >>> binomial_distribution(3, 5, 0.7)
+    0.30870000000000003
+    >>> binomial_distribution (2, 4, 0.5)
+    0.375
+    >>> binomial_distribution(85, 171, 0.5)
+    0.060749764626165485
+    """
+    if successes > trials:
+        raise ValueError("""successes must be lower or equal to trials""")
+    if trials < 0 or successes < 0:
+        raise ValueError("the function is defined for non-negative integers")
+    if not isinstance(successes, int) or not isinstance(trials, int):
+        raise ValueError("the function is defined for non-negative integers")
+    if not 0 < prob < 1:
+        raise ValueError("prob has to be in range of 1 - 0")
+    probability = (prob**successes) * ((1 - prob) ** (trials - successes))
+    # Calculate the binomial coefficient: n! / k!(n-k)!
+    # math.comb is exact and avoids converting huge factorials to float.
+    coefficient = comb(trials, successes)
+    return probability * coefficient
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
+    print("Probability of 2 successes out of 4 trials")
+    print("with probability of 0.75 is:", end=" ")
+    print(binomial_distribution(2, 4, 0.75))