feat: added addition_rule.cpp for probability (#732)

* added addition_rule.cpp for probability

* fixed cpplint errors

* Disable actions-gh-pages

Co-authored-by: Christian Clauss <cclauss@me.com>

probability/addition_rule.cpp

@@ -0,0 +1,28 @@
+#include <iostream>
+
+// calculates the probability of the events A or B for independent events
+
+double addition_rule_independent(double A, double B) {
+    return (A + B) - (A * B);
+}
+
+// calculates the probability of the events A or B for dependent events
+// note that if value of B_given_A is unknown, use chainrule to find it
+
+double addition_rule_dependent(double A, double B, double B_given_A) {
+    return (A + B) - (A * B_given_A);
+}
+
+int main() {
+    double A = 0.5;
+    double B = 0.25;
+    double B_given_A = 0.05;
+
+    std::cout << "independent P(A or B) = "
+    << addition_rule_independent(A, B) << std::endl;
+
+    std::cout << "dependent P(A or B) = "
+    << addition_rule_dependent(A, B, B_given_A) << std::endl;
+
+    return 0;
+}