Fix Modulo zero.

src/calc.cpp

@@ -8,7 +8,12 @@ int plus(    int a, int b) { return a + b; }
 int minus(   int a, int b) { return a - b; }
 int multiply(int a, int b) { return a * b; }
 int power(   int a, int b) { return std::pow(a, b); }
-int modulo(  int a, int b) { return a % b; }
+int modulo(  int a, int b) { 
+  if(b == 0) {
+    throw std::domain_error("Modulo by 0 not defined.");
+  }
+  return a % b;
+}
 int divide(  int a, int b) { 
   if(b == 0) {
     throw std::domain_error("Division by 0.");

test/test.cpp

@@ -67,3 +67,9 @@ TEST_CASE( "Unrecognized characters are detected and exception is thrown", "[cal
   REQUIRE_THROWS( evaluate("11+3?4") );
   REQUIRE_THROWS( evaluate("1F+") );
 }
+
+TEST_CASE( "modulo by zero is detected and exception is thrown", "[calc]") {
+   REQUIRE_THROWS( evaluate("40%") );
+   REQUIRE_THROWS( evaluate("00%") );
+   REQUIRE_THROWS( evaluate("90%") );
+}