fix self-tests and unsigned comparision to zero

refer #897 and https://lgtm.com/projects/g/TheAlgorithms/C-Plus-Plus/rev/pr-f6e7cda8faf908e87511f30e782190233bdee68c

math/double_factorial.cpp

@@ -32,10 +32,33 @@ uint64_t double_factorial_recursive(uint64_t n) {
     return n * double_factorial_recursive(n - 2);
 }
 
-/// main function
-int main() {
-    uint64_t n;
-    std::cin >> n;
-    assert(n >= 0);
-    std::cout << double_factorial_iterative(n);
+/** Wrapper to run tests using both recursive and iterative implementations.
+ * The checks are only valid in debug builds due to the use of `assert()`
+ * statements.
+ * \param [in] n number to check double factorial for
+ * \param [in] expected expected result
+ */
+void test(uint64_t n, uint64_t expected) {
+    assert(double_factorial_iterative(n) == expected);
+    assert(double_factorial_recursive(n) == expected);
 }
+
+/**
+ * Test implementations
+ */
+void tests() {
+    std::cout << "Test 1:\t n=5\t...";
+    test(5, 15);
+    std::cout << "passed\n";
+
+    std::cout << "Test 2:\t n=15\t...";
+    test(15, 2027025);
+    std::cout << "passed\n";
+
+    std::cout << "Test 3:\t n=0\t...";
+    test(0, 1);
+    std::cout << "passed\n";
+}
+
+/// main function
+int main() { tests(); }