diff --git a/math/sum_of_digits.cpp b/math/sum_of_digits.cpp
index d3e7d3291..b259e48da 100644
--- a/math/sum_of_digits.cpp
+++ b/math/sum_of_digits.cpp
@@ -1,7 +1,11 @@
 /**
- * A simple C++ Program to find the Sum of Digits of input integer.
+ * Copyright 2020 @author iamnambiar
+ * 
+ * @file
+ * A C++ Program to find the Sum of Digits of input integer.
  */
 #include <iostream>
+#include <cassert>
 
 /**
  * Function to find the sum of the digits of an integer.
@@ -9,6 +13,9 @@
  * @return Sum of the digits of the integer.
  */
 int sum_of_digits(int num) {
+    if (num < 0) {
+        num = -1 * num;
+    }
     int sum = 0;
     while (num > 0) {
         sum = sum + (num % 10);
@@ -17,12 +24,18 @@ int sum_of_digits(int num) {
     return sum;
 }
 
-int main() {
-    int num;
-    std::cin >> num;
-    if (num < 0) {
-        num = -1 * num;
+/**
+ * Test function.
+ */
+void test() { 
+    int test_case_1 = sum_of_digits(119765);
+    int test_case_2 = sum_of_digits(-12256);
+    assert(test_case_1 == 29);
+    assert(test_case_2 == 16);
     }
-    std::cout << sum_of_digits(num);
+
+int main() {
+    test();
+    std::cout << "Success.";
     return 0;
-}
\ No newline at end of file
+}