add tests for exponential function with positive numbers

src/exponent.c

@@ -42,8 +42,8 @@ unsigned long long fac(int x){
 
 double exponential(double x){
     double sum = 0.0;
-    for(int i = 1; i<=21; i++){
-        sum += (1/fac(i)*(expIN(i,x)));
+    for(int i = 0; i<=21; i++){
+        sum += (1.0/fac(i)*(expIN(i,x)));
     }
     return sum;
 }

test/test_exponent.c

@@ -40,5 +40,22 @@ void test_DoubleBase_ForNegativeExponent(void){
     TEST_ASSERT_EQUAL_FLOAT(0.25, r3);
 }
 
+void test_Exponent_with_positive_number(void){
+    double r1, r2, r3;
+
+    r1 = exponential(0.0);
+    r2 = exponential(1.0);
+    r3 = exponential(2.0);
+
+    printf("Result 1: %8.lf\n", r1);
+    printf("Result 2: %lf\n", r2);
+    printf("Result 3: %lf\n", r3);
+
+    TEST_ASSERT_EQUAL_DOUBLE(1.0, r1);
+    TEST_ASSERT_DOUBLE_WITHIN(0.0001, 2.718282, r2);
+    TEST_ASSERT_DOUBLE_WITHIN(0.0001, 7.389056, r3);
+}
+
+
 
 #endif // TEST