added logic to ConLiterToGallon and Unittest for the function

src/main/c/ConvertMode.c

@@ -139,6 +139,21 @@ double getValue(int choice) {
 
             printf("\nThe convertet result is %dlf %d", result, Fluid[endingUnit]);
             break;
+
+
+        case 9://'ml', 'l'
+            printf("\nEnter what the Unit is starting with (0 l, 1 gallon): ");
+            scanf("%d", &startingUnit);
+
+            printf("\nEnter what the value should it be changed to (0 l, 1 gallon): ");
+            scanf("%d", &endingUnit);
+
+            result = ConLiterToGallon(value, startingUnit, endingUnit);
+
+            printf("\nThe convertet result is %dlf %d", result, Fluid[endingUnit]);
+            break;
+
+
         }
     }
 }
@@ -184,9 +199,7 @@ double ConSpeed(double speed, int startingUnit, int endingUnit) {
 
 
 
-double ConLiterToGallon() {
 
-}
 
 double ConData() {
 

src/main/c/main_taschenrechner.c

@@ -517,6 +517,44 @@ double ConLiter(double liter, int startingUnit, int endingUnit) {
     }
 }
 
+double ConLiterToGallon(double fluid, int startingUnit, int endingUnit) {
+    switch (startingUnit)
+    {
+    case 0: //l to x
+        switch (endingUnit)
+        {
+        case 0: //l to l
+            return fluid;
+            break;
+
+        case 1: //l to gallon
+            return fluid * 0.264172;
+            break;
+
+        default:
+            break;
+        }
+
+    case 1: //gallon to x
+        switch (endingUnit)
+        {
+        case 0: //gallon to l
+            return fluid * 3.78541;
+            break;
+
+        case 1: //gallon to gallon
+            return fluid;
+            break;
+
+        default:
+            break;
+        }
+
+    default:
+        break;
+    }
+}
+
 //..
 // graphMode
 void graphMode() {

src/main/c/taschenrechner.h

@@ -71,6 +71,8 @@ double ConTemp(double temp, int startingUnit, int endingUnit);
 
 double ConSpeed(double speed, int startingUnit, int endingUnit);
 
-double ConLiter(double liter, int startingUnit, int endingUnit)
+double ConLiter(double liter, int startingUnit, int endingUnit);
+
+double ConLiterToGallon(double fluid, int startingUnit, int endingUnit);
 
 #endif // TASCHENRECHNER_H

src/test/c/test_taschenrechner.c

@@ -78,6 +78,10 @@ void test_ConLiter(void) {//5 l to ml
     TEST_ASSERT_EQUAL(5000, result);
 }
 
+void test_ConLiterToGallon(void) {//5 l to gallon
+    double result = ConTemp(5, 0, 1);
+    TEST_ASSERT_EQUAL(1.32086, result);
+}
 
 
 #endif // TEST