added testForNumber.c

src/main/c/testForNumber.c

@@ -0,0 +1,23 @@
+double testForNumber() {
+    double num;
+    char storage[25];
+    char* notANumber;
+
+    do {
+        //Emptys the standart output buffer to ensure that this function works in loops
+        //and returns the value of num after every loop
+        fflush(stdout);
+
+        //fgets() reads the user input into the buffer storage (storage)
+        //the if condition is true as long as the input isn't empty
+        if (fgets(storage, sizeof storage, stdin) != NULL) {
+            //strtod() tries to convert the input into a double
+            //if there is anything but a number at any point of the array the pointer notANumber will point to this position
+            num = strtod(storage, &notANumber);
+        }
+        //isspace makes sure that notANumber doesn't point to an empty space
+        //the secount part makes sure that notANumber isn't pointing at 0
+    } while (!isspace(*notANumber) && *notANumber != 0);
+
+    return num;
+}