C basic menu driven program, Gas calculator with no global variables

Question

as part of an assignment I wrote a Gas cost calculator. This program outputs the current mileage, and the total spent on gas.

Here's my code :

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#define PAUSE system("pause")
#define CLS system("cls")

// Declare variables
float currentMilage = 0.0, pricePerGallon = 0.0, MPG = 0.0, totalCost = 0.0;

// Function to calculate the total cost
void calculateTotalCost() {
    totalCost=(currentMilage/MPG)*pricePerGallon;
}

//Function to read input from user
void userInput() {
    printf("Enter the current milage: ");
    scanf("%f", &currentMilage);

    printf("Enter the price per gallon: ");
    scanf("%f", &pricePerGallon);

    printf("Enter the MPG: ");
    scanf("%f", &MPG);
    calculateTotalCost();
}

// Function to calculate the cost for trip
float calculateCost(float miles) {
    float cost;
    cost=(miles/MPG)*pricePerGallon;
    return cost;
}

//Function to display cost
void displayCost(float cost) {
    printf("Cost for trip: %.2f\n", cost);
}

//Function to update the current milage
void updateCurrentMilage(float miles) {
    currentMilage = currentMilage + miles;
}

// Function to calcuate the total spend on gas
void updateTotalSpend(float cost) {
    totalCost=totalCost+cost;
}

// Function takeTrip
void takeTrip() {
// Declare variables
    float miles,cost;

// Read input from user
    printf("Enter the miles driven: ");
    scanf("%f", &miles);

// Caliculate the total cost
    cost=calculateCost(miles);

// Display the cost
    displayCost(cost);

// Update the current milage
    updateCurrentMilage(miles);

// Update the total spend
    updateTotalSpend(cost);
}

// Function to display the menu
int menu() {
    int ch;

    printf("\n\n");
    printf("1: Take a trip\n");
    printf("2: Display the current milage\n");
    printf("3: Display the total spend\n");
    printf("4: Exit\n");

    printf("Enter the choice: ");
    scanf("%d",&ch);
    return ch;
}

// Function to display the current milage
void displayCurrentMilage() {
    printf("The current milage is %.2f\n",currentMilage);
}

// Function to display the total spend
void displayTotalSpend() {
    printf("The total spend on gas is %.2f\n",totalCost);
}

// Main class
int main() {
    int ch=0;

// Read the user input
    userInput();

// while start
    while(ch!=4) {
        ch=menu();
        switch(ch) {
        case 1:
            takeTrip();

            PAUSE;
            CLS;

            break;

        case 2:
            displayCurrentMilage();

            PAUSE;
            CLS;

            break;

        case 3:
            displayTotalSpend();

            PAUSE;
            CLS;

            break;

        case 4:
            break;
        }
    } // while end

    getchar();
    return 0;

} // end main

The program works excellently, I just realized I am not allowed to use Global Variables or goto statements. The teacher said I must not use global variables, prototype the functions, and call functions passing values and returning stuff through the functions. How can I do this? Thanks!

Answer 1

Move global declarations:

float currentMilage = 0.0, pricePerGallon = 0.0, MPG = 0.0, totalCost = 0.0;

to main .

Change:

void calculateTotalCost() { 

        totalCost=(currentMilage/MPG)*pricePerGallon; 
}

to

float calculateTotalCost(float currentMilage, float MPG, float pricePerGallon) { 

      float totalCost = (currentMilage/MPG)*pricePerGallon; 
      return totalCost;
}

Modify functions which do calculations to take more parameters and return the value:

 void updateCurrentMilage(float miles) { 

       currentMilage = currentMilage + miles; 
 }

like:

  float updateCurrentMilage(float miles, float currentMilage) { 

       float current = currentMilage + miles;
       return current; 
 }

Use the returned value to update the appropriate variable;

    currentMilage = updateCurrentMilage(miles,currentMilage); 

Repeat the process for all relevant data which you want to calculate.