嵌套 if else 语句 C++

Question

Although I have a statement that will exit the program if the user enters q or enters an invalid statement, the code after the if and if else statement still displays. For example, if the user enters q, the code compiles and reads: "You have chosen to exit the program. Thank you for using the program!" But then it also displays the weight and travel time as 0.

int main()
{
    string planet;
    char selection;
    double weightEarth = 0;
    double rate = 0;
    double distanceFromSun = 0;
    double surfaceGravity = 0;
    double newWeight = 0;
    double travelTime = 0;
    double travelDistance = 0;
    double distanceFromPlanets = 0;
    double distanceFromEarthToSun = 93.0;

    cout << "INTERPLANETARY TRAVEL MENU\n"
        << "----------------------------\n"
        << "a) Mercury\n"
        << "b) Venus\n"
        << "c) Earth\n"
        << "d) Mars\n"
        << "e) Jupiter\n"
        << "f) Saturn\n"
        << "g) Uranus\n"
        << "h) Neptune\n"
        << "q) Quit\n"
        << "Select a planet to travel to or q to quit the program :\n";

    cin >> selection;

    if (selection <= 'h')
    {

        cout << "Please enter your weight: \n";
        cin >> weightEarth;

        cout << "At what speed would you like to travel at?\n";
        cin >> rate;


        if (selection == 'a')
        {
            planet = "Mercury";
            surfaceGravity = 0.27;
            distanceFromSun = 36;
        }

        else if (selection == 'b')
        {
            planet = "Venus";
            surfaceGravity = 0.86;
            distanceFromSun = 67.0;
        }

        else if (selection == 'c')
        {
            planet = "Earth";
            surfaceGravity = 1.00;
            distanceFromSun = 93.0;
        }

        else if (selection == 'd')
        {
            planet = "Mars";
            surfaceGravity = 0.37;
            distanceFromSun = 141.0;
        }

        else if (selection == 'e')
        {
            planet = "Jupiter";
            surfaceGravity = 2.64;
            distanceFromSun = 483.0;
        }

        else if (selection == 'f')
        {
            planet = "Saturn";
            surfaceGravity = 1.17;
            distanceFromSun = 886.0;
        }

        else if (selection == 'g')
        {
            planet = "Uranus";
            surfaceGravity = 0.92;
            distanceFromSun = 1782.0;
        }

        else
        {
            planet = "Neptune";
            surfaceGravity = 1.44;
            distanceFromSun = 2793.0;
        }

    }

    else if (selection == 'q')
    {
        cout << "You have chosen to quit the program. Thank you for using the program!\n";
    }

    else
    {
        cout << "You have entered an invalid selection.\n";
    }

    newWeight = weightEarth * surfaceGravity;

    distanceFromPlanets = abs(distanceFromSun - distanceFromEarthToSun);

    travelTime = travelDistance / rate;

    cout << "Your weight on " << planet << " :  " << newWeight << endl;
    cout << "Your travel time to " << planet << ":\n";

Answer 1

Your q and invalid selection paths only print to the console, they don't exit the program. You would need a return 0; in each.

A better way, though, is to use a boolean that is set if you want to quit or the selection is invalid.

Here is one way to go about it.

Declare the boolean as true

bool validPlanet = true;

In the quit and invalid paths put this statement

validPlanet = false;

Then wrap your weight calculations in an if condition like so

if( validPlanet)
{
    newWeight = weightEarth * surfaceGravity;

    distanceFromPlanets = abs(distanceFromSun - distanceFromEarthToSun);

    travelTime = travelDistance / rate;

    cout << "Your weight on " << planet << " :  " << newWeight << endl;
    cout << "Your travel time to " << planet << ":\n";

}

I'm assuming you have a return 0; immediately after this, otherwise you'll need to use the validPlanet to assure those statements get called appropriately.

Answer 2

A simple way is to put getch() after the statement followed by exit(0);

The last else if part then becomes:

  else if (selection == 'q')
    {
        cout << "You have chosen to quit the program. Thank you for using the program!\n";
        getch();  //stops the current screen until a key stroke
        exit(0);  //exits the program. (zero value in the argument tells that it was a safe exit)
    }

OR, if you don't want the user to give a key stroke before exiting, you may use the delay(time) function which stops screen for a particular time.

Then the code becomes:

  else if (selection == 'q')
    {
        cout << "You have chosen to quit the program. Thank you for using the program!\n";
        delay(1000)  //stops the current screen for 1 second. (time in miliseconds, 1000ms = 1 second)
        exit(0);  //exits the program. (zero value in the argument tells that it was a safe exit)
    }

Answer 3

Your code reformatted according to your need (since we haven't gone over the usage of booleans in if statements. At the moment I'm trying to re-structure my if and if else statements to fix the problem, but I can't seem to get the correct combination.)

    double weightEarth = 0; //made these variables global so that display_result()can access it.
    double rate = 0;
    double distanceFromSun = 0;
    double surfaceGravity = 0;
    double newWeight = 0;
    double travelTime = 0;
    double travelDistance = 0;
    double distanceFromPlanets = 0;
    double distanceFromEarthToSun = 93.0;

    void display_result()
    {
           newWeight = weightEarth * surfaceGravity;

           distanceFromPlanets = abs(distanceFromSun - distanceFromEarthToSun);

           travelTime = travelDistance / rate;

           cout << "Your weight on " << planet << " :  " << newWeight << endl;
           cout << "Your travel time to " << planet << ":\n";
    }

    int main()
    {
    string planet;
    char selection;
    
    cout << "INTERPLANETARY TRAVEL MENU\n"
        << "----------------------------\n"
        << "a) Mercury\n"
        << "b) Venus\n"
        << "c) Earth\n"
        << "d) Mars\n"
        << "e) Jupiter\n"
        << "f) Saturn\n"
        << "g) Uranus\n"
        << "h) Neptune\n"
        << "q) Quit\n"
        << "Select a planet to travel to or q to quit the program :\n";

    cin >> selection;

    if (selection <= 'h')
    {

        cout << "Please enter your weight: \n";
        cin >> weightEarth;

        cout << "At what speed would you like to travel at?\n";
        cin >> rate;


        if (selection == 'a')
        {
            planet = "Mercury";
            surfaceGravity = 0.27;
            distanceFromSun = 36;
        }

        else if (selection == 'b')
        {
            planet = "Venus";
            surfaceGravity = 0.86;
            distanceFromSun = 67.0;
        }

        else if (selection == 'c')
        {
            planet = "Earth";
            surfaceGravity = 1.00;
            distanceFromSun = 93.0;
        }

        else if (selection == 'd')
        {
            planet = "Mars";
            surfaceGravity = 0.37;
            distanceFromSun = 141.0;
        }

        else if (selection == 'e')
        {
            planet = "Jupiter";
            surfaceGravity = 2.64;
            distanceFromSun = 483.0;
        }

        else if (selection == 'f')
        {
            planet = "Saturn";
            surfaceGravity = 1.17;
            distanceFromSun = 886.0;
        }

        else if (selection == 'g')
        {
            planet = "Uranus";
            surfaceGravity = 0.92;
            distanceFromSun = 1782.0;
        }

        else
        {
            planet = "Neptune";
            surfaceGravity = 1.44;
            distanceFromSun = 2793.0;
        }

        display_result();

    }

    else if (selection == 'q')
    {
        cout << "You have chosen to quit the program. Thank you for using the program!\n";
    }

    else
    {
        cout << "You have entered an invalid selection.\n";
    }
}
    

I created a function display_result() and shifted the code which was running on exiting the if-else statements. Now this is being called every time only when you try to calculate the weight and not when you are exiting the program. This solved the problem of program complexity and lengthiness of program.