How to use do while loop to display menu?

[Zoren]

Hi everyone! I'm new to C++ and this is what I coded in the last few days:

#include <iostream>


using namespace std;

int main()
{ 

int  choice ;
float weight, weightE, weightJ, weightMA, weightME, weightN, weightP, weightS, weightU, weightV ;

	cout<<"*************************\n";
	cout<<"  Welcome to Planetor!\n";
	cout<<"*************************\n";
	cout<<"Please enter your weight in KG: ";
	cin>>weight;
	
	cout<<"***********************************\n";
	cout<<"Please choose one of the following:\n";
	cout<<"|-----------------|--------------------|"<<endl;
	cout<<"| Planet          |  Force of gravity  |"<<endl;
	cout<<"|-----------------|--------------------|"<<endl;
	cout<<"|1. Earth         |    1.00            |"<<endl;
	cout<<"|2. Jupiter       |    2.65            |"<<endl;
	cout<<"|3. Mars          |    0.39            |"<<endl;
	cout<<"|4. Mercury       |    0.38            |"<<endl;
	cout<<"|5. Neptune       |    1.23            |"<<endl;
	cout<<"|6. Pluto         |    0.05            |"<<endl;
	cout<<"|7. Saturn        |    1.17            |"<<endl;
	cout<<"|8. Uranus        |    1.05            |"<<endl;
	cout<<"|9. Venus         |    0.78            |"<<endl;
	cout<<"|-----------------|--------------------|"<<endl;
	cout<<" 10. Exit                               "<<endl;
	cout<<"                                        "<<endl;
	cout<<"Your choice: ";
	cin>>choice; 
	
	//codes
	
	
	else if (choice > 9 || choice < 1)    //user input validation
	{
	cout << "Invalid Input\n";
	//How to use do while loop to return to menu. 
	}
			
return 0;
}

The code is running fine. However, I want to program to re-display menu if user entered "Invalid Input" using do while loop. Is that possible? How?

[lastchance]

There are many ways of doing this.
One such method runs like this in pseudocode:

Start with an invalid result
While ( result is invalid )
   write menu
   read result
   if ( result is invalid ) inform user
End while

Another method is to write a function to input a variable. The function can be called recursively if the input is invalid.


For your code the former method would look like:

	weight = -1;                                      // start with something invalid
	while ( weight <= 0 )                             // keep looping menu and input whilst invalid
	{
	    cout<<"Please enter your weight in KG: ";
	    cin>>weight;
	    if ( weight <= 0 ) cout << "Invalid input; please try again.\n";       // notify if invalid
	}
	
	choice = 0;
	while ( choice < 1 || choice > 10 )
	{
	    cout<<"***********************************\n";
	    cout<<"Please choose one of the following:\n";
	    cout<<"|-----------------|--------------------|"<<endl;
	    cout<<"| Planet          |  Force of gravity  |"<<endl;
	    cout<<"|-----------------|--------------------|"<<endl;
	    cout<<"|1. Earth         |    1.00            |"<<endl;
	    cout<<"|2. Jupiter       |    2.65            |"<<endl;
	    cout<<"|3. Mars          |    0.39            |"<<endl;
	    cout<<"|4. Mercury       |    0.38            |"<<endl;
	    cout<<"|5. Neptune       |    1.23            |"<<endl;
	    cout<<"|6. Pluto         |    0.05            |"<<endl;
	    cout<<"|7. Saturn        |    1.17            |"<<endl;
	    cout<<"|8. Uranus        |    1.05            |"<<endl;
	    cout<<"|9. Venus         |    0.78            |"<<endl;
	    cout<<"|-----------------|--------------------|"<<endl;
	    cout<<" 10. Exit                               "<<endl;
	    cout<<"                                        "<<endl;
	    cout<<"Your choice: ";
	    cin>>choice; 
	    if ( choice < 1 || choice > 10 ) cout << "Invalid input; please try again.\n";       // notify if invalid
	}

Your code is quite repetitive. Consider:
- Eliminating all those different weight variables - you don't need a different one for each planet.
- Use either parallel arrays for planet name, relative gravitational attraction OR (better) structs containing these. Then you won't need a long line of if blocks.


Oh, and just to be pedantic! ... Pluto has been relegated from the list of planets!
http://www.bbc.co.uk/news/science-environment-33462184

[Zoren]

Regarding Pluto, it's my school work so I afraid I could not eliminate it in this case...hahaha.

I have not learnt pseudocode in the class yet, so I think starting with something simple like do-while loop first would be a lot easier.

Anyway, could you show an example to shorten those codes?

[lastchance]

@zoren,

This thread makes zero sense if you remove your original code. Please do not do that. For consistency I include your original code at the bottom of this post. I think my solution did all that was necessary for this.

If you want a simple struct then

struct Planet
{
   string name;
   double relativeGravity;
   // add any other properties, like radius of orbit etc.
};

would be fine.

You can declare an array of planets like

const int NUM_PLANETS = 9;   // if you insist on keeping Pluto
Planet parray[NUM_PLANETS] = { { "Earth", 1.0 }, { "Jupiter", 2.65 }, ... }

You can loop through the array of type Planets just like you would an array of type ints etc. Refer to the ith planet's name as parray[i].name.


"pseudocode" is not something you learn in class. It's just a common-sense way of describing to yourself in your own language or slang the order of operations in a program. It is programming-language agnostic.


Just for the record this was your original code:

#include <iostream>


using namespace std;

int main()
{ 

int  choice ;
float weight, weightE, weightJ, weightMA, weightME, weightN, weightP, weightS, weightU, weightV ;

	cout<<"*************************\n";
	cout<<"  Welcome to Planetor!\n";
	cout<<"*************************\n";
	cout<<"Please enter your weight in KG: ";
	cin>>weight;
	
	cout<<"***********************************\n";
	cout<<"Please choose one of the following:\n";
	cout<<"|-----------------|--------------------|"<<endl;
	cout<<"| Planet          |  Force of gravity  |"<<endl;
	cout<<"|-----------------|--------------------|"<<endl;
	cout<<"|1. Earth         |    1.00            |"<<endl;
	cout<<"|2. Jupiter       |    2.65            |"<<endl;
	cout<<"|3. Mars          |    0.39            |"<<endl;
	cout<<"|4. Mercury       |    0.38            |"<<endl;
	cout<<"|5. Neptune       |    1.23            |"<<endl;
	cout<<"|6. Pluto         |    0.05            |"<<endl;
	cout<<"|7. Saturn        |    1.17            |"<<endl;
	cout<<"|8. Uranus        |    1.05            |"<<endl;
	cout<<"|9. Venus         |    0.78            |"<<endl;
	cout<<"|-----------------|--------------------|"<<endl;
	cout<<" 10. Exit                               "<<endl;
	cout<<"                                        "<<endl;
	cout<<"Your choice: ";
	cin>>choice; 
	
	if (choice == 1) 
	{
   	weightE = weight*1.00;
	cout<<"Your weight on Earth is:  "<<weightE;
	}
	
	
	else if (choice == 2)
	{
	weightJ = weight*2.65;
	cout<<"Your weight on Jupiter is: "<<weightJ;
	}
	
	else if (choice == 3)
	{
	weightMA = weight*0.39;
	cout<<"Your weight on Mars is: "<<weightMA;
	}
	
	else if (choice == 4)
	{
	weightME = weight*0.38;
	cout<<"Your weight on Mercury is: "<<weightME;
	}
	
	else if(choice == 5)
	{
	weightN = weight*1.23;
	cout<<"Your weight on Neptune is: "<<weightN;
	}
	
	else if(choice == 6)
	{
	weightP = weight*0.05;
	cout<<"Your weight on Pluto is: "<<weightP;
	}
	
	else if (choice == 7)
	{
	weightS = weight*1.17;
	cout<<"Your weight on Saturn is: "<<weightS;
	}
	
	else if (choice == 8)
	{
	weightU = weight*1.05;
	cout<<"Your weight on Uranus is: "<<weightU;
	}
	
	else if (choice == 9)
	{
	weightV = weight*0.78;
	cout<<"Your weight on Venus is: "<<weightV;
	}

	else if (choice == 10)
	{
	cout<<"Program exiting..."<<endl;
	return 0;
	}
	
	
	else if (choice > 9 || choice < 1)    //user input validation
	{
	cout << "Invalid Input\n";
	//How to use bool and while loop to prompt menu. 
	}
			
return 0;
}

[Handy Andy]

Hello Zoren,

Another alternative would be to put the menu in a function and call it when needed. Then you could do something like:

do
{
	MainMenu();

	cout << "Your choice: ";
	cin >> choice;

	//codes


	if (choice > 9 || choice < 1)    //user input validation <--- The else will cause an error becuse there is no if.
	{
		cout << "Invalid Input\n";
		//How to use do while loop to return to menu. 
	}
} while (choice > 9 || choice < 1);

Or what I like to do is have "MainMenu" return the choice back to where it was called and put a do.while or while loop in the function, so it only returns a correct answer.

The above code would still apply,but you will have to replace "MainMenu();" with all your cout statements and define "choice" above the do/while loop and return choice after the do.while loop. This way everything is done in one function and the function can be called from anywhere.

Hope that helps,

Andy

[Zoren]

@lastchance

Sorry I was trying to re-code the part you mentioned to be "repetitive" so it was temporary removed.

[Handy Andy]

Hello Zoren,

This is something I like to do. See if it meets what you are looking for:

//  How touse a do while loop to display a menu.cpp Entry point of program.
//
//  Written  01/29/2017
//
//  Finished //2017
//
//  By Zoren http://www.cplusplus.com/forum/beginner/229334/
//
/*
*/
//
//
//
//
//

#include <iostream>
#include <chrono>
#include <thread>

int MainMenu();

int main()
{
	int choice{};  // <--- Always good practice to initialize your variables.
	int weight{};

	std::cout << "*************************\n";
	std::cout << "  Welcome to Planetor!\n";
	std::cout << "*************************\n";
	std::cout << "Please enter your weight in KG: ";
	std::cin >> weight;

	choice = MainMenu();

	// <--- You could use a switch here to use the value of "choice".

	switch (choice)
	{
		case 1:
			// do something.
			break;
		case 10:
			break;  // <--- Do nothing except the switch or put a return 0; here.
		default:
			break;
	}
	return 0;
}  //  End main

int MainMenu()
{
	int choice{};

	do
	{
		std::cout << "\n***********************************\n";  // <--- Added \n at the begining.
		std::cout << "Please choose one of the following:\n";
		std::cout << "|-----------------|--------------------|" << std::endl;
		std::cout << "| Planet          |  Force of gravity  |" << std::endl;
		std::cout << "|-----------------|--------------------|" << std::endl;
		std::cout << "|1. Earth         |    1.00            |" << std::endl;
		std::cout << "|2. Jupiter       |    2.65            |" << std::endl;
		std::cout << "|3. Mars          |    0.39            |" << std::endl;
		std::cout << "|4. Mercury       |    0.38            |" << std::endl;
		std::cout << "|5. Neptune       |    1.23            |" << std::endl;
		std::cout << "|6. Pluto         |    0.05            |" << std::endl;
		std::cout << "|7. Saturn        |    1.17            |" << std::endl;
		std::cout << "|8. Uranus        |    1.05            |" << std::endl;
		std::cout << "|9. Venus         |    0.78            |" << std::endl;
		std::cout << "|-----------------|--------------------|" << std::endl;
		std::cout << " 10. Exit                               " << std::endl;
		std::cout << "                                        " << std::endl;
		std::cout << "Your choice: ";
		std::cin >> choice;

		if (choice > 10 || choice < 1)    //user input validation
		{
			std::cout << "\nInvalid Input\n";  // <--- Added \n at the begining.
			std::this_thread::sleep_for(std::chrono::seconds(3));  // Requires header files "chrono" and "thread"
			//How to use do while loop to return to menu. 
		}

		} while (choice > 10 || choice < 1);

	return choice;
}  // End MainMenu 

I did not see your if/else if statements until after I wrote this, but the switch will work in place of the if/else if statements.

Using the switch long with "choice" you could define the planet gravitates as constexpr double planteGravity[10]{ 0, 1.0, 2.65, 0.39, 0.38, 1.23, 0.05, 1.17, 1.05, 0.78 };. Then in the switch refer to the needed gravity as planteGravity[choice] in your math. in the switch you can do:
std::cout << "Your weight on Earth is: " << weight * planteGravity[choice]; and eliminate the need for all those different variables.

Hope that helps,

Andy