Beginner Question - Exiting while loop, input type double as condition, C++

Question

Hi, I've only just recent began learning C++ and am having a little issue with while loops when the condition for the while loop is an input, of type double, from the user. I understand that if the user doesn't enter a value compatible with the double type then the loop is automatically broken. The issue is my console application exits upon entering anything other than a double.

The current exercise I'm working on has asked to make use of a while loop and if statements to calculate a number of entered measurements and output the total. The user enters a value and then the measuring system (i.e 25 m for 25 metres). It also has to record and output the highest and lowest values entered.

double value = 0;
double total = 0;
double high = 0;
double low = 0;
string unit = " ";

while (cin >> value >> unit && unit != "convert")
{
if (unit == "in") {total = total+(value*2.54);// in = inches converting to cm
        if (value*2.54 > high) high = value;
        if (value*2.54 < low) low = value;
        }

else if (unit == "m"){total = total+(value*100);// m = metres, converting to cm
        if (value*100 > high) high = value;
        if (value*100 < low) low = value;
        }

else if (unit == "ft"){total = total+(value*30.48);// ft = feet, converting to ft 
        if (value*30.48 > high) high = value;
        if (value*30.48 < low) low = value;
        }
else if (unit == "cm"){total = total+value;// cm = centremetres 
        if (value*2.54 > high) high = value;
        if (value*2.54 < low) low = value;
        }

else cout << "Unable to calculate unit type - " << unit << endl;

value = 0;
}

cout << "Total length in centre-metres: " << total << endl
     << "Total length in metres: " << total/100 << endl
     << "Total length in feet: " << total/30.48 << endl
     << "Total length in inches: " << total/2.54 << endl
     << "\nHighest value: " << high << ", Lowest value: " << low << endl;


keep_window_open();
return 0; 

keep_window_open() is part of the custom header file that came with my C++ book, all it does is ask for a user to enter a character to quit.

I know the current state of the program is a bit bloated but I wanted to get it all working correctly first. I had to use a work around of, if the user enters 'convert' as a unit the loop is killed so I could actually see if it was working correctly. The program does finish/complete its task when anything other than an double is entered into value but the results flash up on the console window and then it exits immediately.

Any help is appreciated.

Answer 1

Your problem is that when you say:

cin >> value

and enter anything other than a double, the stream goes bad, because value is expecting a double. Your keep_window_open() function also fails, because the stream is still bad.

There are two ways round this:

• Run your program from an existing command line prompt window - this will stay open after the program ends. This is the best general purpose solution.

• Clear the stream before using your "pause" code. Unfortunately, the endline you typed will still be in the buffer. Replace your "pause" code with:

_

string line;
if ( ! cin ) {
    cin.clear();
    getline( cin, line );
}
getline( cin, line );

Answer 2

Your program exists because it's the way it suppose to work. You can add in the end a call to wait for a char hit... then you will see your response, and when you hit a char it will exit.

std::cin.get();

Answer 3

To exit a loop you can use a break instruction.

To see results before terminating you can use a getchar() function.