c++ loop quit with q issue

[asdfcxz]

i am trying to make a program with a loop that asks the user to input numbers until he/she presses q. Once the q key is pressed the program is supposed to calculate the average of all the numbers. When the user presses q however it just starts outputting "enter a number" an infinite amount of times. If anyone could help i'd be very appreciative
code:
#include <iostream>

using namespace std;

int main()
{
double num = 0, sum = 0, count = 0, avg;
cout << "Enter 'q' to stop entering numbers and calculate average" << endl;
cout << "Enter a number" << endl;
cin >> num;
sum = sum + num;
count = count + 1;
while(cin != 'q')
{
cout << "Enter a number: " << endl;
cin >> num;
sum = sum + num;
count = count + 1;
}
cout << endl;
cout << "Sum: " << sum << endl;
cout << "Count: " << count << endl;
avg = sum / count;
cout << "Average: " << avg << endl;
return 0;
}

[Chervil]

There are numerous ways to approach this. One possibility is to use cin.peek() to take an advance look at the next character before deciding what to do. Another, which I show below, is to read the input as a string. Then, test the contents of the string to decide what to do. I've used a stringstream to extract the number from the string. There are other ways to convert the value, for example the function std::stod().

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
    double num = 0;
    double sum = 0;
    double count = 0;
    
    cout << "Enter 'q' to stop entering numbers and calculate average" << endl;
    cout << "Enter a number" << endl;
    
    string value;
    
    while (cin >> value && value[0] != 'q')
    {
        istringstream ss(value);
 
        if (ss >> num)
        {
            sum = sum + num;
            count = count + 1;
            cout << "Enter a number: " << endl;
        }
    }
    
    cout << endl;
    cout << "Sum:   " << sum << endl;
    cout << "Count: " << count << endl;

    double avg = sum / count;
    cout << "Average: " << avg << endl;
    return 0;
}

[asdfcxz]

@Chervil i actually like your program a lot better but in the class i am in i am only allowed to use the <iostream> stuff. Is there a way to fix it with just that? Is it just a stupid issue I'm overlooking or much harder than that?

[Chervil]

It may depend on the exact specification of the problem you have been set. It's quite possible that I misunderstood what you were supposed to do.

If it is supposed to be like this:

1. input an int
2. process the int
3. input a char
4. if the char is 'q', repeat from the step 1.
5. print out results.

that would be easier than the version I presented.

I was trying to combine step 1 and step 3 into a single operation, which might be easier for the user, but makes the code more complicated.

See for example this thread which illustrates that kind of loop. Note too the useful example from Thomas1965.

http://www.cplusplus.com/forum/general/221638/#msg1017650

[asdfcxz]

@chervil yes i am trying to do steps 1 and 3 in one thing. The idea is that they either enter a number to continue the loop or they enter q which cancels the loop and calculates the average. Sorry i was a little unclear. I'll try to use the Thomas loop. Thanks so much!

[JLBorges]

#include <iostream>

int main()
{
    double sum = 0 ; // sum of all the numbers entered so far
    int count = 0 ;  // count of how many numbers were entered so far

    double next_number ; // the next number entered by the user
    char next_char ; // the next non white space character n the input buffer

    std::cout << "Enter 'q' to stop entering numbers and calculate average\n" ;
    while( std::cout << "next number? " && // prompt the user for the next number
           // read in the next character; if it is 'q', end the loop; otherwise put it back
           // into the input buffer so that we can try to read it in as part of the next number
           std::cin >> next_char && next_char != 'q' && std::cin.putback(next_char) &&
           std::cin >> next_number ) // try to read in the next number
    {
        sum += next_number ; 
        ++count ;
    }

    if( next_char != 'q' ) std::cout << "*** error: unexpected input (not a number and not 'q')\n" ;
    else if( count > 0 ) std::cout << "average: " << sum / count << '\n' ;
}