c ++循环永远运行而无需等待输入

Question

I am trying to write code that will loop and input user input to a class and print out a chart. This is my main method:

int main()
{
    Company productMatrix;
    int inputNumber = 0;
    cout << "enter the salesman id or -1 to quit." << endl;
    cin >> inputNumber;
    while(inputNumber != -1)
    {
        int salesman = inputNumber;
        cout << "enter the product id." << endl;
        cin >> inputNumber;
        int product = inputNumber;
         cout << "enter the amount sold." << endl;
        cin >> inputNumber;
        double dollarValue = inputNumber;
        productMatrix.inputSales(salesman, product, dollarValue);
        cout << "enter the salesman id or -1 to quit." << endl;
        cin >> inputNumber;
    }
    productMatrix.printChart();
    cout << "Goodbye!";
    return 0;
}

when I run the program, it will let me input one set of data and then loop forever without waiting for me to stop. This is what it looks like:

enter the salesman id or -1 to quit.
3
enter the product id.
2
enter the amount sold.
55.99
enter the salesman id or -1 to quit.
enter the product id.
enter the amount sold.
enter the salesman id or -1 to quit.
enter the product id.
enter the amount sold.
// etc...

I'm guessing there is something wrong with my loop. How can I fox this?

Answer 1

You're writing a double 55.99 to an integer, so cin takes 55 and it has '.' in the buffer which is always !=-1 but never gets read into as integer.

Answer 2

The problem is with following line.

double dollarValue = inputNumber;

inputNumber is a integer type and dollar value is a float. So there is a type mismatch. You can create another variable like dollarInput and store the dollar value there

Answer 3

inputNumber is an int . But you entered a value (55.99) that can not be interpreted as an int . This put cin into an error state. Until the error is cleared, all future operations with cin fail. So it doesn't wait for your input, and the variables retain their values, and you can never get that -1 that the loop needs to terminate.

To check for an error, just use a plain old if statement:

if (cin) {
    // cin is okay
}
else {
    // cin is not okay
}

You can also be a little more concise and put your input operation directly in an if statment:

if (cin >> inputNumber) {

To clear the error:

cin.clear();

You will also probably need to clear the input stream, otherwise the erroneous input will remain in the input buffer, and cin will just try to read it again:

cin.ignore(); // discard one character from the input buffer
// or
cin.ignore(N); // discard N characters from the input buffer

Anyway, that's the cause of the infinite loop. But if you had just input directly into a double , instead of an int , you wouldn't have seen this issue. Isn't that what you want anyway?

Answer 4

To add to prajmus's answer, you can see the additional 'junk' in the input stream by adding the following 'cin' read:

...
double dollarValue = inputNumber;
productMatrix.inputSales(salesman, product, dollarValue);
cout << "enter the salesman id or -1 to quit." << endl;

double myDbl;
cin >> myDbl;
cout << "read the following double:" << myDbl << endl;
...

The added "cin >> myDbl" will read the '.99' from the input stream and the added cout will yield:

0.99