Visual C++ using Console: Char/String compatibility issues with while loop

Question

    cout << "Would you like to make another transaction? (y/n)" << endl;
    cin >> repeat_transaction;
    static_cast<char>(repeat_transaction);
    while (repeat_transaction != 'y' && repeat_transaction != 'n')
    {
        cout << "Invalid selection: Please enter y or n";
        cin >> repeat_transaction;
        static_cast<char>(repeat_transaction);
    }

During the Invalid selection loop, I once accidentally pressed "mn". I noticed the console read out Invalid selection..., So, it did in fact finish and re-enter the while loop. However, after this the console terminated the program. If you enter a single character 'a' or 'y' or 'n' it acts just as it should. Ending or not ending. This was before I attempted to use static_cast to force the truncation of the user input.

Show the definition of `repeat_transaction`, and if this code is part of a loop or there is any earlier input, show that too — M.M, Oct 08 '14 at 22:38
`static_cast(repeat_transaction);` doesn't do anything, it is like writing `x + 1;` . — M.M, Oct 08 '14 at 22:39
`static_cast(repeat_transaction)` doesn't "force the truncation of the user input". It is just a compile-time conversion of the variable `repeat_transaction` from whatever type it is (which you do not specify in your question) into `char`. And BTW, since you are not assigning it to anything (e.g., `char x = static_cast(repeat_transaction)`) it has no effect on your program anyway. — barak manos, Oct 08 '14 at 23:19
"Visual C++ using Console: Char/String compatibility issues with while loop" — MyNameIsJeff, Oct 09 '14 at 05:46

score 0 · Accepted Answer · edited May 23 '17 at 12:05

Since you managed to get this program to compile I can only assume that repeat_transaction was specified as a char and not a std::string.

When you use cin to get a character it only gets one character but it doesn't flush the buffer. I believe you understand this issue since you wrote This was before I attempted to use static_cast to force the truncation of the user input. . You can attempt to use cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); instead of static_cast<char>(repeat_transaction); after each call to cin >> repeat_transaction; . There are downsides to this. If you enter 'mn' it will work as expected. It reads the m which is not y or n and then flushes the extra characters until it finds end of line \n. If you do nm, n will match and the m will be thrown away. So in that case it will accept nm as valid and exit the loop.

There are other ways that may be easier and give you the effect closer to what you are looking for. Instead of reading a character at a time you can read an entire line into a string using getline (See the C++ documentation for more information). You can then check if the length of the string is not equal to 1 character. If it's not length 1 then it is invalid input. If it is 1 then you want to check for y and n. Although basic (and not overly complex) this code would do a reasonable job:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    string repeat_transaction;
    cout << "Would you like to make another transaction? (y/n)" << endl;
    getline(cin, repeat_transaction);

    while (repeat_transaction.length() != 1 || (repeat_transaction != "y" && repeat_transaction != "n"))
    {
        cout << "Invalid selection: Please enter y or n";
        getline(cin, repeat_transaction);
    }
    return 0;
}

I said reasonable job since one deficiency you might see is that you want to trim white spaces from the beginning and end. If someone enters n or y with a space or tab in front it will be seen as invalid (whitespace at the end would be similar). This may not be an issue for you, but I thought I would mention it.

On a final note, you may have noticed I used using namespace std;. I did so to match what was in the original question. However, this is normally considered bad practice and should be avoided. These StackOverflow answers try to explain the issues. It is better to not do it and prepend all standard library references with std::. For example string would be std::string, cin would be std::cin etc.

Thank you, the string method should work well. I understand that std:: is preferred and better for compatibility, but this is a simple midterm project for a first semester computer class. I use system() as well, which I understand to be a bad idea for production code. But, for my own personal uses on Windows and within the limits of my knowledge and the course, it won't become an issue. — MyNameIsJeff, Oct 09 '14 at 05:40

Visual C++ using Console: Char/String compatibility issues with while loop

1 Answers1