Output error when input isn't a number. C++

Question

I am making a function that takes a number from the user\'s input and finds the absolute value of it. I want to make it return an error if the user inputs anything other than j

Answer 1

If you are actually trying to program in idiomatic C++, ignore the (intentionally?) bad advice you are being given. Especially the answers pointing you toward C functions. C++ may be largely backwards-compatible with C, but its soul is a totally different language.

Your question is so foundational as to make for a terrible homework assignment. Especially if you're so adrift that you don't know to avoid conio.h and other tragedies. So I'm just going to write out a solution here:

#include 
#include 

// Your function is presumably something like this
// although maybe you are just using integers instead of floats
float myAbs(const float x) {
    if (x >= 0) {
        return x;
    } else {
        return -x;
    }
}

int main(int argc, char* argv[]) {
    // give a greeting message followed by a newline
    std::cout << "Enter values to get |value|, or type 'quit'" << std::endl;

    // loop forever until the code hits a BREAK
    while (true) {
        // attempt to get the float value from the standard input
        float value;
        std::cin >> value;

        // check to see if the input stream read the input as a number
        if (std::cin.good()) {

            // All is well, output it
            std::cout << "Absolute value is " << myAbs(value) << std::endl;

        } else {

            // the input couldn't successfully be turned into a number, so the
            // characters that were in the buffer that couldn't convert are
            // still sitting there unprocessed.  We can read them as a string
            // and look for the "quit"

            // clear the error status of the standard input so we can read
            std::cin.clear();

            std::string str;
            std::cin >> str;

            // Break out of the loop if we see the string 'quit'
            if (str == "quit") {
                break;
            }

            // some other non-number string.  give error followed by newline
            std::cout << "Invalid input (type 'quit' to exit)" << std::endl;
        }
    }

    return 0;
}

This lets you use the natural abilities of the iostream classes. They can notice when they couldn't automatically convert what a user entered into the format you wanted, and give you a chance to just throw up your hands with an error -or- try interpreting the unprocessed input a different way.