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 just a number. How would I go about doing that?

(I know that this is probably an easy question for a lot of you, but I'm taking my first programming class in C++ so I know very little. Any help would be greatly appreciated.)

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 <iostream>
#include <string>

// 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.

Answer 2

Have you checked out atoi, or the even better strtol? I recommend starting there.

Answer 3

Treat user input as std::string or char *, then validate whether it contains a valid digit character.

Answer 4

Pass your number as a reference and return an error code. Using the function argument as an output parameter.

bool parseNumber(int &n)
{
... 
//assign to number to n
//   if number parsing is ok return true;
...
   return false; 
}

int main()
{
   int number=0;

   if(!parseNumber(number))
      std::cout << "Number parsing failed\n";
}

Answer 5

In addition to the great answers here, you could try using std::stringstream:

http://cplusplus.com/reference/iostream/stringstream/stringstream/

It works like any other stream for the most part, so you could do something like:

  int converted;
  string user_input;

  cin >> user_input;

  stringstream converter(user_input);


  if(!(converter >> converted)) {
    cout << "there was a problem converting the input." << endl;
  }
  else {
    cout << "input successfully converted: " << converted << endl;
  }

HTH!

P.S. personally, I would just use boost::lexical_cast<>, but for a homework assignment you probably won't have boost available to you. If you become a professional C++ programmer, Boost will become one of your best friends outside of the STL.