Why an Infinite Loop

Question

I've been learning C++, and this chunk of code is from a simple grading program. But when I try to get the user input, there's a problem. If I enter a number, whether it be less than 0 or more than 100, or anything in between, my loop works fine. But if I type in any letter, or any non-alphanumeric character (ex: +, (, %, etc.) I get an infinite loop with "Please enter a grade value between 0 and 100" printed forever. What am I doing wrong?

Thanks.

int _tmain(int argc, _TCHAR* argv[])
{
using namespace std;

int grade = -1; // grade will hold grade value; initialized to -1
do {
    cout << "Please enter a grade value between 0 and 100." << "\n";
    cin >> grade;
} while (grade < 0 || grade > 100);

cout << grade << "\n";

printGrade(grade);

return 0;
}

Answer 1

if cin>>grade fails (aka can't parse as int) it does not consume the stream. You can try:

int main()
{ using namespace std;

int grade = -1; // grade will hold grade value; initialized to -1
do {
    cout << "Please enter a grade value between 0 and 100." << "\n";
    if (!(cin >> grade))
    {
      cin.clear();
    }
} while (grade < 0 || grade > 100);

cout << grade << "\n";

return 0;

}

But this is only part of the problem. Really, you should use std::getline and parse the grade as a full line for correct input.

Answer 2

If cin does not receive valid input for the data type (int), the variable grade is not changed and remains at -1. You can test whether the input was successful like so

bool success = (cin >> grade);
if (! success)
{
    cin.clear();
    cout << "bad input\n";
    break;
}

You can also use this as a shortcut if (! (cin >> grade))

Note that you need to clear the error state of cin before you use it again.

Answer 3

I'm pretty sure the cin failed so you may need to reset its fail flag or something like that.

Add this to your loop:

if (cin.fail())
{
    cout << "failed";
    cin.clear();
}

Answer 4

Correctly and safely reading until you get valid input is far trickier than you'd think. If there's invalid input, like a letter, the stream is set in a "failure" state, and refuses to read any more characters until you clear the state. But even if you clear the state, that input is still waiting there, in the way. So you have to ignore those characters. The easiest thing to do is simply ignore everything until the next enter key, and then try the input again.

But it gets even more complicated, because if the stream has an error, it gets set in a "bad" state, or if it reaches the end of the stream, it gets set in a "eof" state. Neither of these two are recoverable, so you must detect them and quit the program to avoid an infinite loop. Even more irritating, istreams have a .fail() function, but that checks if it's in fail or bad, which makes it nearly useless in my opinion. So I wrote a little invalid_input which checks if the stream can continue.

Note that get_grade sets the fail flag manually if the input is out-of-range.

#include <iostream>
#include <stdlib.h>
#include <limits>

bool invalid_input(std::istream& in)
{return in.rdstate() == std::ios::failbit;}

std::istream& get_single_grade(std::istream& in, int& grade) {
    std::cout << "Please enter a grade value between 0 and 100." << "\n";
    if (in>>grade && (grade<0 || grade>100))
        in.setstate(std::ios::failbit);
    return in;
}

bool get_grade(std::istream& in, int &grade) {
    while(invalid_input(get_single_grade(in, grade))) { //while we failed to get data
         in.clear(); //clear the failure flag
         //ignore the line that the user entered, try to read the next line instead
         in.ignore(std::numeric_limits<std::streamsize>::max(),'\n'); 
    }
    return in.good();
}

int main(int argc, char* argv[]) {    
    int grade = -1; // grade will hold grade value; initialized to -1
    if (get_grade(std::cin, grade) == false) {
        std::cerr << "unxpected EOF or stream error!\n";
        return false;
    }    
    std::cout << grade << "\n";
    return EXIT_SUCCESS;
}

As you can see here, this doesn't get into an infinite loop when given out of bounds numbers, end of file, a stream failure, or invalid characters.