Thread: infinite looping when 2 large integers divide

relatively new to c++ and to the forum. wrote a program that divides two integers:

Code:

int main()
{
	unsigned long a, b, c;
	do
	{
		cout << "\n\nEnter a and b respectively.\n";
		cin >> a; cin >> b;
		cout << "a/b= " << (double) a/b << "\n";
		cout << "Continue? 1=yes, 0=no\n";
		cin >> c;
	}while(c);

	return 0;
}

i understand that a digit that is more than 10 digits long cannot be fed into a or b. But what I don't understand is why would the program loop infinitely without pausing for an input in c when a huge number is entered (besides c doesn't have a value for while to respond). have search the faqs and threads but couldn't find what i wanted.

Well, I remember vaguely having this kind of issue... And yes, I have tried program on my machine and it shows the symptoms.

I maybe wrong here...

You have to remember that input and output using cin and cout are buffered. This means that until EOF (ctrl + d on unix) is sent, cin doesn't know where the current input ends. Now, to be able to tell if you have met the end of input when using cin, you should be using cin.eof() to check it.

When you/I enter a large number to the program, it gets buffered, cin reads in as much as it can for a and b. Then reads in c. Now if you are lucky and there's 0 somewhere on the input buffer, and c becomes 0 by some luck, then the code terminates. Otherwise, c will take a value greater than 0 (probably). This implies that the loop has to keep going.

After reading everything from input buffer,

Code:

cin >> c;

will simply not read in anything. So, c gets to keep its old value, and there we have infinite loop.

Please note that you cannot check for error with cin until you actually have tried to read in from the stdin. So rather than looping on the value c, you might as well check to see if EOF has been sent from the stdin.

Another note, EOF is NOT a character. It's just a convinient way to denote the end of a file. And, I think ctrl + d sends an EOF signal.

Originally Posted by Horse22

relatively new to c++ and to the forum. wrote a program that divides two integers:


i understand that a digit that is more than 10 digits long cannot be fed into a or b. But what I don't understand is why would the program loop infinitely without pausing for an input in c when a huge number is entered (besides c doesn't have a value for while to respond). have search the faqs and threads but couldn't find what i wanted.

When using cin to accept numerical data, it is wise to test to see if everything is OK (what if the user inters a non-numeric character or does someting else to cause cin to enter a "fail" state).

Try this:

Code:

#include <iostream>
using namespace std;
int main()
{
  unsigned long a, b, c;
  do
  {
    cout << "\n\nEnter a and b respectively." << endl;
    cin >> a; 
    cout << "You entered a = " << a << endl;
    if (!cin) {
      cout << "cin is NULL after reading a" << endl;
      break;
    }
    cin >> b;
    cout << "You entered b = " << b << endl;
    if (!cin) {
      cout << "cin is NULL after reading b" << endl;
      break;
    }
    cout << "a/b= " << (double)a / b << endl;
    cout << "Continue? 1=yes, 0=no"<< endl;
    cin >> c;
    cout << "You entered " << c << endl;
    if (!cin) {
      cout << "cin is NULL after reading c" << endl;
      break;
    }
  }while(c);

  return 0;
}

Then enter 11111111111 and 22222222222, or whatever large numbers (or non-numbers) that you feel like

Regards,

Dave