从istream读取输入，直到EOF

Question

I am having trouble understanding how to read from istream for my code. The program assignment I am trying to do is retrieve input from user or from an input file from the cmdline (ie. ./ < input.txt). The user input values are passed to a function that determines if it is a prime number or not. The issue is when I pass an input file (ie. input.txt) with multiple integers or characters, it only reads the first one and the program ends. I have read many questions and answers, but many of the solutions I have tried do not work.

For example, input.txt holds:

2 3 4 5

or

2
3
4
5

Here is my program, I won't provide my isPrime function as I believe it working just fine. It is just the issue with passing the input file to be read until end-of-file. Should I be using ifstream isntead? I was given a hint to use a while loop to read till end-of-file but that just keeps spitting the same first data I input in the program.

#include <iostream>
#include <cstdlib>
#include <limits>

using namespace std;

bool isPrime(int) { // Example return for isPrime
        return false;
}

int main(int argc, char *argv[]){
        // Initialize input integer
        int num = 0;

        cout << "Enter number to check if prime: ";
        cin >> num;

        // while loop to detect bad input
        while(!(cin >> num)){
                cin.clear(); // clear error flag
                cin.ignore(numeric_limits<streamsize>::max(), '\n'); //ignores bad input
        } // end while

        while(cin >> num){ // while there is valid input, do isPrime
            if(isPrime(num)){
                    cout << "prime\n";
            } else {
                    cout << "composite\n";
            }
        } // end while
        return 0;

} // end main

Answer 1

You are using the while loop incorrectly.

Use:

int main(int argc, char **argv)
{
   int num = 0;

   // Keep reading from stdin until EOF is reached or
   // there is bad data in the input stream.
   while ( cin >> num )
   {
      if(isPrime(num)){
         cout << "prime\n";
      } else {
         cout << "composite\n";
      }
   }

   return 0;

} // end main