How to read a multiple line input from command line in c or C++?

Question

For Example: If I need to read a multiple line input like(and I dont know How many lines would be there!!):

1 20

2 31

3 41

I am using something like

int main()
{
  string line;

  while(getline(cin,line) != NULL)
  {
       // some code
       // some code    
  }


}

Now the program never stops- i.e always it expects some input. How do i beak the loop when there are no more input lines ?

Answer 1

The scanf function reads from stdin.

Answer 2

while (true)
   {
   long value1;
   long value2;
   int nofValuesRead;
   nofValuesRead = scanf("%ld %ld\n",&value1,&value2);
   if (nofValuesRead==0) break;
   if (nofValuesRead>=1)
      {
      // Process value1
      }
   if (nofValuesRead>=2)
      {
      // Process value2
      }
   }

Answer 3

Note that the use of scanf directly on stdin is not very safe. For example, entering anything that can't be parsed as a number will make the loop hang. Here's a more robust implementation that reads whole lines first and then tries to parse the numbers from it.

#include <stdio.h>
#include <stdlib.h>

int main(void) {
        char * line = NULL;
        size_t sz = 0;

        while(!feof(stdin)) {
                ssize_t ln = getline(& line, & sz, stdin);

                if(ln > 0) {
                        int x, y;

                        if(sscanf(line, "%d %d", & x, & y) == 2)
                                printf("x = %i, y = %i\n", x, y);
                        else
                                puts("invalid input");
                }
        }

        return EXIT_SUCCESS;
}

Answer 4

Just insert a special end-of-input command, and parse the rest line-by-line. You can't automatically detect end-of-input, because there's no way to know if the user is genuinely finished inputting or just browsing or speaking or whatever- it's a totally system-external circumstance.

Answer 5

On linux - C-d (or Ctrl+D) outputs the EOF character, which will terminate your loop.

It's much easier to do something like...

~ $ cat sample.input | my_cool_program
output will be displayed here.

Answer 6

Just test the variable line for empty each time you read a line. If the use presses enter with no other data, then line will be empty.

#include <iostream>
#include <string>

using std::cin;
using std::getline;
using std::string;

int main(int argc, char *argv[]) {

    string line;

    while (true) {
        getline(cin, line);
        if (line.empty()) {
            break;
        }
        // some code
    }
    return 0;
}