C: Reading pairs of ints from user until newline (scanf, sscanf, fgets and friends)

Question

I'm trying to read unknown pairs from stdin, until a new line is entered.

Input example:

Enter something:

5

Enter pairs:

1 3 4 2 5 2 5 4

What's the best method of doing that? I've tried several of methods, including fgets & sscanf - but couldn't get the expected outcome.

Here's what I've tried, but I keep missing the \n:

Method #1:

while (1)
{
    scanf("%d %d", &a, &b);

    // do something with a,b

    if (getchar() == '\n')
        break;
}

Method #2:

while (scanf("%d %d", &a, &b) == 2)
{
    // do something with a,b

    if (getchar() == '\n')
        break;
}

I keep getting into an infinite loop - what am I doing wrong?

Answer 1

I believe the easiest way to handle spaces at the end of the line -- which is what probably causes your problem -- is to read the line in advance and parse it with sscanf. It could look roughly like this:

#include <stdio.h>

int main() {
  char line[1024];
  char const *p;
  int x, y;
  int n;

  fgets(line, 1024, stdin);

  for(p = line; sscanf(p, " %d %d%n", &x, &y, &n) == 2; p += n) {
    printf("%d %d %d\n", x, y, n); // to show you what happens
  }

  return 0;
}

Here the %n makes sscanf tell you the number of characters that has been processed up to that point, and we use that number to advance our read pointer in every iteration.

This handles an uneven number of numbers in a line by ignoring the last one, which may or may not be what you want to happend.