Why is scanf() causing infinite loop in this code?

Question

I've a small C-program which just reads numbers from stdin, one at each loop cycle. If the user inputs some NaN, an error should be printed to the console and the input prompt should return again. On input of "0", the loop should end and the number of given positive/negative values should be printed to the console. Here's the program:

#include <stdio.h>

int main()
{
    int number, p = 0, n = 0;

    while (1) {
        printf("-> ");
        if (scanf("%d", &number) == 0) {
            printf("Err...\n");
            continue;
        }

        if (number > 0) p++;
        else if (number < 0) n++;
        else break; /* 0 given */
    }

    printf("Read %d positive and %d negative numbers\n", p, n);
    return 0;
}

My problem is, that on entering some non-number (like "a"), this results in an infinite loop writing "-> Err..." over and over. I guess it's a scanf() issue and I know this function could be replace by a safer one, but this example is for beginners, knowing just about printf/scanf, if-else and loops.

I've already read the answers to this question and skimmed through other questions, but nothing really answer this specific problem.

Answer 1

scanf consumes only the input that matches the format string, returning the number of characters consumed. Any character that doesn't match the format string causes it to stop scanning and leaves the invalid character still in the buffer. As others said, you still need to flush the invalid character out of the buffer before you proceed. This is a pretty dirty fix, but it will remove the offending characters from the output.

char c = '0';
if (scanf("%d", &number) == 0) {
  printf("Err. . .\n");
  do {
    c = getchar();
  }
  while (!isdigit(c));
  ungetc(c, stdin);
  //consume non-numeric chars from buffer
}

edit: fixed the code to remove all non-numeric chars in one go. Won't print out multiple "Errs" for each non-numeric char anymore.

Here is a pretty good overview of scanf.