C - scanf 变量，但它不是整数

Question

I want my function to read my variable while it is not an integer. My teacher taught us to do it using the form var=scanf("%d", &x) and if it is a string, it will be equal to zero. However, when I enter a string, the while loop repeats without asking me to re-enter a value.

Here's my algorithm:

int returnValue(int a, int b)
{
    int x, r;
    do{
        printf("Enter a value between %d and %d.\n", a, b);
        r=scanf("%d", &x);
    }while(x<a || x > b || r==0);
    return x;
}

If someone has any idea of the problem, it would be great.

Answer 1

while loop repeats without asking me to re-enter a value.

The non-numeric input that did not covert to an int remains in stdin for the next I/O operation. Same offending input is read again each loop in OP's code. Code should read and discard the non-numeric input.

I recommend to not use scanf() until you know why it is bad.
In the meantime, use fgets() to read line of user input.

int returnValue(int a, int b) {
  char buf[40];  // Suggest a size twice the expected max.
  int x, r;
  do {
    printf("Enter a value between %d and %d [inclusive].\n", a, b);
    if (fgets(buf, sizeof buf. stdin) == NULL) {
      fprintf(stderr, "End-of-file or input error\n");
      return INT_MIN;  // Or some other invalid value.    
    }
    int r = sscanf(buf, "%d", &x);  // or look into strtol() as a more robust solution
  } while(r != 1 || x < a || x > b); // test r first else x is undefined.
  return x;
}