I am trying to validate the user input. But if i entered an invalid character the program goes to an infinite loop

Question

I am trying validate the user input. In case of an invalid input i am trying to ask the user to reinsert a correct numeric(double) value.

The program is not working, its going to an infinite loop.

Could you please give me any suggestion how can i do this ? Thanks.!!

int main() {

double t; /* Input from user */

int  check;
check = 0;

/* This loop is use to validate the user input.                 *
 * For example: If the user insert a character value "x".       *
 * i am trying to ask the user to insert a valid numeric value. */

while (check == 0)
{
    printf("Insert the value: ");
    if (scanf(" %lf", &t) == 1) {
        check = 1;          /* Everythink okay. No loop needed */
    }
    else
    {
        printf("Failed to read double. ");
        check = 0;          /* loop aganin to read the value */
        fflush( stdout );
    }
}

return 0;


}

expected results: $ ./a.out
Insert the value: X
Failed to read double.
Insert the value: 5


actual results :
$ ./a.out
Insert the value: X
Insert the value: Failed to read double. Insert the value: Failed to read double. ( loop )...

Answer 1

if i entered an invalid character the program goes to an infinite loop... if i entered an invalid character the program goes to an infinite loop

OP's code is simply re-attempting to convert the same failed data endlessly.

When scanf(" %lf", &t) == 0, non-numeric input remains in stdin and needs to be removed. @Eugene Sh..

int conversion_count = 0;
while (conversion_count == 0) {
  printf("Insert the value: ");
  // Note: lead space not needed. "%lf" itself consumes leading space.
  // if (scanf(" %lf", &t) == 1) {  
  conversion_count = scanf("%lf", &t); 

  // conversion_count is 1, 0 or EOF
  if (conversion_count == 0) {
    printf("Failed to read double.\n");
    fflush(stdout);

    int ch;
    // consume and discard characters until the end of the line.
    while ( ((ch = getchar()) != '\n') && (ch != EOF)) {
      ; 
    }
    if (ch == EOF) {
      break;
    }
  }
}

if (conversion_count == 1) {
  printf("Read %g\n", t);
}  else {
  printf("End-of-file or input error\n");
}