How to reject letter when requesting int in C

Question

When I enter a letter the loop runs infinitely. Does a letter store as a zero when it is input as an int? How can I reject a non digit answer, just I have rejected an answer outside the range?

int main(int argc, const char * argv[]) {
// insert code here...

int categoryToScore;
int categoryScores = 6;

printf("Enter category to save score: ");
scanf("%d", &categoryToScore);

while (categoryToScore >= categoryScores || categoryToScore <= 0) {
    printf("Error: invalid command. Enter 1-5 to save to an unused category\n");
    printf("Enter category to save score: ");
    scanf("%d", &categoryToScore);
}

return 0;

}

Just for background

I want to:

1. print a request an input that is between 1 and an upper bound
2. scanf for the input
3. check if the input is of a correct type and within the correct range
4. if it isn't then print an error message and go back to 1.
5. if it is then proceed

Answer 1

You are asking scanf to read a number from standard input. Scanf finds a non-digit character in the standard input and does not remove it from the standard input. Scanf fails and returns 0 (the number of fields successfully processed).

The next time you call scanf, it finds the same character at the start of standard input. So the process repeats indefinitely.

One solution is to read stdin one character at a time.

Another solution is to read (and discard) the one character from stdin before calling scanf again.

int main(int argc, const char * argv[]) {
// insert code here...

int categoryToScore;
int categoryScores = 6;
int scantRetVal;

printf("Enter category to save score: ");
scantRetVal = scanf("%d", &categoryToScore);
if (scantRetVal != 1) {
    getchar(); // read and discard one character from stdin
    categoryToScore = 0;
}

while (categoryToScore >= categoryScores || categoryToScore <= 0) {
    printf("Error: invalid command. Enter 1-5 to save to an unused category\n");
    printf("Enter category to save score: ");
    scantRetVal = scanf("%d", &categoryToScore);
    if (scantRetVal != 1) {
        getchar(); // read and discard one character from stdin
        categoryToScore = 0;
    }
}

return 0;
}

Answer 2

Rather than fix this particular program I will show how to solve ANY similar problem using a concept called an "exit condition".

The idea of an exit condition is that you have an infinite loop and it has various exit conditions. Often there are two exit conditions: one for success and one for an error.

while( true ){   /* infinite loop */
   char c = ...  /* get the character you want */
   if( c < '0' || c > '9' ){ 
      printf( "invalid character, not a digit\n" );
      continue; // get another character
   }
   ... /* do whatever you with valid data */
   if( c == '3' ) break; /* your exit condition, whatever it is */
   if( c == '7' ) exit(0); /* exit the whole program */
}

---

Note: If you are accepting free form input (numbers and strings), scanf is probably not a good idea. scanf accepts very specific, well-formatted input. So if you ask for a %d, then there better be a %d (decimal number) in the input or you will have problems.

For example, if you accept numbers and strings, you should take everything as strings using fgets or something like that.

---

Here is a complete program that does what you want:

#include <stdio.h>
#include <stdbool.h>

int main(int argc, const char * argv[]) {
  int iMaxScore = 6;
  int charInput = 0;
  int iInputValue = 0;

  while( true ){
      printf("Enter category to save score: ");
GetInput:
      charInput = getchar();
      if( charInput == 10 || charInput == 13 ) goto GetInput; /* ignore enter key */
      if( charInput == 'q' ) break;
      if( charInput < '0' || charInput > '9' ){
          printf( "invalid entry, not a digit %d\n", charInput );
          break;
      }
      iInputValue = charInput - '0';
      if( iInputValue > iMaxScore ){
          printf( "Error, input value exceeds maximum category %d\n", iMaxScore );
          continue; /* try again */
      }
      printf( "you entered category %d\n", iInputValue );
      /* continue ... */
  }

  return 0;
}