Thread: How to determine if input is numerical

You can check the return value of scanf(). If it is not 1, you handle the input error accordingly.

Alternatively, you can use say, fgets() in conjunction with sscanf(), in which case "banana" would be read as a string, so there would be no need to discard the invalid input remaining on the standard input stream, and you can still check the return value of sscanf().

Call atoi or atof on the string; if it fails, you are dealing w/non-numeric data...

Originally Posted by jeffcobb

Call atoi or atof on the string; if it fails, you are dealing w/non-numeric data...

There is no string variable

Anyway, although that will work in this case (if the input was stored in a string), more generally there is no way to distinguish between failure and a legitimate zero with atoi and company, so strtol and company should normally be used instead where input validation is desired.

Originally Posted by Fdisk

But I don't think this will work... In fact it doesn't seem to do anything at all right now lol.

It works, except that your error handling is incomplete. You need to discard the invalid input remaining on the standard input stream. This is why just reading it as a string to begin with is easier.

If you want to read the input as a string and then parse numbers from it:

Code:

char string[BUFSIZ];
int number;
if(!fgets(string, sizeof string, stdin)) {
    /* end of input detected, perhaps you want to quit the program */
}
else if(sscanf(string, "%d", &number) != 1) {
    /* invalid number entered */
}
else {
    printf("You entered %d.\n", number);
}

If you want to simply strip the remaining characters from the input: Trouble with assignment in C

You'll probably want to do some range checking also...

Code:

/* return 0 on success */
int parse_ulint(const char * buffer, unsigned long int * dest)
{
   unsigned long int tmp = 0;
   char * ep = NULL;

   assert(buffer != NULL);
   assert(dest != NULL);

   /* reset errno and attempt to parse */
   errno = 0;
   tmp = strtoul(buffer, &ep, 10);

   if(buffer[0] == '\0' || *ep != '\0')
   {
      return -2;        /* not a number */
   }else if(errno == ERANGE && tmp == ULONG_MAX){
      return -3;        /* range error */
   }else if(errno != 0){
      return -1;        /* some other error */
   }

   /* store the parsed value */
   *dest = tmp;
   return 0; /* success */
}

Of course if you just want integers, then just check for INT_MAX or INT_MIN (and use strtol instead of strtoul) from limits.h and return failure if the long int is outside of that range.

Then all you need to do is read with fgets() as others suggest.