Intger or string

[countdrak]

On reading a value from a file, using fgets, I need to determine if the value is a string or integer before I do an "atoi".

I have noticed on "atoi"ing a string, I get zero all the time. But there has to be a better check....is there?

Thanks.

 

[DaveSinkula]

man [tt]strtol[/tt]

 

[countdrak]

It will still return a 0 if not able to convert.

My situation is

if fgets_val == string
req_value = DEFAULT;
else
req_value == covert_to_int(fgets_val);

So if the fgets_val is 0 or string should take me to different values, unlike in strtol.

 

[DaveSinkula]

Attempt to convert a string to an [tt]int[/tt] using [tt]strtol[/tt]. Use the extra parameters to assist the validation. If you successfully convert the text to an [tt]int[/tt] it's an [tt]int[/tt]; if you don't it's not.

http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?answer=1043372399&id=1043284385#opt3

(look at the strtol example)

 

[cpjust]

That example is good, but shouldn't you also check if errno was set to ERANGE (in case the user entered a number too big or small for a long)?

 

[DaveSinkula]

Yep.

http://home.att.net/~jackklein/c/code/strtol.html

 

[ArkM]

Well, implement your own int parser, it's so simple - for example:

/** Returns:
 *  - 0 - not an integer.
 *  - 1 - integer + garbage tail.
 *  - 2 - true integer!
 */
int isint(const char* str)
{
  static const char ws[] = " \t\r\n";
  static const char dd[] = "0123456789";
  int	n, res = 0;
  if (!str || !*str)
     return 0;
  str += strspn(str,ws);
  if (*str == '-' || *str == '+')
     ++str;
  n = strspn(str,dd);
  if (n)
  {
     str += n;
     str += strspn(str,ws);
     res = (*str?1:2);
  }
  return res;
}

You may replace strspn() call by an explicit scan loops.
On rc 1 and 2 case atoi() returns int value (for example, it returns 3 for 3.14 arg).

 

[CataUTM]

Of course U mean

req_value = covert_to_int(fgets_val);

NOT

req_value == covert_to_int(fgets_val);