Determine if a C string is a valid int in C

Question

I need to check if a C string is a valid integer.

I tried both

int num=atoi(str);

and

int res=sscanf(str, "%d", &num);

But sending the string "8 -9 10" in both of the lines returned simply 8, without indicating the invalidity of this string.

Can anyone suggest an alternative?

`atoi` does no error checking; its behavior if `str` doesn't contain a representation of an `int` value is undefined. (Typical implementations return `0`.) `sscanf` is better, but its behavior is undefined if the value in the string is not representable as an `int`. Also, your title says you want to check for a valid int, but the body of your question says "valid integer"; they aren't the same thing. `int` is one of several *integer* types. — Keith Thompson, Mar 18 '12 at 00:22

blueshift · Accepted Answer · 2012-03-17T20:34:38.627

30

Have a look at strtol(), it can tell you about invalid parts of the string by pointer return.

And beware of enthusiastic example code.. see the man page for comprehensive error-handling.

edited Mar 17 '12 at 20:34

answered Mar 17 '12 at 20:16

blueshift

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strtol() just sets the pointer to the next "object" in the string. It does not validate anything. – Mar 17 '12 at 20:41
1

@g24l If that pointer ends up pointing to NULL, then you know the entire string was valid. If not, not. Tadaa! – blueshift Mar 17 '12 at 20:45
If it is a null terminated string. – Mar 17 '12 at 20:47
2

ok, you are right I just noticed that she specified C string. – Mar 17 '12 at 20:52
4

Note that `strtol()` returns a `long` result. If you want to check for a valid `int`, you'll also need to check that the result is in the range `INT_MIN` .. `INT_MAX`. – Keith Thompson Mar 18 '12 at 00:23
The problem is that `strtol` even returns a correct value and set `errno = 0 ` if you input "123dfds" which is not an correct number. – M.K. aka Grisu Feb 19 '15 at 16:16

asveikau · Answer 2 · 2012-03-17T20:52:45.377

8

Maybe I'll get flamed for not using strtol or similar libc functions, but reasoning about this problem is not that hard:

#include <stdbool.h>  // if using C99...  for C++ leave this out.
#include <ctype.h>

bool is_valid_int(const char *str)
{
   // Handle negative numbers.
   //
   if (*str == '-')
      ++str;

   // Handle empty string or just "-".
   //
   if (!*str)
      return false;

   // Check for non-digit chars in the rest of the stirng.
   //
   while (*str)
   {
      if (!isdigit(*str))
         return false;
      else
         ++str;
   }

   return true;
}

[NB: I might have otherwise done isdigit(*str++) instead of the else to keep it shorter but my recollection is that the standards say it's possible that isdigit is a macro.]

I guess one limitation is that this does not return false if the number in the string won't fit in an integer. That may or may not matter to you.

edited Mar 17 '12 at 20:52

answered Mar 17 '12 at 20:47

asveikau

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2

"not that hard", but you had to come back and change it ;) – blueshift Mar 17 '12 at 20:52
2

@blueshift Point taken, but 1 edit in 5 minutes when typing code into a web form is not that bad to me. :-) – asveikau Mar 17 '12 at 20:54
In this case I don't know if it's more error-prone to roll your own or to try and get the error-handling for strtol() right.. I'd probably still recommend strtol. – blueshift Mar 17 '12 at 20:57
1

No, you will not get flamed, and I upvote you, but I think strtol is safer. – sara Mar 17 '12 at 21:21
6

@asveikau: So, this will accept `'-37'` but it will reject `'+42'`? – ypercubeᵀᴹ Mar 17 '12 at 23:44

score 3 · Answer 3 · answered Mar 20 '12 at 17:19

3

A simple way to do this robustly would be to read the int and make sure it's string representation is identical to the input string, for example combining atoi and itoa:

int is_int(char const* p)
{
    return strcmp(itoa(atoi(p)), p) == 0;
}

answered Mar 20 '12 at 17:19

Inverse

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Nice idea :). However, this will require trimming leading and trailing spaces as well, in order to make it work. – sara Mar 21 '12 at 08:55
4

Note that this solution is not ANSI compliant. – Samy Dindane Apr 26 '13 at 22:55
Nice solution! Just be careful with frees after. – Caio V. Nov 28 '21 at 19:49

score 0 · Answer 4 · answered May 14 '16 at 07:31

For checking if string contains a valid number you can use a regular expression. For example for integers use:

[-+]?[0-9]+

and a general case for floating point numbers:

[+-]?[0-9]+[.]?[0-9]*([eE][-+]?[0-9]+)?

In the case of C++11 regular expression functions are avalaible in library, e.g. "std::regex_match(.....)" gives the exact match. The code should look like this:

#include <regex>
.....
std::string strnumber("-1.234e+01");
float number;
if(regex_match(strnumber,std::regex("[+-]?[0-9]+[.]?[0-9]*([eE][-+]?[0-9]+)?"))
number=std::stof(strnumber);
else
std::cout<<"error, string is not a valid number";

ncarrier · Answer 5 · 2017-12-27T08:46:42.993

Sorry for digging up the topic, but for the sake of completeness and because this thread is the first match when doing a google search...

It is possible to use something like:

ret = sscanf(string, "%d%n", &number, &idx);
if (ret == 0 || string[idx] != '\0')
    /* handle the error */

the %n directive, which seems to be standard C according to the man page, counts the number of characters processed .

[Edit] sscanf doesn't seem to provide a way to detect overflows, so the strtoX functions family should be preferred IMHO.

Determine if a C string is a valid int in C

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