Thread: atoi return error

ok, i've got one for ya!

i've just looked at a refernce for the atoi function in stdlib.h.

it states that atoi returns a 0 if it cannot convert the char*.

how, then, is "0" supposed to be converted with confirmation?

therefore, aoti cannot be used to check for a valid int in a character array. is the best way to achieve this to convert to ascii and test for the int range for each char in the array?

or is there a function that can do this?

TIA, rotis23

Or just use the isdigit function...

Code:

#include <ctype.h>

bool IsNumeric(char* String)
{
   char *ptr = String;

   while(*ptr && isdigit(*ptr))
      ptr++;

   return (*ptr ? false : true);
}

Use strtol instead of atoi. One way to think of atoi might be as follows.

Code:

int atoi(const char *s)
{
    return (int)strtol(s, NULL, 10);
}

So if you want the error detection, use strtol but do error handling using the second parameter instead of passing it NULL.

i think i might just check manually - using an ascii range.

aswell as atoi, strtol returns a 0 if an error occurs. i presume the second parameter will not point to NULL if it finds a non-numerical char. hence, the error check.

is isdigit part of standard c?

thanks guys.

rotis23>is isdigit part of standard c?
Yes.

rotis23>i think i might just check manually - using an ascii range

FWIW I have a few comments.

Checking the digits first may work fine. But consider that you will be doing the checking up front, and if the string contains only digits it sounds as if you will use atoi. It is quite possible that atoi itself calls strtol, which performs another check on the validity as it attempts to convert the data (but its error-checking results are thrown away). If performance is an issue, this double-checking may be a problem. My suggestion was that since you want error checking and strtol does this, maybe you could use it.

Code:

#include <stdlib.h>
#include <stdio.h>

int myatoi(const char *s, int *value)
{
    if ( s != NULL && *s != '\0' && value != NULL )
    {
        char *endptr = s;
        *value = (int)strtol(s, &endptr, 10);
        if ( *endptr == '\0' )
        {
            return 1;
        }
    }
    return 0; /* failed to convert string to integer */
}

int main(void)
{
    int i, value = 2;
    const char *text[] =
    {
        "0", "0x0", "0.0", "00", "15", "", NULL, "a", " 0", "-0", " +0",
    };
    for ( i = 0; i < sizeof(text)/sizeof(*text); ++i )
    {
        int result = myatoi(text[i], &value);
        printf("text[%d] = \"%s\", value = %d (%s)\n", i,
               text[i] ? text[i] : "<NULL>", value,
               result ? "confirmed" : "error");
    }
    return 0;
}

/* my output
text[0] = "0", value = 0 (confirmed)
text[1] = "0x0", value = 0 (error)
text[2] = "0.0", value = 0 (error)
text[3] = "00", value = 0 (confirmed)
text[4] = "15", value = 15 (confirmed)
text[5] = "", value = 15 (error)
text[6] = "<NULL>", value = 15 (error)
text[7] = "a", value = 0 (error)
text[8] = " 0", value = 0 (confirmed)
text[9] = "-0", value = 0 (confirmed)
text[10] = " +0", value = 0 (confirmed)
*/

Some of these results may not be what you want, so first checking for all digits may be necessary. But if the behavior of strtol is what you need, then you could skip the double-checking.

<nitpicks>
Magos>#define FirstDigit 48
This is less portable than #define FirstDigit '0'.

Monster,Magos>bool, true, false
C++, not C.
</nitpicks>