How can I extract an integer from within a string?

Question

I'm working on an assignment and as part of it I need to extract the integer from a string.

I've tried using the atoi() function, but it always returns a 0, so then I switched up to strtol(), but it still returns a 0.

The goal is to extract the integers from the string and pass them as arguments to a different function. I'm using a function that then uses these values to update some data (update_stats).

Please keep in mind that I'm fairly new to programming in the C language, but this was my attempt:

void get_number (char str[]) {
    char *end;
    int num;
    num = strtol(str, &end, 10);
    update_stats(num);
    num = strtol(end, &end, 10);
    update_stats(num);
}

The purpose of this is in a string "e5 d8" (for example) I would extract the 5 and the 8 from that string.

The format of the string is always the same.

How can I do this?

Answer 1

strtol doesn't find a number in a string. It converts the number at the beginning of the string. (It does skip whitespace, but nothing else.)

If you need to find where a number starts, you can use something like:

const char* nump = strpbrk(str, "0123456789");
if (nump == NULL) /* No number, handle error*/

(man strpbrk)

If your numbers might be signed, you'll need something a bit more sophisticated. One way is to do the above and then back up one character if the previous character is -. But watch out for the beginning of the string:

if ( nump != str && nump[-1] == '-') --nump;

Just putting - into the strpbrk argument would produce false matches on input like non-numeric7.