Question

I want to write a function that will take a string which represents a hexadecimal number and converts it to an integer, I want to put the hexadecimal digits in an enum, but when I use an element from the enum I get an error while compiling.

Here is the code:

#include <stdio.h>

int htoi (char h[]);
enum HexDigits {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F};

int main () {
    enum HexDigits h = 9;
    
    return 0;
}

int htoi (char h[]) {
    
}

and here is the error im getting:

C:\Users\KiKo-SaMa\Desktop\C>gcc hello.c -o hello

hello.c:4:17: error: expected identifier before numeric constant

What could be the problem with what I’m doing?

Answer 1

Enum members have to be named and their names (like any other name) aren’t allowed to start with a digit. If you want to use digits, you have to add something in front, e.g. a letter or an underscore (e.g. H0 or _0).

Also, as an additional suggestion, it’s a lot easier to simply add your valid characters into a string and then use strchr() for conversion:

const char *hexchars = "0123456789ABCDEF";

// this takes a single hex character and returns its decimal representation (or -1 in case of an invalid character)
int hex_to_dec(char h) { return strchr(hexchars, h) ? strchr(hexchars, h) - hexchars : -1; }

Also note that you might as well just use the hexadecimal representation of numeric constants: 0x0, 0x1,… 0xF.