使用枚举常量时发生C错误

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

What an enum does is to assign symbolic names to what is essentially an integer, by default starting at index 0. For example;

enum BoolValue { FALSE, UNKNOWN, TRUE };

essentially does something close to;

const int FALSE   = 0;
const int UNKNOWN = 1;
const int TRUE    = 2;

Your problem is that you're trying to put numbers in your enum instead of symbolic names, which makes sense that it gives an error if you rewrite it as essentially;

const int 0 = 0;
const int 1 = 1;

...etc.

I'm not sure an enum is a solution to the problem you're trying to solve.

Answer 2

It is incorrect to use such a enum definition, it should be like this

enum Hex
    {       
        ZERO = 0,
        ONE = 1,
        A = 10,
        B = 11,
    };

if your problem is to convert string to int use atoi

Answer 3

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, eg a letter or an underscore (eg 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 .

Answer 4

Use strtol() to convert strings from any number base 2-36.

strtol(const char *nptr, char **endptr, int base)

Base 0 is a special case that accepts "0x" prefix for base 16, "0" prefix for 8, and no prefix for 10.

char a[5]="0x0a";
char b[3]="0b";
printf ("%li\n",strtol(a,NULL,0));
printf ("%li\n",strtol(b,NULL,16));

See the man page for longer examples with error checking.

Answer 5

enum HexDigits h = 9; // assigning numerical 9 or enum variable

the 9 in this case can be taken as the numerical 9 and the '9' in enum HexDigits.

The Variables used inside enum definition should follow the variable naming conventions of the language. Only valid variable names are allowed.

But why do you have to define enum?? Doesnt c++ support Hexadecimal numbers on its own?

check out about enum at MSDN page