Because 1 isn't 0? enums are just integers, their definitions don't change the laws of mathematics.

I think you might be confused about what an enum is. They're integers, pure and simple. Declaring a variable to a certain type of enum is just telling the programmer which set of values it belongs to, the compiler doesn't do anything special with it. Their labels are just labels, they can't redefine the value of '1'. You can't even convert a number back into an enum's string. There's no connection.

If you must avoid a loop and get it exactly right the first look every time, you need a lookup table. This can be done in as little as one instruction sometimes. If you know for a fact you won't have any numbers higher than 255, it wouldn't even be a very large lookup table...

Code:

int lookup(unsigned char v)
{
        static const int l_val[256]={ // static locals are like globals, initialized once on program load
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 5, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

        return(l_val[v]);
}

If you're using the GNU compiler, you can write it much more tersely(though the table will still be 256 elements):

Code:

int lookup(unsigned char v)
{
        static const int l_val[256]={ [10]=5, [20]=4, [40]=8 }
        return(l_val[v]);
}

I had thought you wanted a general-purpose sparse array.

The CPU will never psychically know which in an array of 3 elements has the one you want. It has to look. If you make it a lookup table, the problem is solved, since to find element 3, you look inside index 3...

enum's are integers. Their labels don't have any magic conversion properties. You can't even get an enum's label back from its integer. They lose all meaning when compiled, becoming pure numbers.

Never put data in your header files. (Or function definitions, for that matter.) All you need to say in the header is that the variable exists at all like extern int lookup_table[256]; Put its actual definition somewhere else.