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So far, we've learned how to use strings and numbers, and how the computer uses booleans (bool) to represent yes or no as true or false. However, in many cases that is not enough. Often you have concepts that don't fit into the strict form of true or false, and using a string of text seems awfully wasteful. After all, if you compare two strings, the computer has to compare each character of them against each other.

You could use numbers instead, that would be faster. Simply equate 1 to "yes", 2 to "no" and 3 to "maybe". But of course, you'd have to remember this always during coding.

Also, what if you make a typo? One of the nice things about C is that if you type twue instead of true, it will complain. But if you compare a string against another string, C will simply tell you they don't match. Same for a number.

That's why the authors of C invented enumerated symbolic constants, or enums, for short. An enum is simply a name for a number, or a series of numbers. To create an enum, you write it like the following:

enum ThreefoldLogic // Pick any name you like here.

{

    YES = 1,

    NO = 2,

    MAYBE = 3

};

from now on, you can use these three words just like you can use true and false. And C will see them as if you'd typed the numbers they're equivalent to. But if you mistype one of them, it will not recognize the misspelling and balk at you. Cool, isn't it? They don't have to be all-uppercase either. Any valid identifier will do. Whether it be kThisIsMyCoolConstant, or this_is_a_constant.

But there's more. You can even use this as a type for your variables:

enum TreefoldLogic   myVar;



myVar = YES;

The nice part about this is that variables declared like this will not accept any other values but the ones in this one enum. So in this example, it'll accept YES, NO, MAYBE, but not true, false or 5.

Note that these symbolic enumerated constants (sometimes just referred to as "constants") are limited to being integers or characters (characters are just a different way of writing an integer to the computer)

enum EXAMPLES

{

    A_NEWLINE_CHARACTER = '\n',

    kTab = '\t',

    kFirstCapitalLetter = 'A',

    zero = 0,

    one,

    two

};

Also, as you can see in this example, if you don't give a number, it will simply add one to the previous constant's value. This goes so far that you can write:

enum {

    zero,

    one,

    two

};

because when you don't give a number for the first constant in an enum, it will get the number 0.

An Alternative

One other way of declaring constants that you may encounter a lot is the old preprocessor method:

#define kConstant     15

This is basically just a global search-and-replace. Whatever is on the left side will be replaced with whatever is on the right side. This works even for function calls, but is very low-level. There are a few cases where you need #define, like for defining a constant for a string literal or a floating point number:

#define DEFAULT_DISK_NAME    "Macintosh HD"

And then there are a few more rather advanced uses that won't be covered here. But in general you should just use enum in your code, because it's safer.

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