Before you can do anything in any language, you must at least know how you name an identifier. An indentifier is used for any variable, function, data definition, etc. In the programming language C, an identifier is a combination of alphanumeric characters, the first being a letter of the alphabet or an underline, and the remaining being any letter of the alphabet, any numeric digit, or the underline. Two rules must be kept in mind when naming identifiers.

1. The case of alphabetic characters is significant. Using "INDEX" for a variable is not
the same as using "index" and neither of them is the same as using "InDex" for a variable. All three refer to different variables.

2. As C is defined, up to eight significant characters can be used and will be considered
significant. If more than eight are used, they may be ignored by the compiler. This may or may not be true of your compiler. You should check your reference manual to find out how many characters are significant for your compiler. The HiTech C compiler used with the Applix 1616 allows 31 significant characters, and prepends an underscore (_)

It should be pointed out that some C compilers allow use of a dollar sign in an identifier name, but since it is not universal, it will not be used anywhere in this tutorial. Check your documentation to see if it is permissible for your particular compiler.