Storing Information: Variables and Constants in C (二)

Storing Information with Variables

A variable is a named data storage location in your computer's memory. By using a variable's name in your program, you are, in effect, referring to the data stored there.

Variable Names

To use variables in your C programs, you must know how to create variable names. In C, variable names must adhere to the following rules:

• The name can contain letters (a to z and A to Z), digits (0 to 9), and the underscore character (_).
• The first character of the name must be a letter. The underscore is also a legal first character, but its use is not recommended at the beginning of a name. A digit (0 to 9) cannot be used as the first character.
• Case matters (that is, upper- and lowercase letters). C is case-sensitive, thus, the names count and Count refer to two different variables.
• C keywords can't be used as variable names. A keyword is a word that is part of the C language. (A complete list of the C keywords can be found in Appendix B, "Reserved Words.")

The following list contains some examples of legal and illegal C variable names:

Variable Name	Legality
Percent	Legal
y2x5__fg7h	Legal
annual_profit	Legal
_1990_tax	Legal but not advised
savings#account	Illegal: Contains the illegal character #
double	Illegal: Is a C keyword
4sale	Illegal: First character is a digit

Because C is case-sensitive, the names percent, PERCENT, and Percent would be considered three different variables. C programmers commonly use only lowercase letters in variable names, although this isn't required. Using all-uppercase letters is usually reserved for the names of constants (which are covered later today).

For many compilers, a C variable name can be up to 31 characters long. (It can actually be longer than that, but the compiler looks at only the first 31 characters of the name.) With this flexibility, you can create variable names that reflect the data being stored. For example, a program that calculates loan payments could store the value of the prime interest rate in a variable named interest_rate. The variable name helps make its usage clear. You could also have created a variable named x or even ozzy_osborne; it doesn't matter to the C compiler. The use of the variable, however, wouldn't be nearly as clear to someone else looking at the source code. Although it might take a little more time to type descriptive variable names, the improvements in program clarity make it worthwhile.

Many naming conventions are used for variable names created from multiple words. You've seen one style: interest_rate. Using an underscore to separate words in a variable name makes it easy to interpret. The second style is called camel notation. Instead of using spaces, the first letter of each word is capitalized. Instead of interest_rate, the variable would be named InterestRate. Camel notation is gaining popularity, because it's easier to type a capital letter than an underscore. The underscore is used in this book because it's easier for most people to read. You should decide which style you want to adopt.

DO

use variable names that are descriptive.

DO

adopt and stick with a style for naming your variables.

DON'T

start your variable names with an underscore unnecessarily.

DON'T

name your variables with all capital letters unnecessarily.