Constants

Summary: in this tutorial, you will learn how use C constants in your code to make your code more readable and easy to maintain.

Introduction to C constants

C constants using C preprocessor

There are a lot of cases you want to use a constant in your code for example to calculate the square of a circle you code as follows:

square = 3.14159 * diameter;

In the statement, the constant 3.14159 represents the constant PI. Instead of putting constant in literal form, we can use symbolic constant as below:

square = PI * diameter;

As you see, the C constants make your code readable and the symbolic constants tell you not only the value but also its meaning. In addition, if a constant is used in several places, it is better to use symbolic constant because when you want to change its value you just have to change it in one place that define the constant, see the below example:

1	total =  net + TAXRATE * net;

Later on if the tax rate changes, you only have to change it in where it is defined.But how do you define symbolic C constants? The first idea of defining a C constant is using C preprocessor as follows:

	#define PI 3.14159
	#define TAXRATE 0.1

By doing this, when your program compiled, the constant values 1.14159 and 0.1 will be substituted everywhere you used PI and TAXRATE. This feature is known as compile time substitution. Those defined constants are called manifest constants because when you run the program, all the substitutions have already been made.

Note that there is no equal sign between constant’s name and value and there is no semicolon (;) at the end of the statement.

It is highly recommended that you use constant name is upper case so when you see it in code you know that it is a constant. The formula of defining a constant is as follows:

#define CONSTANT_NAME value

C constants using const keyword

C90 officially added a second way to create symbolic constants using the constkeyword to declare a constant. This approach is more flexible that using C preprocessor #define such as you can use keyword const to declare an array, pointer…etc.Once the value of a constant using const keyword has been initialized, its value cannot be changed through the program. See the below example:

	const double PI = 3.14159;
	const double TAXRATE = 0.1;