7 Constants

Constants simply allow the Forth programmer to use a word to represent a number. Once declared, their value cannot be changed. There are three reasons for using Constants in Forth programs:

Using a Constant instead of a number in your code helps to convey more meaning, making the code easier to understand. For example, KEY_A is more intuitive than 65;
If a number is used repeatedly throughout a program, using a Constant means that you can change the number, easily, in one step, by editing the constant, helping to reduce bugs;
Constants when referenced in colon definitions actually require less memory than plain numbers. Thus if a number is used repeatedly in code, replacing them with constants can save memory.

7.1 Declaring Constants with CONSTANT

Constants should be declared outside of a colon definition, like this:

    99 CONSTANT NinetyNine

As can be seen, CONSTANT requires a number on the stack, followed by the name of the constant to create.

7.2 Using Constants

Using constants in your code could not be simpler; simply name them. For example:

    99 CONSTANT NinetyNine
    : TEST NinetyNine 0 DO I . LOOP ;

<-- Back to Tutorials