About Constants

A constant tells the compiler to find a given word, and replace it with another word or number. Define directives create constants.

Constants are useful for situations where a routine needs to be easily altered. For example, a define could be used to specify the amount of time to run an alarm for once triggered.

It is also possible to use defines to specify ports - thus defines can be used to aid in the creation of code that can easily be adapted to run on a different microcontroller with different ports.

Great Cow BASIC makes considerable use of defines internally. For instance, the LCD code uses defines to set the ports that it must use to communicate with the LCD.

About Defines

To create a define is a matter of using the #define directive. Here are some examples of defines:

    #define Line 34
    #define Light PORTB.0
    #define LightOn Set PORTB.0 on

Line is a simple constant - Great Cow BASIC will find Line in the program, and replace it with the number 34. This could be used in a line following program, to make it easier to calibrate the program for different lighting conditions.

Light is a port - it represents a particular pin on the microcontroller. This would be of use if the program had many lines of code that controlled the light, and there was a possibility that the port the light was attached to would need to change in the future.

LightOn is a define used to make the program more readable. Rather than typing Set PORTB.0 on over and over, it would then be made possible to type LightOn, and have the compiler do the hard work.

Great Cow BASIC Defined constants

    #define ON 1
    #define OFF 0
    #define TRUE 255
    #define FALSE 0

    'Names for symbols
    #define AND &
    #define OR |
    #define XOR #
    #define NOT !
    #define MOD %

Great Cow BASIC special constant

Forever is a special constant. For Great Cow Graphical BASIC users think of this as 'false'. For those not using Great Cow Graphical BASIC think of this as a non numeric value that has no value. You can use Forever in a DO-LOOP but not in a REPEAT-END REPEAT loop, as the in the later case the REPEAT will have no value and you will create an error condition.

Precedence of Constants within Great Cow BASIC.

The #define command creates constants, and, a script can creates constants.

The precedence is as follows:

#define in the main program are read first,

then, the #define in the include files. Constants defined in the include files will be ignored if they conflict or are different to another constant in the main program,

then, the scripts are processed. Scripts that create constants always override any constant value previously defined.

Scripts are highest priority, then constants in the main program, then constants in include files from the main program, then constants in the standard libraries.

See #define