#ifdef Constant | Constant Value | Var(VariableName)


The #ifdef directive is used to selectively enable sections of code.

There are several ways in which it can be used:

  • Checking if a constant is defined
  • Checking if a constant is defined and has a particular value
  • Checking if a system variable exists
  • Checking if a system bit has been defined

The advantage of using #ifdef rather than an equivalent series of IF statements is the amount of code that is downloaded to the chip. #ifdef controls what code is compiled and downloaded, IF controls what is run once on the chip. #ifdef should be used whenever the value of a constant is to be checked.

Great Cow BASIC also supports the #ifndef directive - this is the opposite of the #ifdef directive - it will remove code that #ifdef leaves, and vice versa.

Note: The code in the following sections will not compile, as it is missing #chip directives and Dir commands. It is intended to act as an example only.

Example 1: Enabling code if a constant is defined

    #define Blink1

    #ifdef Blink1
        PulseOut PORTB.0, 1 sec
        Wait 1 sec
    #ifdef Blink2
        PulseOut PORTB.1, 1 sec
        Wait 1 sec

This code will pulse PORTB.0, but not PORTB.1. This is because Blink1 has been defined, but Blink2 has not. If the line was added at the start of the program, then both pins would be pulsed.

    #define Blink2

The value of the constant defined is not important and can be left off of the #define.

Example 2: Enabling code if a constant is defined and has a given value

    #define PinsToFlash 2

    #ifdef PinsToFlash 1,2,3
    PulseOut PORTB.0, 1 sec
    #ifdef PinsToFlash 2,3
    PulseOut PORTB.1, 1 sec
    #ifdef PinsToFlash 3
    PulseOut PORTB.2, 1 sec

This program uses a constant called PinsToFlash that controls how many lights are pulsed. PORTB.0 is pulsed when PinsToFlash is equal to 1, 2 or 3, PORTB.1 is pulsed when PinsToFlash equals 2 or 3, and PORTB.2 is flashed when PinsToFlash is 3.

Example 3: Enabling code if a system variable is defined

    #ifdef NoVar(ANSEL)
    #ifdef Var(ANSEL)
    ANSEL = 0

The above section of code has been copied directly from a-d.h. It is used to disable the A/D function of pins, so that they can be used as standard digital I/O ports. If ANSEL is not declared as a system variable for a particular chip, then the program uses ADCON1 to control the port modes. If ANSEL is defined, then the chip is newer and its ports can be set to digital by clearing ANSEL.

Example 4: Enabling code if a system bit is defined

Similar to above, except with Bit and NoBit in the place of Var and NoVar respectively.

See Also Defines, #define