You can use #ifdef to check if a constant was previously defined with a #define directive or CONST statment. If the constant has been previously defined, the statements inside the #if…#endif block will be compiled, otherwise they will be discarded.

#define  LOWMODEL 0
#ifdef LOWMODEL
Low 0
#endif

In the above example, since LOWMODEL is defined, the statement LOW 0 is compiled.

#else #elseifdef may be used for more complex expressions as shown below:

#ifdef LOWMODEL
Low 0
#elseifdef HIGHMODEL
High 0
#else
Low 1
#endif

#ifndef is the opposite of the #ifdef directive. If a constant has not been defined, the statements inside a #ifndef…#endif block will be compiled, otherwise the statements will be discarded.

#define  LOWMODEL 0
#ifndef LOWMODEL
Low 0
#endif

#elseifndef and #elsemay be used for more complex expressions as shown below:

#ifndef LOWMODEL
Low 0
#elseifndef HIGHMODEL
High 0
#else
Low 1
#endif

Finally, the directives may be mixed as shown below:

#if MODELNO = 0
Low
 0
#elseifdef HIGHMODEL
High
 0
#else
Low
 1
#endif

Nested #ifdef/#ifndef directives are not supported; #ifdef/#ifndef may not be used inside another #ifdef/#ifndef.

See also #if...#endif.

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