====== #ifdef name ... #endif ======

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.

<code xbasic>
#define  LOWMODEL 0
#ifdef LOWMODEL
Low 0
#endif
</code>

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:

<code xbasic>
#ifdef LOWMODEL
Low 0
#elseifdef HIGHMODEL
High 0
#else
Low 1
#endif
</code>

====== #ifndef name .. #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.

<code xbasic>
#define  LOWMODEL 0
#ifndef LOWMODEL
Low 0
#endif
</code>

''#elseifndef'' and  ''#else''may be used for more complex expressions as shown below:

<code xbasic>
#ifndef LOWMODEL
Low 0
#elseifndef HIGHMODEL
High 0
#else
Low 1
#endif
</code>

Finally, the directives may be mixed as shown below:

<code xbasic>
#if MODELNO = 0
Low
 0
#elseifdef HIGHMODEL
High
 0
#else
Low
 1
#endif
</code>

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

See also [[:cubloc::if...endif:|#if...#endif]].

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