A 
Syntax
%ifdef expression %then
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.
%elseifdef expression %then
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%endif
Comments
If the expression in %ifdef expression %then is false, pc skips over the %then part and executes the %elseifdef part instead. expression consists of a conditional variable and the optional boolean operators, and, or, and not. See the %else listing for examples of expression.
Example
The Pascal program, ifdef.p, which first checks if bird1 has been defined. If not, it defines it with a %var directive. If bird1 has been defined, the program checks whether or not it needs to define bird2.
program ifdef_example(output);
%include 'bird.h';
begin
%ifdef not(bird1) %then %var bird1
%elseifdef not(bird2) %then %var bird2
%endif;
%if bird1 %then
writeln('Bird one is a ', a, '.'); %elseif bird2 %then
writeln('Bird two is a ', b, '.') %endif
end. { ifdef_example }
Pascal Preprocessor | 253 |