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Why
Conditionals are Used
Generally
there are three kinds of reason to use a conditional.
- A program may need to use
different code depending on the machine or operating system it is to run
on. In some cases the code for one operating system may be erroneous on
another operating system; for example, it might refer to library routines
that do not exist on the other system. When this happens, it is not enough
to avoid executing the invalid code: merely having it in the program makes
it impossible to link the program and run it. With a preprocessing conditional,
the offending code can be effectively excised from the program when it
is not valid.
- You may want to be able
to compile the same source file into two different programs. Sometimes
the difference between the programs is that one makes frequent time-consuming
consistency checks on its intermediate data, or prints the values of those
data for debugging, while the other does not.
- A conditional whose condition
is always false is a good way to exclude code from the program but keep
it as a sort of comment for future reference.
Most
simple programs that are intended to run on only one machine will not need
to use preprocessing conditionals.