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Why Conditionals are Used
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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.
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