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| Next: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="n" rel="next">Conditional Syntax</a>, Up: <a href="Conditionals.html#Conditionals" accesskey="u" rel="up">Conditionals</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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| <a name="Conditional-Uses-1"></a> |
| <h3 class="section">4.1 Conditional Uses</h3> |
| |
| <p>There are three general reasons to use a conditional. |
| </p> |
| <ul> |
| <li> 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 data types or constants that do not exist on |
| the other system. When this happens, it is not enough to avoid |
| executing the invalid code. Its mere presence will cause the compiler |
| to reject the program. With a preprocessing conditional, the offending |
| code can be effectively excised from the program when it is not valid. |
| |
| </li><li> You may want to be able to compile the same source file into two |
| different programs. One version might make frequent time-consuming |
| consistency checks on its intermediate data, or print the values of |
| those data for debugging, and the other not. |
| |
| </li><li> A conditional whose condition is always false is one way to exclude code |
| from the program but keep it as a sort of comment for future reference. |
| </li></ul> |
| |
| <p>Simple programs that do not need system-specific logic or complex |
| debugging hooks generally will not need to use preprocessing |
| conditionals. |
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