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71 will be processed normally by the compiler or ignored completely. The compiler supports the definition of conditional identifiers on the command line but you can also define them in your source code as well using the following syntax: #define <identifier> [ [=] <expression> ] Here, <identifier> is the name of the conditional identifier that you want to define. You may also give it a value, represented by <expression>, which may be an integral or string type. If you do not specify a value, a default value of 1 is used. The expression may include literals or identifiers previously defined using #define and some built-in constants (see Section 3.9). Examples #define EXPERIMENTAL #define Version 23 #define Greeting = "Hello" If you attempt to define a conditional identifier that is already defined, you will get an error message to that effect. If you want to redefine a conditional identifier you must first “undefine” the existing one using the directive: #undef <identifier> If the specified identifier is not actually defined, no error message will be issued so you may freely use this directive to ensure that no definition exists prior to defining a conditional identifier. Note that undefining an identifier that was defined on the command line only has effect in the current module. All other modules will see the original value. Once you have defined your conditional identifiers, you may use them in conditional directives that are similar to If statements. The first two forms presented below are complementary. #ifdef <identifier> <other-text> #endif #ifndef <identifier> <other-text> #endif The first form specifies that if the given <identifier> is defined, the compiler should process the text up to the matching #endif but if the <identifier> is not defined, the compiler should ignore the text up to the matching #endif. The second form has the opposite effect. Example #ifdef EXPERIMENTAL Call TestSetup(i) #endif This allows the subroutine call to be compiled into the application if EXPERIMENTAL is defined, otherwise it is left out. As you might have already guessed, the conditional syntax also allows an #else clause. #ifdef EXPERIMENTAL ver = "X006" #else ver = "V1.4" #endif