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68 lines
3.1 KiB
Plaintext
68 lines
3.1 KiB
Plaintext
'''$IF''' is precompiler [[Metacommand|metacommand]], which determines which sections of code inside its blocks are included into the final code for compliing.
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{{PageSyntax}}
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:[[$IF]] variable = expression THEN
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:.
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:[[$ELSEIF]] variable = expression THEN
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:.
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:[[$ELSE]]
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:.
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:[[$END IF]]
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* $IF is the start of a precompiler code block which includes or excludes sections of code from being compiled.
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* There is no single line $IF statement. $IF must be in a valid $IF THEN...$END IF block to work properly.
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* Like all other metacommands, you can not use more than one metacommand per line. '''Use of : to separate statements in a single line is not allowed.'''
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* Variable names can contain numbers, letters, and periods -- in any order.
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* Expressions can contain one set of leading and/or trailing quotes; and any number of numbers, letters, and periods, in any order.
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* The precompiler comes with some preset values which can be used to help determine which code blocks to include/exclude for us. These are: '''WIN''' or '''WINDOWS''' if the user is running QB64 in a Windows environment. '''LINUX''' if the user is running QB64 in a Linux environment. '''MAC''' or '''MACOSX''' if the user is running QB64 in a macOS environment. '''32BIT''' if the user is running a 32-bit version of QB64. '''64BIT''' if the user is running a 64-bit version of QB64.
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* [[$END IF]] denotes the end of a valid precompiler $IF block.
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* [[$ELSEIF]] must follow a valid $IF or $ELSEIF statement.
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* If [[$ELSE]] is used, it must be used as the last conditional check before $END IF. $ELSEIF cannot come after $ELSE.
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** There can only be one $ELSE in an '''$IF-$ELSEIF-$ELSE-$END IF''' block, and it must be the last block selection before the $END IF. $ELSEIF cannot follow $ELSE.
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{{PageExamples}}
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''Example 1:''
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{{CodeStart}} '' ''
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{{Cl|$LET}} ScreenMode = 32
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{{Cl|$IF}} ScreenMode = 0 THEN
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{{Cl|CONST}} Red = 4
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{{Cl|$ELSEIF}} ScreenMode = 32 THEN
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{{Cl|CONST}} Red = _RGB32(255,0,0)
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{{Cl|$END IF}}
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{{Cl|COLOR}} Red
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{{Cl|PRINT}} "Hello World"
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{{CodeEnd}}
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''Explanation:'' The same CONST is defined twice inside the program. Normally, defining a CONST more than once generates an error, but the $IF condition here is choosing which CONST will be inside the final program.
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As long as Screenmode is 0, the program will exclude the code where CONST Red is defined as color 4. If Screenmode is 32, CONST Red will be defined as _RGB32(255, 0, 0).
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The [[$LET]] and $IF statements let the programmer control the code that actually gets compiled, while excluding the other blocks completely.
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''Example 2:''
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{{CodeStart}} '' ''
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{{Cl|$IF}} WIN THEN
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{{Cl|CONST}} Slash = "\"
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{{Cl|$ELSE}}
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{{Cl|CONST}} Slash = "/"
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{{Cl|$END IF}}
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{{Cl|PRINT}} "The proper slash for your operating system is "; Slash
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{{CodeEnd}}
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''Explanation:'' For the above, the CONST slash is defined by the automatic internal flags which returns what operating system is being used at compile time. On a Windows PC, the Slash will be the backslash; for any other OS it will be the forward slash.
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{{PageSeeAlso}}
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* [[$LET]]
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* [[Metacommand]]s
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{{PageNavigation}}
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