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63 lines
2.7 KiB
Text
63 lines
2.7 KiB
Text
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'''$IF''' is precompiler command, which determines which sections of code inside its blocks are included into our code for compliing.
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{{PageSyntax}}
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:: $IF variable = expression THEN...
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::.
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::.
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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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* Currently 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 multi-line statements onto one line will not work with $IF (or any other metacommand).
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* Variable names can contain numbers, letters, and periods -- in any order!
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* Expressions can contain one set of leading and/or trailing quotes; and any number of numbers, letters, and periods, in any order.
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* The precompiler comes with some preset values which can be used to help determine which code blocks to include/exclude for us. These are: '''WIN''' or '''WINDOWS''' if the user is running QB64 in a windows environment. '''LINUX''' if the user is running QB64 in a Linux environment. '''MAC''' or '''MACOSX''' if the user is running QB64 in a Mac environment. '''32BIT''' if the user is running a 32-bit version of QB64. '''64BIT''' if the user is running a 64-bit version of QB64.
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''Example 1:''
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{{CodeStart}} '' ''
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{{Cl|$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:
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If you look at the code above, you'll see that we have the same CONST defined twice inside the program. Normally, we get an error if we try to define a CONST more than once, but the $IF condition here is CHOOSING which CONST we want inside our 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 $IF statements let us control the code that actually gets compiled, while excluding the other blocks completely.
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''Example 2:''
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{{CodeStart}} '' ''
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{{Cl|$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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For the above, the CONST slash is defined by the automatic internal flags which tell us what operating system we're using. On a windows PC, the Slash will be the backslash; for any other OS it will be the forward slash.
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''See also:''
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* [[$LET]]
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* [[$ELSE]]
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* [[$ELSEIF]]
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* [[$END IF]]
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{{PageNavigation}}
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