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64 lines
3 KiB
Plaintext
64 lines
3 KiB
Plaintext
The '''BYVAL''' statement is used to pass a numerical parameter's value with procedures made in other programming languages.
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{{PageSyntax}}
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:: SUB Procedure_Name ('''BYVAL''' ''variable1'', '''BYVAL''' ''variable2'')
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{{PageDescription}}
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* '''QB64''' can only use BYVAL in [[DECLARE LIBRARY]] procedures that add DLL or Operating System API functions.
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* Supported with [[DECLARE LIBRARY]] [[SUB]] and [[FUNCTION]] procedure declarations when passing '''numerical values only'''.
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* Passing numerical values BYVAL assures that the original numerical variable value is not changed by another procedure.
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* Use [[parenthesis]] around program [[SUB]] or [[FUNCTION]] parameters passed BY VALUE. Ex: ''CALL Procedure ((x&), (y&))''
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''Historical:''
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* Qbasic versions below 7 do not use BYVAL unless the [[SUB]] program referred to is from a different programming language.
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* PDS versions can use BYVAL as it is intended in any [[SUB]] or [[FUNCTION]] parameters.
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* BYVAL can also be used with [[ABSOLUTE]] in Qbasic only.
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{{PageErrors}}
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* '''Do not use BYVAL before [[STRING]] or [[TYPE]] variables or in regular prograam [[SUB]] or [[FUNCTION]] parameters!'''
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* '''Many Qbasic keyword variable names CAN be used with a [[STRING]] suffix($) ONLY! You CANNOT use them without the suffix, use a numerical suffix or use [[DIM]], [[REDIM]], [[_DEFINE]], [[BYVAL]] or [[TYPE]] variable [[AS]] statements!'''
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''Example 1:'' BYVAL is used to preserve the values sent to an external procedure so they remain the same after they are used:
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{{CodeStart}} '' ''
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{{Cl|DECLARE LIBRARY}}
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{{Cl|SUB}} MouseMove {{Cl|ALIAS}} SDL_WarpMouse ({{Cl|BYVAL}} xoffset&, {{Cl|BYVAL}} yoffset&)
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{{Cl|DECLARE LIBRARY|END DECLARE}}
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{{CodeEnd}}
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: ''Note:'' Since the DLL library was already used by QB64 SDL(up to v.954), it did not require a DLL name. GL would!
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''Example 2:'' Passing parameters "by value" using [[parenthesis|brackets]] when calling a [[SUB]] or [[FUNCTION]] in Qbasic or QB64.
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{{CodeStart}} '' ''
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{{Cl|CALL}} MySUB (a%, (b%), (c%)) 'CALL SUB b and c stay 0 after sub
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MySUB a%, b%, (c%) 'call SUB again without CALL only c stays 0 after sub
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{{Cl|PRINT}} "After procedures: "; a%, b%, c%
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{{Cl|SUB}} MySUB (a%, b%, c%)
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a% = a% + 1: b% = b% + 1: c% = c% + 1
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{{Cl|PRINT}} "Inside procedure: "; a%, b%, c%
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{{Cl|END SUB}} '' ''
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{{CodeEnd}}
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{{OutputStart}}
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Inside procedure: 1 1 1
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Inside procedure: 2 1 1
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After procedures: 2 1 0
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{{OutputEnd}}
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:''Explanation:'' Both SUB calls pass just the '''values''' of b% and c% to the procedure. The first variable, a%, is passed by '''reference''' (the default) so the value was changed by the SUB procedure. Brackets can only be used in the [[CALL]] or function reference!
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{{PageSeeAlso}}
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* [[DECLARE LIBRARY]]
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* [[SUB]], [[FUNCTION]]
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* [[CALL]], [[ALIAS]]
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* [[DECLARE]], [[DECLARE (non-BASIC statement)]] {{text|(not used by QB64)}}
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* [[Libraries#C++_Variable_Types|C++ Variable Types]]
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{{PageNavigation}} |