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