QB64-PE/internal/help/FUNCTION.txt

A [[FUNCTION]] block statement is used to create a function procedure to return a calculated value to a program.


{{PageSyntax}}
: '''FUNCTION procedureName'''[type-suffix] [(''parameters'')]
:: ''{code}''
:: 'variable definitions and procedure statements
:: ⋮
:: procedureName = returnValue
: '''END FUNCTION'''


{{PageDescription}}
* The function type can be any variable type that it will return to the program and is represented by the type suffix.
* Functions hold one return value in the function's name which is a variable type. Other values can be passed through ''parameters''.
* Functions are often referred to in program calculations, not called like SUB procedures. [[CALL]] cannot be used with functions.
* If there are no parameters passed or they are [[SHARED]] the ''parameters'' and parenthesis are not required.
* Variable names within the procedure do not have to match the names used in the reference parameters, just the value types.
* All [[$DYNAMIC|dynamic]] variable values return to 0 or null strings when the procedure is exited except when a variable or the entire function is [[STATIC]]. This can save program memory as all [[$DYNAMIC|dynamic]] memory used in a FUNCTION is released on procedure exit.
* FUNCTION procedure code can use [[GOSUB]] and [[GOTO]] line numbers or labels inside of the procedure when necessary. 
* For early function exits use [[EXIT]] [[FUNCTION]] before [[END FUNCTION]] and [[GOSUB]] procedures using [[RETURN]].
* '''QB64 ignores all procedural DECLARE statements.''' Define all ''parameter'' [[Data types|types]] in the FUNCTION procedure.
*  '''Images are not deallocated when the [[SUB]] or [[FUNCTION]] they are created in ends. Free them with [[_FREEIMAGE]].'''
* The IDE can create the FUNCTION and END FUNCTION lines for you. Use the ''New FUNCTION...'' option in the Edit Menu. A box will come up for you to enter a name for the FUNCTION. Enter all code between the FUNCTION and [[END FUNCTION]] lines.


==QBasic/QuickBASIC==
* Once a FUNCTION was created and used, the QBasic IDE would DECLARE it when the file was saved. '''QB64 doesn't need these declarations.'''
* QBasic's IDE could place a [[DEFINT]], [[DEFSNG]], [[DEFLNG]], [[DEFDBL]] or [[DEFSTR]] statement before the FUNCTION line if it is used in the main module. It may even be the wrong variable type needed.
* QBasic allowed programmers to add DATA fields anywhere because the IDE separated the main code from other procedures.


{{PageExamples}}
''Example 1:'' A simple function that returns the current path. Place [[FUNCTION]] or [[SUB]] procedures after the program [[END]].
{{CodeStart}} '' ''
{{Cl|PRINT}} "Current path = "; PATH$
{{Cl|END}}

{{Cl|FUNCTION}} PATH$
    f% = {{Cl|FREEFILE}}
    file$ = "D0Spath.inf" 'file name uses a zero to prevent an overwrite of existing file name
    {{Cl|SHELL}} {{Cl|_HIDE}} "CD > " + file$ 'send screen information to a created text file
    {{Cl|OPEN}} file$ {{Cl|FOR (file statement)|FOR}} {{Cl|INPUT (file mode)|INPUT}} {{Cl|AS}} #f% 'file should exist with one line of text
    {{Cl|LINE INPUT (file statement)|LINE INPUT}} #f%, PATH$ 'read file path text to function name
    {{Cl|CLOSE}} #f%
    {{Cl|KILL}} file$
{{Cl|END FUNCTION}} '' ''
{{CodeEnd}}


''Example 2:'' Returns a [[LONG]] array byte size required for a certain sized graphics screen pixel area [[GET (graphics statement)|GET]].
{{CodeStart}} '' ''
{{Cl|INPUT}} "Enter a screen mode: ", mode%
{{Cl|INPUT}} "Enter image width: ", wide&
{{Cl|INPUT}} "Enter image depth: ", deep&
IntegerArray& = ImageBufferSize&(wide&, deep&, mode%) \ 2 ' returns size of an {{Cl|INTEGER}} array.
{{Cl|PRINT}} IntegerArray&
{{Cl|END}}

{{Cl|DEFINT}} A-Z
{{Cl|FUNCTION}} ImageBufferSize& (Wide&, Deep&, ScreenMode%)
    {{Cl|SELECT CASE}} ScreenMode%
        {{Cl|CASE}} 1: BPPlane = 2: Planes = 1
        {{Cl|CASE}} 2, 3, 4, 11: BPPlane = 1: Planes = 1
        {{Cl|CASE}} 7, 8, 9, 12: BPPlane = 1: Planes = 4
        {{Cl|CASE}} 10: BPPlane = 1: Planes = 2
        {{Cl|CASE}} 13: BPPlane = 8: Planes = 1
        {{Cl|CASE ELSE}}: BPPlane = 0
    {{Cl|END SELECT}}
    ImageBufferSize& = 4 + {{Cl|INT}}((Wide& * BPPlane + 7) / 8) * (Deep& * Planes) 'return the value to function name.
{{Cl|END FUNCTION}} '' ''
{{CodeEnd}}
:''Explanation:'' Function calculates the array byte size required when you [[GET (graphics statement)|GET]] an area of a graphics [[SCREEN]]. Each mode may require a different sized array. Since graphics uses [[INTEGER]] arrays, 2 byte elements, the size returned is divided by 2 in the IntegerArray& calculation function reference. Function returns only 4 for [[SCREEN]] 0 which is a text only mode.


{{PageSeeAlso}}
* [[SUB]], [[SCREEN (statement)]] 
* [[EXIT]] (statement), [[END]]
* [[_EXIT (function)]]


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