Iterators


The overload Operators For, Next, and Step, allowing to construct User-Defined Types Iterators (instead of only intrinsic scalar types iterators) for a For...Next loop

Syntax (declaration)
{ Type | Class | Union } typename
' For...Next statement with implicit step (1st version of operators)
declare Operator For ( )
declare Operator Next ( [ byref | byval ] cond as typename ) as Integer
declare Operator Step ( )
' For...Next statement with explicit step (2nd version of operators)
declare Operator For ( [ byref | byval ] stp as typename )
declare Operator Next ( [ byref | byval ] cond as typename, [ byref | byval ] stp as typename ) as Integer
declare Operator Step ( [ byref | byval ] stp as typename )
End { Type | Class | Union }

Usage
For iterator [ As typename ] = start_value To end_value [ Step step_value ]
[ ...statements... ]
Next

The first version of operators is used if no step_value is given in the For...Next statement.
If a step_value is given, the second version is used and a step object (initialized with step-value) is passed through the stp parameter:
- to Operator For because eventual additional initialization may use it,
- to Operator Next because testing for iterating end may depend on it,
- to Operator Step to increment the iterator object.
Both versions of the operators can coexist (thanks to member overloading) in the same user-defined type (to be able to both use and not use the explicit increment in For...Next statements of the user code).

Parameters
typename
name of the Type, Class, or Union
stp, step_value
a typename object used as an incremental value
iterator
a typename object used as an iterator
cond, end_value
a typename object used as a loop-terminating value
start_value
a typename object used to copy construct or assign to the iterator initially

Description
Operator For, Operator Next and Operator Step can be overloaded in user-defined type definitions to allow objects of that type to be used as iterators and step values in For...Next loops (instead of the pre-defined for intrinsic scalar types).

As all non-static member procedures, the 3 operators have passed a hidden this parameter that allows to access by reference to the iterator object (initialized to the start_value argument value from the For...Next statement).
The cond parameter of the Operator Next allows to access the end_value argument value from the For...Next statement.
If a step_value is given (as argument) in the For...Next statement, the stp parameter allows to access this value in the 3 operators.

Note: If no step_value is given in the For...Next statement (implicit step), the user-defined type must have a default constructor (implicit or explicit) or a conversion constructor. It is a bug at the moment because if the user defines a default constructor, the compiler does not even use it when initializing the For...Next loop!

Operator For
Operator For is called once immediately after copy constructing or assigning to the iterator object (with the start_value), constructing the end object (with the end_value), and constructing the step object (with step_value if defined in the For...Next statement).
Operator For allows to perform any additional initialization needed in preparation for the loop.

Operator Next
Operator Next is called every time the iterator object needs to be checked against the end value. This happens immediately after the call to the Operator For, and then immediately after any calls to the Operator Step.
Operator Next should return zero (0) if the loop should be terminated, or non-zero if the loop should continue iterating.
The first time Operator Next is called, no statements in the For...Next body have been executed yet.
Operator Next also allows to perform some processing before the execution of all statements in the For...Next body.

Operator Step
Operator Step is called to increment the iterator object immediately after all statements in the For...Next body are executed.

Algorithm
For...Next loop algorithm around the 3 overload operators:
'                       FOR...NEXT loop
'                              V
'                              |
'            constructing/assigning iterator object
'       (This = start_value from For...Next statement)
'                              |
'                   constructing end object
'       (cond = end_value from For...Next statement)
'                              |
'                   if step_value is defined >---------------------.
'                            else                                  :
'                              v                                   v
'                              :                        constructing step object
'                              :               (stp = step_value from For...Next statement)
'                              :                                   :
'                              :<----------------------------------'
'                              |
'                    calling Operator For
'                              |
'     .----------------------->|
'     |                        |
'     |              calling Operator Next
'     |     (if end-condition verified: =0 returned) >-------------.
'     |               (else: <>0 returned)                         |
'     |                        v                                   |
'     |                        |                                   |
'     |            executing For...Next body                       |
'     |                        |                                   |
'     |              calling Operator Step                         |
'     |                        |                                   |
'     '------------------------'                                   |
'                                                                  |
'                                                                  V

Example
Type for iterating through screen resolutions, with implicit step-value:
Type screenResolution
    ' user interface
        Declare Constructor (ByVal colorBit As Long)
        Declare Property colorDepth () As Long
        Declare Property screenWidth () As Long
        Declare Property screenHeigth () As Long
    ' overload iteration operators when Step is not defined in For...Next statement
        Declare Operator For ()
        Declare Operator Next (ByRef iterateCondition As screenResolution) As Integer
        Declare Operator Step ()
    ' internal variables
        Dim As Long colorBit, resolutionWH
End Type

Constructor screenResolution (ByVal colorBit As Long)
    This.colorBit = colorBit
End Constructor

Property screenResolution.colorDepth () As Long
    Return This.colorBit
End Property

Property screenResolution.screenWidth () As Long
    Return HiWord(This.resolutionWH)
End Property

Property screenResolution.screenHeigth () As Long
    Return LoWord(This.resolutionWH)
End Property

Operator screenResolution.For ()
    This.resolutionWH = ScreenList(This.colorBit)
End Operator

Operator screenResolution.Next (ByRef iterateCondition As screenResolution) As Integer
    While This.resolutionWH = 0
        If This.colorBit < iterateCondition.colorBit Then
            This.colorBit += 1
            This.resolutionWH = ScreenList(This.colorBit)
        Else
            Exit While
        End If
    Wend
    Return (This.resolutionWH <> iterateCondition.resolutionWH)
End Operator

Operator screenResolution.Step ()
    This.resolutionWH = ScreenList()
End Operator


Print "Screen resolutions supported within [1 bpp , 64 bpp]:"
For iterator As screenResolution = screenResolution(1) To screenResolution(64)
    Print "    " & iterator.colorDepth & " bpp ",
    Print ":" & iterator.screenWidth & "x" & iterator.screenHeigth
Next iterator
Print "End of supported screen resolutions"

Sleep
Output example:
Screen resolutions supported within [1 bpp , 64 bpp]:
	24 bpp    :320x200
	24 bpp    :320x240
	24 bpp    :400x300
	24 bpp    :512x384
	24 bpp    :640x400
	24 bpp    :640x480
	24 bpp    :800x600
	24 bpp    :1024x768
	24 bpp    :1152x864
	24 bpp    :1280x600
	24 bpp    :1280x720
	24 bpp    :1280x768
	24 bpp    :1280x800
	24 bpp    :1280x960
	24 bpp    :1280x1024
	24 bpp    :1360x768
	24 bpp    :1366x768
	24 bpp    :1400x1050
	24 bpp    :1440x900
	24 bpp    :1600x900
	24 bpp    :1680x1050
	24 bpp    :1920x1080
	32 bpp    :320x200
	32 bpp    :320x240
	32 bpp    :400x300
	32 bpp    :512x384
	32 bpp    :640x400
	32 bpp    :640x480
	32 bpp    :800x600
	32 bpp    :1024x768
	32 bpp    :1152x864
	32 bpp    :1280x600
	32 bpp    :1280x720
	32 bpp    :1280x768
	32 bpp    :1280x800
	32 bpp    :1280x960
	32 bpp    :1280x1024
	32 bpp    :1360x768
	32 bpp    :1366x768
	32 bpp    :1400x1050
	32 bpp    :1440x900
	32 bpp    :1600x900
	32 bpp    :1680x1050
	32 bpp    :1920x1080
End of supported screen resolutions

Type for iterating through fractions, with explicit step-value used in the 3 operators:
(improved example compared to the one of Operator Step page)
Type fraction
    ' user interface
        Declare Constructor (ByVal n As Integer, ByVal d As Integer)
        Declare Operator Cast () As String
    ' overload iteration operators when Step is defined in For...Next statement
        Declare Operator For (ByRef iterateStep As fraction)
        Declare Operator Next (ByRef iterateCondition As fraction, ByRef iterateStep As fraction) As Integer
        Declare Operator Step (ByRef step_var As fraction)
    ' internal variables and cast operator
        As Integer num, den
        Declare Operator Cast () As Double
End Type

Constructor fraction (ByVal n As Integer, ByVal d As Integer)
    This.num = n
    This.den = d
End Constructor

Operator fraction.Cast () As String
    ' search for the highest common factor (a) between numerator and denominator
        Dim As Integer a = Abs(This.num), b = Abs(This.den)
        If a <> 0 Then
            While a <> b
                If a > b Then
                    a -= b
                Else
                    b -= a
                End If
            Wend
        Else
            a = 1
        End If
    ' reduce the fraction
        Return num \ a & "/" & den \ a
End Operator

Operator fraction.Cast () As Double
    Return This.num / This.den
End Operator

Operator fraction.For (ByRef iterateStep As fraction)
    ' search for the least common multiple (a) between the two denominators
        Dim As Integer a = Abs(This.den), b = Abs(iterateStep.den), c = a, d = b
        While a <> b
            If a > b Then
                b += d
            Else
                a += c
            End If
        Wend
    ' align at the same denominator the 2 fractions
        This.num *= a \ This.den
        This.den = a
        iterateStep.num *= a \ iterateStep.den
        iterateStep.den = a
End Operator

Operator fraction.Next (ByRef iterateCondition As fraction, ByRef iterateStep As fraction) As Integer
    If iterateStep.num < 0 Or iterateStep.den < 0 Then
        Return This >= iterateCondition
    Else
        Return This <= iterateCondition
    End If
End Operator

Operator fraction.Step (ByRef iterateStep As fraction)
    This.num += iterateStep.num
End Operator


Print "iteration from 1/8 to 1/2 by step of 1/12:"
For iterator As fraction = fraction(1, 8) To fraction(1, 2) Step fraction(1, 12)
    Print "    " & iterator;
Next
Print
Print
Print "iteration from 7/10 to -8/5 by step of -8/15:"
For iterator As fraction = fraction(7, 10) To fraction(-8, 5) Step fraction(-8, 15)
    Print "    " & iterator;
Next
Print

Sleep
Output:
iteration from 1/8 to 1/2 by step of 1/12:
	1/8    5/24    7/24    3/8    11/24
	
iteration from 7/10 to -8/5 by step of -8/15:
	7/10    1/6    -11/30    -9/10    -43/30

See also
Back to Programmer's Guide
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