VB6 Language Enhancement


Author: Dave
Date: 04.09.17 - 5:34am



article source: https://github.com/dzzie/libs/tree/master/experiments/vb6_enumerate


so in python there is a shortcut

for index,value in enumerate(array):

this is shorthand for

Code:

for i = lbound(array) to ubound(array)
   value = array(i)
   ...

I know we have for each but then you dont have the index which you might need to set array(i) sometimes

so I am wondering if we can do something similar and use it like a built in language feature.

features:
  • supports arrays and collections
  • simple usage: Do While enumerate(index, value, aryOrCollection, [collectionKey])
  • no pre setup required just call it, do not need to initialize the index,
  • single call to walk object, it sets index, value and collection key
  • not limited to variant type variables like for each (so you keep intellisense for class objects)
  • put this in a global class in your dll and it will be available like a new language feature
  • limitation: can not be used nested Code:
    Private Sub Form_Load()
        Dim i, v, sample, colkey
        Dim c As New Collection
        Dim l As Long
        
        List1.AddItem "Experiment using a user function like a built in language operator"
        
        sample = Array("apple", "orange", "cat", "dog")
        
        List1.AddItem "----- [ walk simple array ] -------"
        Do While enumerate(i, v, sample)
            List1.AddItem "  Index: " & i & " Value: " & v
            c.Add v
            sample(i) = i
        Loop
        
        List1.AddItem "   Array Values now: " & Join(sample, ", ")
        
        List1.AddItem "----- [ walk array with long var enumerator ] -------"
        
        'walk changed array, use long as enum variable (variant not required like for each)
        Do While enumerate(i, l, sample)
            List1.AddItem "  Index: " & i & " Value: " & l
        Loop
        
        List1.AddItem "----- [ enum collection of simple values ] -------"
        
        Do While enumerate(i, v, c)
            List1.AddItem "  Collection Index: " & i & " Value: " & v
        Loop
        
        Dim c1 As Class1 'class source: Public id
        Set c = New Collection
        
        'build up a collection of class objects..
        For i = 0 To 3
            Set c1 = New Class1
            c1.id = i
            c.Add c1, "key:" & i
        Next
        
        'note the i we pass in here will be 4 and not initilized...
        'enumerate will detect the change in object and reset..
        'there can be a case if we were using same collection as last
        'enumerate call and i was not 0 we would be undefined behavior..
        'note we can use a Class1 variable as our enumerator and is not required to be variant
        'enumerator also supports enumerating the items collection key..not possible with for each..
        
        List1.AddItem "----- [ walk obj collection with key and class obj var ] -------"
        List1.AddItem "----- [ note i = " & i & " is unitilized  ] -------"
        Do While enumerate(i, c1, c, colkey)
            List1.AddItem "  Objs Index: " & i & " Class1 id: " & c1.id & " ColKey: " & colkey
        Loop
    
        sample = Array("apple", "orange", "cat", "dog")
        
        List1.AddItem "----- [ bug test 1 - incomplete loop exit + same obj enum ] -------"
        
        Do While enumerate(i, v, sample)
            List1.AddItem "  Index: " & i & " Value: " & v
            If i = 1 Then Exit Do
        Loop
        
        List1.AddItem "----- [ first enum call exited, enum again same i same obj ] -------"
        
        Do While enumerate(i, v, sample)
            List1.AddItem "  Index: " & i & " Value: " & v
        Loop
        
        List1.AddItem "----- [ ok maybe that is actually a feature..could be useful ] -------"
        
    '    List1.AddItem "----- [ bug test 2 - nesting = endless loop :( ] -------"
    '    'and this will not work and do weird stuff..
    '    Do While enumerate(i, v, sample)
    '
    '        List1.AddItem "  Array Index: " & i & " Value: " & v
    '
    '        Do While enumerate(i, c1, c, colkey)
    '            List1.AddItem "  Objs Index: " & i & " Class1 id: " & c1.id & " ColKey: " & colkey
    '        Loop
    '
    '    Loop
    End Sub
    


    bonus is that the value set as the enumerator does not have to be a variant as with for each it can be a specific class type so intellisense is available for it Code:
    Dim c1 As Class1
    
        Do While enumerate(i, c1, c)
            List1.AddItem "Objs Index: " & i & " class 1 id: " & c1.id
        Loop
    


    trying to use a user function as if it was a built in language feature is tricky..you dont want extra calls to initialize it or close it, you dont want to require class instances so it supports nesting..so you can do pretty good as a language add on "feature" without overhead, but not bullet proof..





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