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Filter Collections / Maps

Filtering a collection or map is done using a CollectionFilter or a MapFilter. The steps that need to be done are:

  1. Create a class extending either CollectionFilter or MapFilter.
  2. Override the filter() method. This takes a single parameter (a variant for the current value in the collection, or a Pair for the current key/value pair in the map). It returns a boolean, for whether the entry should be copied into the new collection or map.
  3. Create an instance of the new class.
  4. Pass the instance to the Collection's or Map's filter() method.

The filter methods will return a new Collection or Map, with the same settings as the original, containing the entries that passed the filter criteria. This means calls can be chained.

Sample Filter

A CollectionFilter class can be constructed like so:

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Class EvenFilterer as CollectionFilter

    Function filter(source as Variant) as Boolean
        If (IsNumeric(source)) Then
            If (source Mod 2 = 0) Then
                Return True
            End If
        End If
    End Function

End Class

This EvenFilterer will only pass even numbers to the new Collection. The CollectionFilter class takes an empty constructor, so the only function that needs to be added is the filter(source as Variant) function that is being overridden, from line 3. The function needs to return a Boolean value, true if the element should be added to the new Collection and false if it should be omitted (filtered out).

Line 4 checks the value passed in is numeric. Line 5 uses Mod to return the remainder from a division by 2 - any even number will return 0, any odd number will return 1. Thus only even numbers will return true from the function.

This would then be implemented with the code:

Dim coll1 as New Collection("INTEGER", Nothing, false, false)
Dim coll2 as Collection
Dim collFilter as New EvenFilterer()

Dim i as Integer
For i = 0 to 9
    ' Create a random number, multiply by 100 and add just the integer portion
    Call coll1.add(Fix(Rnd() * 100))
Next

Set coll2 = coll1.filter(collFilter)

The MapFilter is constructed and used in the same way, except its filter() method takes a key/value Pair object.