Operator Step (Iteration)


Increments the iterator of a For...Next loop

Syntax:
{ Type | Class | Union } typename
declare operator Step ()
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

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
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.
As all non-static member procedures, they have passed a hidden this parameter that allows to access by reference to the iterator object in the code body of the 3 operators.

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

The first version of Operator Step 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 is passed the step value to increment the iterator object.

Examples:
'' Example Type
Type T
  '' value is set by the constructor
  value As Double
  Declare Constructor( ByVal x As Double = 0 )

  Declare Operator For( ByRef stp As T )
  Declare Operator Step( ByRef stp As T )
  Declare Operator Next( ByRef cond As T, ByRef stp As T ) As Integer
End Type

Constructor T ( ByVal x As Double )
  Print "T iterator constructed with value " & x
  value = x
End Constructor

Operator T.For( ByRef stp As T )
End Operator

Operator T.Step( ByRef stp As T )
  Print " incremented by " & stp.value & " in step."
  value += stp.value
End Operator

Operator T.Next( ByRef cond As T, ByRef stp As T ) As Integer
  '' iterator's moving from a high value to a low value (step >= 0)
  If( stp.value < 0 ) Then
    Return( value >= cond.value )
  Else
  '' iterator's moving from a low value to a high value (step < 0)
    Return( value <= cond.value )
  End If
End Operator

'' Example Usage. It looks like we are working with numbers, but the iterators
'' have overloaded constructors. The 10, 1, and -1 are all of type T.
For i As T = 10 To 1 Step -1
  Print i.value;
Next i

A more practical example demonstrating file iteration based on cha0s' file iteration class.
'' a class which iterates through files
Type FileIter
    As String pathName, fileName
    Declare Constructor( ByRef pathName As String )

    Declare Operator For()
    Declare Operator Step()
    Declare Operator Next( ByRef endCond As FileIter) As Integer
End Type

Constructor FileIter( ByRef pathName As String )  
    This.pathName = pathName
End Constructor

Operator FileIter.For( )  
    fileName = Dir(pathName & "/*.*")  
End Operator

Operator FileIter.Step( )  
    fileName = Dir("")
End Operator

Operator FileIter.Next( ByRef endCond As FileIter ) As Integer
    Return(fileName <> endCond.pathName)  
    '' the c'tor sets the path name and so we check against that
End Operator

'' example code
'' change it to any directory
For i As FileIter = "./" To ""
    Print i.fileName
Next

Another example working with strings:
Type CharIterator
    '' used to build a step var
    Declare Constructor( ByVal r As ZString Ptr )
   
    '' implicit step versions
    Declare Operator For ( )
    Declare Operator Step( )
    Declare Operator Next( ByRef end_cond As CharIterator ) As Integer
   
    '' explicit step versions
    Declare Operator For ( ByRef step_var As CharIterator )
    Declare Operator Step( ByRef step_var As CharIterator )
    Declare Operator Next( ByRef end_cond As CharIterator, ByRef step_var As CharIterator ) As Integer
   
    '' give the current "value"    
    Declare Operator Cast( ) As String
   
    Private:   
        '' data
        value As String
       
        '' This member isn't necessary - we could use
        '' the step variable on each iteration -
        '' but we choose this method, since we have
        '' to compare strings otherwise. See below.
        is_up As Integer
End Type

Constructor CharIterator( ByVal r As ZString Ptr )
    value = *r
End Constructor

Operator CharIterator.Cast( ) As String
    Operator = value
End Operator

'' implicit step versions
''
'' In this example, we interpret implicit step
'' to always mean 'up'
Operator CharIterator.For( )
    Print "implicit step"
End Operator

Operator CharIterator.Step( )
    value[0] += 1
End Operator

Operator CharIterator.Next( ByRef end_cond As CharIterator ) As Integer
    Return This.value <= end_cond.value
End Operator

'' explicit step versions
''
'' In this example, we calculate the direction
'' at FOR, but since the step var is passed to
'' each operator, we have the choice to also calculate
'' it "on-the-fly". For strings such as this, repeated comparison
'' may penalize, but if you're working with simpler types,
'' then you may prefer to avoid the overhead of
'' an 'is_up' variable.
Operator CharIterator.For( ByRef step_var As CharIterator )
    Print "explicit step"
    is_up = (step_var.value = "up")
End Operator

Operator CharIterator.Step( ByRef step_var As CharIterator )
    If( is_up ) Then
        value[0] += 1
    Else
        value[0] -= 1
    End If
End Operator

Operator CharIterator.Next( ByRef end_cond As CharIterator, ByRef step_var As CharIterator ) As Integer
    If( This.is_up ) Then
        Return This.value <= end_cond.value
    Else
        Return This.value >= end_cond.value
    End If
End Operator

For i As CharIterator = "a" To "z"
    Print i; " ";
Next
Print "done"

For i As CharIterator = "a" To "z" Step "up"
    Print i; " ";
Next
Print "done"

For i As CharIterator = "z" To "a" Step "down"
    Print i; " ";
Next
Print "done"

For i As CharIterator = "z" To "a" Step "up"
    Print i; " ";
Next
Print "done"

Iterating with fractions:
Type fraction
    '' Used to build a step var
    Declare Constructor( ByVal n As Integer, ByVal d As Integer )

    '' Implicit step versions
    Declare Operator For ( )
    Declare Operator Step( )
    Declare Operator Next( ByRef end_cond As fraction ) As Integer

    '' Explicit step versions
    Declare Operator For ( ByRef step_var As fraction )
    Declare Operator Step( ByRef step_var As fraction )
    Declare Operator Next( ByRef end_cond As fraction, ByRef step_var As fraction ) As Integer

    '' Give the current "value"    
    Declare Operator Cast( ) As Double
    Declare Operator Cast( ) As String

    Private:
        As Integer num, den
End Type

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

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

Operator fraction.Cast( ) As String
    Operator = num & "/" & den
End Operator

'' Some fraction functions
Function gcd( ByVal n As Integer, ByVal m As Integer ) As Integer
    Dim As Integer t
        While m <> 0
            t = m
            m = n Mod m
            n = t
        Wend
    Return n
End Function

Function lcd( ByVal n As Integer, ByVal m As Integer ) As Integer
    Return (n * m) / gcd( n, m )
End Function

''
'' Implicit step versions
''
'' In this example, we interpret implicit step
'' to mean 1
''
Operator fraction.For( )
    Print "implicit step"
End Operator

Operator fraction.Step( )
    Var lowest = lcd( This.den, 1 )

    Var mult_factor = This.den / lowest
    Dim As fraction step_temp = fraction( 1, 1 )
   
    This.num *= mult_factor
    This.den *= mult_factor
   
    step_temp.num *= lowest
    step_temp.den *= lowest
   
    This.num += step_temp.num
End Operator

Operator fraction.Next( ByRef end_cond As fraction ) As Integer
    Return This <= end_cond
End Operator

''
'' Explicit step versions
''
Operator fraction.For( ByRef step_var As fraction )
    Print "explicit step"
End Operator

Operator fraction.Step( ByRef step_var As fraction )
    Var lowest = lcd( This.den, step_var.den )
    Var mult_factor = This.den / lowest
    Dim As fraction step_temp = step_var

    This.num *= mult_factor
    This.den *= mult_factor

    mult_factor = step_temp.den / lowest

    step_temp.num *= mult_factor
    step_temp.den *= mult_factor

    This.num += step_temp.num
End Operator

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

For i As fraction = fraction(1,1) To fraction(4,1)
    Print i; " ";
Next
Print "done"

For i As fraction = fraction(1,4) To fraction(1,1) Step fraction(1,4)
    Print i; " ";
Next
Print "done"

For i As fraction = fraction(4,4) To fraction(1,4) Step fraction(-1,4)
    Print i; " ";
Next
Print "done"

For i As fraction = fraction(4,4) To fraction(1,4)
    Print i; " ";
Next
Print "done"

Dialect Differences:
See also:
Back to Iterating Operators
Back to Operators
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