[BMX] Custom For Loop Iterators

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Pixel_Outlaw	Created : 02 July 2025 Edited : 03 July 2025 [BMX] Custom For Loop Iterators
Here is an example of how to implement your own Iterator for BlitzMax NG for loops. I've never seen somebody do this so I present it here. The For loops are designed this way and left open to the user such that you can make your own kind of iterator. This is a very common technique in other languages too. Especially C++, Java, C#, Rust - Anything with loops and an iterator interface. Beyond simple generators like below, it also provides a mechanism to use For loops with any new type of collection you define (think things like Red/Black trees, R-Trees etc) SuperStrict 'old habit - sue me ' Example of how to construct a custom iterator for your For loops. ' First you need an iterator that implements IIterator<type> ' It needs to have Current, and MoveNext Override the base class' virtual methods ' The new Iterator must be given by an IIterable<type> class you override. ' It will provide an IITerator implementation to the for loop which "pumps" it each loop ' This kind of behavior is useful for various generators taking the behavior OUT ' of the for loop. I can give you these classes and now you can use it without ' knowing how it works. ' Further ideas: What about an x,y iterator that kills those nested for loops? ' Fibonacci iterator: produces Fibonacci numbers up to a fixed count Type FibonacciIterator Implements IIterator<Int> ' These don't /have/ to be private but mutation would break the calculation Private Field lastValue:Int = -1 Private Field nextValue:Int = 1 Private Field currentIteration:Int = 0 Private Field iterations:Int = 10 Method New(iterations:Int) Self.iterations = iterations End Method ' Return the current Fibonacci number ' we need this to be public to match the call to Current Public Method Current:Int() Override Return nextValue End Method ' Move to next Fibonacci number, return True if available Method MoveNext:Int() Override If currentIteration >= iterations Then Return False End If Local temp:Int = nextValue + lastValue lastValue = nextValue nextValue = temp currentIteration :+ 1 Return True End Method End Type ' Fibonacci iterable: returns a new iterator each time Type FibonacciIterable Implements IIterable<Int> Field iterations:Int Method New(iterations:Int = 10) Self.iterations = iterations End Method Method GetIterator:IIterator<Int>() Override Return New FibonacciIterator(iterations) End Method End Type ' Here we use the new FibonacciIteratable ' You don't even need to know the formula to use it. ' The advantage here is that the For loop is /extensible/ ' We can't get here using the "step" value as it's constant. ' I can provide generators and other niceties for common looping patterns. ' The idea here is modularity. No more copy pasting iteration behavior ' over and over and over again. We've described a non linear computation ' in an IIterable instead. ' Now, if you're constantly writing loops for x and y you might ' consider a new IIterable that steps through those such that you ' never write that copy/pasted loop again. This would return an ' Point object. and you'd take the x and y there. ' It could even be generalized into an <n> dimensional one. For Local fibNum:Int = EachIn New FibonacciIterable(15) Print "Calling next Fibonacci number!" + fibNum Next --v

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Thursday, 03 July 2025, 17:39
realtime	not a blitz max user. vibe coding asking AI to create the grid iterator Type Point Field x:Int Field y:Int Method New(x:Int, y:Int) Self.x = x Self.y = y End Method End Type Type GridIterator Implements IIterator<Point> Field width:Int, height:Int Field currentX:Int = 0 Field currentY:Int = 0 Method New(width:Int, height:Int) Self.width = width Self.height = height End Method Method Current:Point() Override Return New Point(currentX, currentY) End Method Method MoveNext:Int() Override currentX :+ 1 If currentX >= width Then currentX = 0 currentY :+ 1 End If Return currentY < height End Method End Type Type GridIterable Implements IIterable<Point> Field width:Int, height:Int Method New(width:Int, height:Int) Self.width = width Self.height = height End Method Method GetIterator:IIterator<Point>() Override Return New GridIterator(width, height) End Method End Type --v sample use: For Local pt:Point = EachIn New GridIterable(5, 5) Print "Point: " + pt.x + ", " + pt.y Next --v
Thursday, 03 July 2025, 17:53
Pixel_Outlaw	That's exactly the idea for another type. Looks to compile and pass. (but the AI seems to have made a logic mistake for 0,0) Somebody making a 3D game might even add a z. Turning what would be 3 nested loops into a single iterator to populate a cube. It's a lot of code, but you only ever need to define it once to replace a bunch of nested stuff forever. This could even be extended to provide an offset x and y. Well done. Point: 1, 0 Point: 2, 0 Point: 3, 0 Point: 4, 0 Point: 0, 1 Point: 1, 1 Point: 2, 1 Point: 3, 1 Point: 4, 1 Point: 0, 2 Point: 1, 2 Point: 2, 2 Point: 3, 2 Point: 4, 2 Point: 0, 3 Point: 1, 3 Point: 2, 3 Point: 3, 3 Point: 4, 3 Point: 0, 4 Point: 1, 4 Point: 2, 4 Point: 3, 4 Point: 4, 4 --v
Thursday, 03 July 2025, 18:20
Pixel_Outlaw	Looks a the fix is just to prime the x with -1 instead of 0. Then it works just fine. (It will then iterate 0 through N - 1) Field currentX:Int = -1 --v If you want inclusive iteration then the MoveNext method changes a bit. Becomes Method MoveNext:Int() Override currentX :+ 1 If currentX > width Then currentX = 0 currentY :+ 1 End If Return currentY - 1 < height End Method --v Result: (with the inclusive tweak) Point: 0, 0 Point: 1, 0 Point: 2, 0 Point: 3, 0 Point: 4, 0 Point: 5, 0 Point: 0, 1 Point: 1, 1 Point: 2, 1 Point: 3, 1 Point: 4, 1 Point: 5, 1 Point: 0, 2 Point: 1, 2 Point: 2, 2 Point: 3, 2 Point: 4, 2 Point: 5, 2 Point: 0, 3 Point: 1, 3 Point: 2, 3 Point: 3, 3 Point: 4, 3 Point: 5, 3 Point: 0, 4 Point: 1, 4 Point: 2, 4 Point: 3, 4 Point: 4, 4 Point: 5, 4 Point: 0, 5 Point: 1, 5 Point: 2, 5 Point: 3, 5 Point: 4, 5 Point: 5, 5 --v Now, what a powerful thing it would be to define an iterator to create things like Bezier curves. The user doesn't even need to know how it works, he just feeds it a new BezierCurve iterator and gets his <n> Points back. Another idea for an Interator might take a collection and only return the "even" indexes etc. All kinds of fun to be had. Languages that let you extend the basic loops are quite nice.

[BMX] Custom For Loop Iterators

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