grasshopper draw only one line...why? simple example help

Replies to This Discussion

Permalink Reply by David Rutten on September 9, 2010 at 4:26am

Hi Daniel,

the problem is that you are adding the same line over and over again to the list. OnLine is a class, meaning that when you do this:

arr_line.Add(line)

you end up with a list filled with references to the same line. And

line.Create(pt1, pt2)

thus changes all the elements in arr_line.

You should make sure you create a new line for every solution. So instead of:

line.Create(pt1, pt2)

You should use:

arr_line.Add(New OnLine(pt1, pt2))

--
David Rutten
david@mcneel.com
Poprad, Slovakia

Permalink Reply by Daniel de Berna on September 9, 2010 at 5:18am

thanks a lot! it works! :) but now i need to understand the concept of this (new online...)

Permalink Reply by David Rutten on September 9, 2010 at 6:00am

Let me try. Hope I don't go overboard on this...

There are two types of data you can have in .NET (and many other languages). Reference Types and Value Types.

A Value Type is the easiest to understand. It's basically just the data directly in memory. For example if you declare a new integer, that integer value is stored 'inside' the variable:

Dim val As Integer

val contains zero by default, and no matter what you do, val always contains a number. You can of course assign a value directly to it at declaration like so:

Dim val As Integer = 55

Whenever you pass this integer to a function, val is copied (unless you pass it by reference, but let's assume now we use the default mechanism). This means that a function cannot screw with 'our' val:

Dim val As Integer = 55
SquareInteger(val)
...
...

Public Function SquareInteger(ByVal int As Integer) As Integer
   int *= int
   Return int
End Function

val goes into SquareInteger(), it is copied into the int parameter, so when the function multiplies int by itself, it does not affect val.


to be continued >>>

Permalink Reply by David Rutten on September 9, 2010 at 6:10am

OnLine on the other hand is a Reference Type. The difference is that the actual data that represents the line (the x, y, z coordinates of the start and end points, 6 numbers in total) is not stored 'inside' the OnLine variable. It is instead stored somewhere else in memory, and the OnLine variable only contains the address of this chunk of far-away memory. In other words, it contains not the value, but a reference to the value.

So when I say:

Dim ln As New OnLine()

then the computer creates a new OnLine instance (the 6 numbers) somewhere deep down in the RAM, and ln merely holds the address of this instance.

When I call a function that takes an OnLine as an input, it is in fact given an address, meaning that any changes to the OnLine from within the function can have far reaching effects:

Dim ln As New OnLine()
ln.From = New On3dPoint(10, 10, 0)
ln.To = New On3dPoint(15, 20, 3)
...
...

Public Sub FlattenLine(ByVal line As OnLine)
   Dim project As New OnXForm()
   project.PlanarProjection(OnPlane.World_xy)
   line.Transform(project)
End Sub

This function changes the line parameter, and since OnLine is a Reference Type, it also affects every other variable that points to the same memory.


to be continued >>>

Permalink Reply by David Rutten on September 9, 2010 at 6:26am

So to backtrack to your original problem of filling a list with OnLines. Let's say we create a list and fill it with integers, like so:

Dim val As Integer = 1
Dim lst As New List(Of Integer)
For i As Integer = 0 To 9
   val *= 2
   lst.Add(val)
Next

Since val is a ValueType, when we assign it to the list we actually put a copy of val into the list. Thus, the list contains the following memory layout:

[0] = 2
[1] = 4
[2] = 8
[3] = 16
[4] = 32
[5] = 64
[6] = 128
[7] = 256
[8] = 512
[9] = 1024

Now let's assume we do the same, but with OnLines:

Dim ln As New OnLine(A, B)
Dim lst As New List(Of OnLine)
For i As Integer = 0 To 9
   ln.Transform(xform)
   lst.Add(ln)
Next

When we declare ln on line 1, it is assigned an address in memory, say "24 Bell Ave." Then we modify that one line over and over, and keep on adding the same address to lst. Thus, the memory layout of lst is now:

[0] = "24 Bell Ave."
[1] = "24 Bell Ave."
[2] = "24 Bell Ave."
[3] = "24 Bell Ave."
[4] = "24 Bell Ave."
[5] = "24 Bell Ave."
[6] = "24 Bell Ave."
[7] = "24 Bell Ave."
[8] = "24 Bell Ave."
[9] = "24 Bell Ave."

To do this properly, we need to create a unique line for every element in lst:

Dim lst As New List(Of OnLine)
For i As Integer = 0 To 9
   Dim ln As New OnLine(A, B)
   ln.Transform(xform)
   lst.Add(ln)
Next

Now, ln is constructed not just once, but whenever the loop runs. And every time it is constructed, a new piece of memory is reserved for it and a new address is created. So now the list memory layout is:

[0] = "24 Bell Ave."
[1] = "12 Pike St."
[2] = "377 The Pines"
[3] = "3670 Woodland Park Ave."
[4] = "99 Zoo Ln."
[5] = "13a District Rd."
[6] = "2 Penny Lane"
[7] = "10 Broadway"
[8] = "225 Franklin Ave."
[9] = "420 Paper St."

--
David Rutten
david@mcneel.com
Poprad, Slovakia

Permalink Reply by David Rutten on September 9, 2010 at 6:28am

ps. In the new Rhino SDK (RhinoCommon), a lot of geometry types are in fact Value Types. Point3d, Vector3d, Interval, Line, Arc, Circle, Plane, Transform etc. etc. These now all behave like Integers.

This can be quite confusing and it often requires very different kinds of methods to deal with Lines and Curves (Curves are still Reference Types). However, Value Types allow us to work with these objects much faster than before.

--
David Rutten
david@mcneel.com
Poprad, Slovakia

Permalink Reply by Daniel de Berna on September 9, 2010 at 6:36am

perfect :) now i think to understand all :D thanks a lot :)