.
Things have been working swimmingly in many areas of the plugin, but one particular problem has been tough to solve. I have two components that are trying to read/write to the same memory at the same time, causing Rhino exceptions and crashes.
The conflicts appear to be happening between two components -- one is a "Layer Events Listener" that reports essentially what type of layer event just happened. The other is a "Set Layer Visibility" component that toggles the visibility of a list of layers.
The code:
public class LayerTools_LayerEventsListener : GH_Component { /// <summary> /// Initializes a new instance of the LayerTools_LayerListener class. /// </summary> public LayerTools_LayerEventsListener() : base("Layer Events Listener", "Layer Listener", "Get granular information about the layer events happening in the Rhino document.", "Squirrel", "Layer Tools") { }
/// <summary> /// Registers all the input parameters for this component. /// </summary> protected override void RegisterInputParams(GH_Component.GH_InputParamManager pManager) { pManager.AddBooleanParameter("Active", "A", "Set to true to listen to layer events in the Rhino document.", GH_ParamAccess.item, false); pManager.AddTextParameter("Exclusions", "E", "Provide a list of exclusions to stop reading specific events (Added, Deleted, Moved, Renamed, Locked, Visibility, Color, Active).", GH_ParamAccess.list); pManager[1].Optional = true; }
/// <summary> /// Registers all the output parameters for this component. /// </summary> protected override void RegisterOutputParams(GH_Component.GH_OutputParamManager pManager) { pManager.AddBooleanParameter("Initialized", "I", "Whether the listener changed from passive to active.", GH_ParamAccess.item); pManager.AddTextParameter("Document Name", "doc", "Name of the Rhino document that is changing.", GH_ParamAccess.item); pManager.AddTextParameter("Layer Path", "path", "Path of the modifed layer.", GH_ParamAccess.item); pManager.AddIntegerParameter("Layer Index", "ID", "Index of the modified layer.", GH_ParamAccess.item); pManager.AddIntegerParameter("Sort Index", "SID", "Sort index of the modified layer.", GH_ParamAccess.item); pManager.AddTextParameter("Event Type", "T", "Type of the modification.", GH_ParamAccess.item); pManager.AddBooleanParameter("Added", "A", "If the layer has been added.", GH_ParamAccess.item); pManager.AddBooleanParameter("Deleted", "D", "If the layer has been deleted.", GH_ParamAccess.item); pManager.AddBooleanParameter("Moved", "M", "If the layer has been moved.", GH_ParamAccess.item); pManager.AddBooleanParameter("Renamed", "R", "If the layer has been renamed.", GH_ParamAccess.item); pManager.AddBooleanParameter("Locked", "L", "If the layer locked setting has changed.", GH_ParamAccess.item); pManager.AddBooleanParameter("Visibility", "V", "If the layer's visibility has changed.", GH_ParamAccess.item); pManager.AddBooleanParameter("Color", "C", "If the layer's color has changed.", GH_ParamAccess.item); pManager.AddBooleanParameter("Active", "Act", "If the active layer has changed.", GH_ParamAccess.item); }
/// <summary> /// This is the method that actually does the work. /// </summary> /// <param name="DA">The DA object is used to retrieve from inputs and store in outputs.</param> protected override void SolveInstance(IGH_DataAccess DA) { bool active = false; List<string> exclusions = new List<string>();
DA.GetData(0, ref active); DA.GetDataList(1, exclusions);
RhinoDoc thisDoc = null;
bool initialize = false;
string dName = null; string activePath = null; int layerIndex = -1; int sortIndex = -1; string eventType = null; bool added = false; bool deleted = false; bool moved = false; bool renamed = false; bool locked = false; bool visibility = false; bool color = false; bool current = false;
if (active) { thisDoc = RhinoDoc.ActiveDoc;
initialize = (!previouslyActive) ? true : false;
RhinoDoc.LayerTableEvent -= RhinoDoc_LayerTableEvent; RhinoDoc.LayerTableEvent += RhinoDoc_LayerTableEvent; previouslyActive = true;
} else {
RhinoDoc.LayerTableEvent -= RhinoDoc_LayerTableEvent; previouslyActive = false; }
if (ev != null) { dName = ev.Document.Name; layerIndex = ev.LayerIndex; eventType = ev.EventType.ToString();
if (!exclusions.Contains("Active")) { if (ev.EventType.ToString() == "Current") { // active layer has just been changed current = true; }
}
if (!exclusions.Contains("Moved")) { if (ev.EventType.ToString() == "Sorted") { // active layer has just been changed moved = true; }
}
if (!exclusions.Contains("Added")) { if (ev.EventType.ToString() == "Added") { // layer has just been added activePath = ev.NewState.FullPath; added = true; }
}
if (!exclusions.Contains("Active")) { if (ev.EventType.ToString() == "Deleted") { // layer has just been added
deleted = true; } }
if (ev.EventType.ToString() == "Modified") { // layer has been modified activePath = ev.NewState.FullPath;
//skip sortindex eventType = ev.EventType.ToString();
if (ev.OldState != null && ev.NewState != null) { if (!exclusions.Contains("Locked")) { if (ev.OldState.IsLocked != ev.NewState.IsLocked) locked = true;
} if (!exclusions.Contains("Visibility")) { if (ev.OldState.IsVisible != ev.NewState.IsVisible) visibility = true; }
if (!exclusions.Contains("Moved")) { if (ev.OldState.ParentLayerId != ev.NewState.ParentLayerId) moved = true; }
//if (ev.OldState.SortIndex != ev.NewState.SortIndex) moved = true; if (!exclusions.Contains("Renamed")) { if (ev.OldState.Name != ev.NewState.Name) renamed = true; }
if (!exclusions.Contains("Color")) { if (ev.OldState.Color != ev.NewState.Color) color = true; } }
} }
DA.SetData(0, initialize); DA.SetData(1, dName); DA.SetData(2, activePath); DA.SetData(3, layerIndex); DA.SetData(4, sortIndex); DA.SetData(5, eventType); DA.SetData(6, added); DA.SetData(7, deleted); DA.SetData(8, moved); DA.SetData(9, renamed); DA.SetData(10, locked); DA.SetData(11, visibility); DA.SetData(12, color); DA.SetData(13, current);
}
static bool previouslyActive = false; Rhino.DocObjects.Tables.LayerTableEventArgs ev = null;
void RhinoDoc_LayerTableEvent(object sender, Rhino.DocObjects.Tables.LayerTableEventArgs e) { ev = e;this.ExpireSolution(true); }
And for the layer visibility component:
public LayerTools_SetActiveLayer() : base("Set Active Layer", "SetActiveLayer", "Set the active layer in the Rhino document.", "Squirrel", "Layer Tools") { }
/// <summary> /// Registers all the input parameters for this component. /// </summary> protected override void RegisterInputParams(GH_Component.GH_InputParamManager pManager) { pManager.AddBooleanParameter("Active", "A", "Set to true to change the active layer in Rhino.", GH_ParamAccess.item, false); pManager.AddTextParameter("Path", "P", "Full path of the layer to be activated.", GH_ParamAccess.item); }
/// <summary> /// Registers all the output parameters for this component. /// </summary> protected override void RegisterOutputParams(GH_Component.GH_OutputParamManager pManager) { pManager.AddIntegerParameter("Layer ID", "ID", "Index of layer that has been activated.", GH_ParamAccess.item); pManager.AddBooleanParameter("Status", "St", "True when the layer has been activated.", GH_ParamAccess.item); }
/// <summary> /// This is the method that actually does the work. /// </summary> /// <param name="DA">The DA object is used to retrieve from inputs and store in outputs.</param> protected override void SolveInstance(IGH_DataAccess DA) { bool active = false; string path = "";
if (!DA.GetData(0, ref active)) return; if (!DA.GetData(1, ref path)) return;
int layer_index = -1; bool status = false;
if (path != null) {
Rhino.RhinoDoc doc = Rhino.RhinoDoc.ActiveDoc; Rhino.DocObjects.Tables.LayerTable layertable = doc.Layers;
layer_index = layertable.FindByFullPath(path, true);
if (layer_index > 0) { // if exists RhinoDoc.ActiveDoc.Layers.SetCurrentLayerIndex(layer_index, true); status = true; } }
DA.SetData(0, layer_index); DA.SetData(1, status); }
Now originally I was getting exceptions when changing multiple layers' visibility properties, which would cause the Event Listener to fire and try to read the Visibility property before the memory has been released by the Set Layer Visibility component. That led me to add an "Exceptions" input, that would allow me to disable the reading of Visibility events at the source in the Layer Events listener. That helped me manage about 95% of the crashes I was getting, but I still get strange crashes in other event properties, even when that property shouldn't be affected. For instance, I am getting a crash here on the Name property in the event from the delegate function, even though I am only changing Visibility at any one time:
I have a few ideas but they all seem pretty hacky. One is to try to set a flag that is readable by any component in the plugin -- so that the event listener can see if a "set" component is currently running and abort before causing an exception. The other is creating a delay in the event listener, somthing like 200ms, to allow any set components to finish what they are doing before reading the event. Neither seems super ideal.
Any ideas?
Thanks,
Marc
…
currently within a fake euphoria framework - blame China/UAE) and a potential decision about doing/developing this or doing that … well …anyway … read and enjoy.
AEC matters: The good, the bad and the ugly.
The bad news: Rhino is NOT suitable for the job (although some use it … but only in the sense that people use Modo for the so called “hard modeling”). By job I mean things up to shop drawing level + specs + you and me (we call it Final level) – nothing to do with sketches and outlines of some abstract “schematic” topology.
The ugly news: The so called Design-Construct approach gains exponentially momentum especially in countries the likes of China/UAE/BRICS (95% of the whole AEC activity worldwide happens there). DeCo means: AEC engineers deliver some kind of study in a preliminary level and the main contractor splits (outsourcers) the job and assigns the study completion AND the construction to various sup-constructors. That thing appeared first – in a large scale - in Dubai 15 years ago. This means that the era of Sergio Pininfarina is over and out: welcome anonymous Toyota designer. In plain English: days of construction corporations fast replacing practices. Dead men walking.
The good news: All AEC related apps (Revit, AECOsim, Allplan and the likes) are in a lethargic state as regards the brave new world (based on the archaic level driven organization schemas etc etc). Of course they all claim the exact opposite and point that support BIM (nobody mention PLM) better than the other guy. But the 21st century – helped by 2 forthcoming unavoidable crisis (a) about shortage of water (b) about transition from carbon to hydrogen economy – isn’t about bureaucracy: think cost/resources optimization and “fitness” rather than China/UAE type of liquid trend. Days of euphoria fast approaching the Wall.
Top to bottom and visa versa.
Old days Titans (Oscar, Mies, Walter, Pierre Luigi, Frank, Eero, blah blah) outlined things (mostly using crayons) and the rest were struggling to translate these in reality in an one way vector like process : Top to bottom that is. These days the inverse gains momentum : when in the whole consider the part … validate … redo … validate … redo. This means bottom to top geared with top-to-bottom. In plain English : child imposes rules to parent and parent imposes rules to child. This means classic MCAD feature/history modeling (CATIA/NX/MS). This is something that Rhino can’t do (not to mention that Rhino is a surface modeler – a rather critical fact).
The parts that are bigger than the whole.
Go there ( http://www.behance.net/gallery/2885057/a-myriad-of-cables) are inspect the whole thing: it’s a parametric nightmare made with the other guy (Generative Components – slower than a Skoda + bugs + why bother?). But the whole (masts and membranes and the likes) means nothing here: focus to the details that are critical for connecting this with that. Complex feature driven solids that are made with internal (on a per se basis) parameters (like fillets required for casting or radius for cable anchoring) whilst they comply with external rules/parameters (cable angles, topology clash issues etc etc). So the whole outlines possibilities … and the part either can follow…or the part must change…or the whole must change. Can you do that with GH/Rhino? And if not what’s missing? (lot’s of things to be honest).
Some other "similar" things:
The narrow picture.
I agree with what others already said and with pretty much all Ola’s points – especially the visual drag-and-drop path mapper (i.e. a visual data manipulator so to speak) and the enable/disable components in groups capability.
Some other suggestions:
A multi canvas capability. As things are right now…it’s like working in Rhino in one view (rather unsuitable I guess). In fact …since overlapping views they don’t work in Rhino…well…you know, he he.
A working auto profile arrangement capability (non twisted Loft/Sweep and the likes). Worth 1Bn dollars that one.
Ability to locate components that caused this or that in the Rhino view: meaning a 2 way communication approach : GH makes things happen in Rhino and things can indicate their cause in the GH canvas.
A robust collection of components that bake stuff in nested blocks (emulating some primitive assembly/component way of thinking). Why may you ask? Well … the whole objective is to talk to CATIA (via STEP) don’t you agree? CATIA makes things happen in real-life not Rhino.
A robust collection of components that can create real-life parametric tensile membrane solutions (get some inspiration from FormFinder: useless because it’s academic but good to point the way). Membranes (and geodesics) are the future.
I could continue at infinitum but IMHO the big picture is worth 12345,67 “focused” GH improvements.
May the Force (the dark option) be with us all.
…
ld be the best UI.
I think difference is made by 'Slider = 10' vs 'Slider = 10.000' more than by simple input/component initialization so, why to stop when it could be even more powerful?
Slider = 0 To 5 --- Slider in [0, 5]
Slider = {3; 0 To 5}
Slider = {3;0;5}
Slider = 3;0;5
Slider = 3 0:5
Slider = 3,0,5
Slider = 3 0 5 --- Value and range (min max)
3 0.0 5 --- 3.0 0.0 5.0
3 0 5.0 --- 3.0 0.0 5.0
3.0 0 5 --- 3.0 0.0 5.0
-1 0 5 --- 0 0 5 (-1 -1 5)
6 0 5 --- 5 0 5 (6 0 6)
Slider = 0:2:6 --- Even numbers: 0, 2, 4, 6.
Slider = 1:2:7 --- Odd numbers: 1, 3, 5, 7.
0:2:5 --- 0:2:4 (or 0:2:6)
3:2:8 --- 3:2:7 (or 3:2:9)
3 1:2:7 --- 1 3 5 7 (value 3)
Bang! = 7 --- 7 outputs
Merge = 5 --- 5 inputs
What's your opinion about Bang! = 7? As it's setting number of inputs, should it use different format? Bang! 7? Bang! (7)? Bang! i7?
+ * - / \ % ^ & | ! = > --- Addition, Multiplication, Subtraction, Division, Integer Division, Modulus, Power, AND, OR, NOT, Larger than, &c.
= could be a problem.
\ Integer division or Set difference?
! could be NOT but also Factorial.
| could mean intersection.
& could mean concatenate.
1+ --- Addition: input A = 1
2* --- Multiplication: input A = 2
+{0,1,1} --- Addition: input B = {0,1,1}
0-, 1/, 2^, 10^, e^ have their own components
Flatten = {7} or Flatten = 7 --- Input P = {7} (off-topic: Why can’t P be a list?)
Pt = {1, 2, 3} --- Point XYZ, X = 1, Y = 2, Z = 3.
Swatch = 129,239,231 (102)
Swatch = 129 139 231 102
F2 = "x^2+y"
"List Length" and "List Insert" don't work properly: "Value List" is choosen. Why? What's the reason to this choice? Well, I'd like to know how the whole thing (search by keywords) works, David.
Name and nickname can be now used as keywords. "Larger" works for ">" but "greater" doesn't. Could it be improved? Could synonyms be used? Could a short description even be used (I know this could be a bit weird)?
more than --- >
more or less --- Similarity
more less --- Similarity
red green --- Sets.List components should be showed
lightning --- Split Tree
What about use Curve.Analysis or Math.Boolean to display those Tab.Panel components? Param, Math, Sets, Vector? Primitive, Special, Util? Tab, Panel, and Tab.Panel as keywords.
At the moment that I write this, I check that ignoring accents in keywords has almost been included (0.8.0009): p`anel, pañel, pánel --- panel (almost)
Shouldn’t 'Dom2' work for Dom²?
What about nested search? You type some keywords (say 'Params' or 'Params.Geometry', or 'red green', or 'lst') and then you make a fine-tunning search over previous results/keywords. Tab.Panel and/or nested geometry could be useful when search by plug-in is desired or when you want to search among .ghuser components (first 'ghuser' or 'Extra.MyPlugIn' or 'lst' keyword and then fine-tunning, specific, search).
Is 'list length' performing this nested search right now ('lst' > 'length')? Anyway, I am thinking about UI (graphical) changes; successive searches.
As I said, description (and even words from the help info) could be used to search. What about "some kind of tags"? I mean that if 'list l' to finally choose List Length has been used for a while, that could be learned. Eventually, an XML file could store these tags, so you could even edit them. That could implement description, name, nickname, help info, Tab.Panel, .ghuser, synonyms (lots of them), tags/shortcuts or wathever.
How could flatten/graft/reverse be used? Initialize graft+Simplify or graft+Bang! could be really useful.
What about expressions? I don't how could it be done properly: would Slider = x^2 (expression) work? I mean, aren't expressions parsed when initializing?
Is Panel somehow doing this? 'panel = wathever' always suppose that wathever is a string, so you can't use 'panel = <pi>'. Sets.Strings components also do this.
I've been about to write several paragraphs about height/width (resizable components: Panel, Graph Mapper, Slider, &c.), input/output names (Scripts, F components; or any component with editable input/output names), orientation (Scribble), type hint and access option, nickname, &c. but, to sum up: being able to set any property when initializing would be really useful. I'd like to know the best choice of syntax but I'm sure that, David, you're closer to the answer. What do you think about this?
Slider: 3 0 5 "MySlider" "Slider^2"
Panel: "This is the content" "This is the title"
VB: "N" List Integer 7 "r" Item Double
Addition: A 1 B 2
I guess that any unified syntax would be elegant and useful, but additional ad hoc syntax (per component) could be even better (cleaner).
What about use lists of values? I'm not sure about format: panel = ("Hello", "Bonjour", "Hola")? If any valid format/syntax is found, maybe more sophisticated fetaures could be achieved: panel = {0;0} ("A", "B", "C") {0;1} ("1", "2", "3") How would you like this to be implemented?
There is a much simpler and interesting feature that would be useful, in my opinion: being able to initialize more than one component. I mean say 7xSlider = 10.0 and get 7 sliders and I also mean multiline (multi-component) initialization: Ctrl+Intro when you want to start a new line and Intro (or even some Accept/Cancel buttons when you activate multiline mode) to initialize (every line/component), for example. I mean:
3 x Slider = 1
Panel
Mass addition
Panel
And the whole bunch of components that were in mind (pre-thinked definition) is initialized. It speeds up the workflow, making more dynamic to add components that are only available via the drop-down panels.
Should this multiplier be something like a text box adjacent to search field more than '7x'?
These are some of my thoughts about intitializing. Please let me know your opinion :]
…
Introduction to Grasshopper Videos by David Rutten.
Wondering how to get started with Grasshopper? Look no further. Spend an some time with the creator of Grasshopper, David Rutten, to learn the
ng is deciding how and where to store your data. If you're writing textual code using any one of a huge number of programming languages there are a lot of different options, each with its own benefits and drawbacks. Sometimes you just need to store a single data point. At other times you may need a list of exactly one hundred data points. At other times still circumstances may demand a list of a variable number of data points.
In programming jargon, lists and arrays are typically used to store an ordered collection of data points, where each item is directly accessible. Bags and hash sets are examples of unordered data storage. These storage mechanisms do not have a concept of which data comes first and which next, but they are much better at searching the data set for specific values. Stacks and queues are ordered data structures where only the youngest or oldest data points are accessible respectively. These are popular structures for code designed to create and execute schedules. Linked lists are chains of consecutive data points, where each point knows only about its direct neighbours. As a result, it's a lot of work to find the one-millionth point in a linked list, but it's incredibly efficient to insert or remove points from the middle of the chain. Dictionaries store data in the form of key-value pairs, allowing one to index complicated data points using simple lookup codes.
The above is a just a small sampling of popular data storage mechanisms, there are many, many others. From multidimensional arrays to SQL databases. From readonly collections to concurrent k-dTrees. It takes a fair amount of knowledge and practice to be able to navigate this bewildering sea of options and pick the best suited storage mechanism for any particular problem. We did not wish to confront our users with this plethora of programmatic principles, and instead decided to offer only a single data storage mechanism.*
Data storage in Grasshopper
In order to see what mechanism would be optimal for Grasshopper, it is necessary to first list the different possible ways in which components may wish to access and store data, and also how families of data points flow through a Grasshopper network, often acquiring more complexity over time.
A lot of components operate on individual values and also output individual values as results. This is the simplest category, let's call it 1:1 (pronounced as "one to one", indicating a mapping from single inputs to single outputs). Two examples of 1:1 components are Subtraction and Construct Point. Subtraction takes two arguments on the left (A and B), and outputs the difference (A-B) to the right. Even when the component is called upon to calculate the difference between two collections of 12 million values each, at any one time it only cares about three values; A, B and the difference between the two. Similarly, Construct Point takes three separate numbers as input arguments and combines them to form a single xyz point.
Another common category of components create lists of data from single input values. We'll refer to these components as 1:N. Range and Divide Curve are oft used examples in this category. Range takes a single numeric domain and a single integer, but it outputs a list of numbers that divide the domain into the specified number of steps. Similarly, Divide Curve requires a single curve and a division count, but it outputs several lists of data, where the length of each list is a function of the division count.
The opposite behaviour also occurs. Common N:1 components are Polyline and Loft, both of which consume a list of points and curves respectively, yet output only a single curve or surface.
Lastly (in the list category), N:N components are also available. A fair number of components operate on lists of data and also output lists of data. Sort and Reverse List are examples of N:N components you will almost certainly encounter when using Grasshopper. It is true that N:N components mostly fall into the data management category, in the sense that they are mostly employed to change the way data is stored, rather than to create entirely new data, but they are common and important nonetheless.
A rare few components are even more complex than 1:N, N:1, or N:N, in that they are not content to operate on or output single lists of data points. The Divide Surface and Square Grid components want to output not just lists of points, but several lists of points, each of which represents a single row or column in a grid. We can refer to these components as 1:N' or N':1 or N:N' or ... depending on how the inputs and outputs are defined.
The above listing of data mapping categories encapsulate all components that ship with Grasshopper, though they do not necessarily minister to all imaginable mappings. However in the spirit of getting on with the software it was decided that a data structure that could handle individual values, lists of values, and lists of lists of values would solve at least 99% of the then existing problems and was thus considered to be a 'good thing'.
Data storage as the outcome of a process
If the problems of 1:N' mappings only occurred in those few components to do with grids, it would probably not warrant support for lists-of-lists in the core data structure. However, 1:N' or N:N' mappings can be the result of the concatenation of two or more 1:N components. Consider the following case: A collection of three polysurfaces (a box, a capped cylinder, and a triangular prism) is imported from Rhino into Grasshopper. The shapes are all exploded into their separate faces, resulting in 6 faces for the box, 3 for the cylinder, and 5 for the prism. Across each face, a collection of isocurves is drawn, resembling a hatching. Ultimately, each isocurve is divided into equally spaced points.
This is not an unreasonably elaborate case, but it already shows how shockingly quickly layers of complexity are introduced into the data as it flows from the left to the right side of the network.
It's no good ending up with a single huge list containing all the points. The data structure we use must be detailed enough to allow us to select from it any logical subset. This means that the ultimate data structure must contain a record of all the mappings that were applied from start to finish. It must be possible to select all the points that are associated with the second polysurface, but not the first or third. It must also be possible to select all points that are associated with the first face of each polysurface, but not any subsequent faces. Or a selection which includes only the fourth point of each division and no others.
The only way such selection sets can be defined, is if the data structure contains a record of the "history" of each data point. I.e. for every point we must be able to figure out which original shape it came from (the cube, the cylinder or the prism), which of the exploded faces it is associated with, which isocurve on that face was involved and the index of the point within the curve division family.
A flexible mechanism for variable history records.
The storage constraints mentioned so far (to wit, the requirement of storing individual values, lists of values, and lists of lists of values), combined with the relational constraints (to wit, the ability to measure the relatedness of various lists within the entire collection) lead us to Data Trees. The data structure we chose is certainly not the only imaginable solution to this problem, and due to its terse notation can appear fairly obtuse to the untrained eye. However since data trees only employ non-negative integers to identify both lists and items within lists, the structure is very amenable to simple arithmetic operations, which makes the structure very pliable from an algorithmic point of view.
A data tree is an ordered collection of lists. Each list is associated with a path, which serves as the identifier of that list. This means that two lists in the same tree cannot have the same path. A path is a collection of one or more non-negative integers. Path notation employs curly brackets and semi-colons as separators. The simplest path contains only the number zero and is written as: {0}. More complicated paths containing more elements are written as: {2;4;6}. Just as a path identifies a list within the tree, an index identifies a data point within a list. An index is always a single, non-negative integer. Indices are written inside square brackets and appended to path notation, in order to fully identify a single piece of data within an entire data tree: {2,4,6}[10].
Since both path elements and indices are zero-based (we start counting at zero, not one), there is a slight disconnect between the ordinality and the cardinality of numbers within data trees. The first element equals index 0, the second element can be found at index 1, the third element maps to index 2, and so on and so forth. This means that the "Eleventh point of the seventh isocurve of the fifth face of the third polysurface" will be written as {2;4;6}[10]. The first path element corresponds with the oldest mapping that occurred within the file, and each subsequent element represents a more recent operation. In this sense the path elements can be likened to taxonomic identifiers. The species {Animalia;Mammalia;Hominidea;Homo} and {Animalia;Mammalia;Hominidea;Pan} are more closely related to each other than to {Animalia;Mammalia; Cervidea;Rangifer}** because they share more codes at the start of their classification. Similarly, the paths {2;4;4} and {2;4;6} are more closely related to each other than they are to {2;3;5}.
The messy reality of data trees.
Although you may agree with me that in theory the data tree approach is solid, you may still get frustrated at the rate at which data trees grow more complex. Often Grasshopper will choose to add additional elements to the paths in a tree where none in fact is needed, resulting in paths that all share a lot of zeroes in certain places. For example a data tree might contain the paths:
{0;0;0;0;0}
{0;0;0;0;1}
{0;0;0;0;2}
{0;0;0;0;3}
{0;0;1;0;0}
{0;0;1;0;1}
{0;0;1;0;2}
{0;0;1;0;3}
instead of the far more economical:
{0;0}
{0;1}
{0;2}
{0;3}
{1;0}
{1;1}
{1;2}
{1;3}
The reason all these zeroes are added is because we value consistency over economics. It doesn't matter whether a component actually outputs more than one list, if the component belongs to the 1:N, 1:N', or N:N' groups, it will always add an extra integer to all the paths, because some day in the future, when the inputs change, it may need that extra integer to keep its lists untangled. We feel it's bad behaviour for the topology of a data tree to be subject to the topical values in that tree. Any component which relies on a specific topology will no longer work when that topology changes, and that should happen as seldom as possible.
Conclusion
Although data trees can be difficult to work with and probably cause more confusion than any other part of Grasshopper, they seem to work well in the majority of cases and we haven't been able to come up with a better solution. That's not to say we never will, but data trees are here to stay for the foreseeable future.
* This is not something we hit on immediately. The very first versions of Grasshopper only allowed for the storage of a single data point per parameter, making operations like [Loft] or [Divide Curve] impossible. Later versions allowed for a single list per parameter, which was still insufficient for all but the most simple algorithms.
** I'm skipping a lot of taxonometric classifications here to keep it simple.…
Added by David Rutten at 2:22pm on January 20, 2015