Lists, Sets, Strings & Trees

Replies to This Discussion

Permalink Reply by David Rutten on June 21, 2012 at 12:54am

• Lists. A List is an ordered (but not necessarily sorted) collection of data. Lists may contain zero elements, a single element, or (at most) 2,147,483,647 elements. Furthermore, the same element can occur more than once in the same list, and different elements with identical values can occur more than once. Also, a list may contain lack of elements, referred to as "nulls".
• Sets. Strictly speaking a Set is a mathematical construct which adheres to a strict collection of rules and limitations. Basically, a Set is the same as a List, with the exception that it cannot contain the same element more than once, or indeed two or more different elements with the same values. You see, in mathematics there is no difference between a value and an instance of that value, they are the same thing. In programming however it is possible to store the number 7 in more than one spot in the RAM. Grasshopper does not enforce this rule very strongly though, you can use a lot of Set components on lists that have multiple occurrences of the same value. The big difference between Lists and Sets in Grasshopper is that Sets are only defined for simple data types that have trivial equality comparisons. Basically: booleans, integers, numbers, complex numbers, strings, points, vectors, colours and intervals. Lists can contain all kinds of data.
• Strings. Strings are text. There's nothing more to it. I don't know why early programmers chose to call them strings, but I suppose it's a better description of the memory representation of them. Strings are essentially sequences of individual characters.
• Trees. Trees are the way all data is stored in Grasshopper. Even when you only have a single item, it will still be stored in a tree. A tree is a sorted collection of lists, where each list is identified by a path. A specific path can only occur once in a tree, when you merge two trees together, lists with identical paths are appended to each other. Trees are an attempt to losslessly represent not just the data itself, but also the history of that data. Imagine you have 4 curves {A,B,C,D} and you divide each into 3 points {X,Y,Z}. Then, for each of those points you create a new line segment {X',Y',Z'} and then divide each of those line segments again into 5 points each {K,L,M,N,O}. The way data is stored in trees, it should be possible to figure out whether a point M belongs to X' or to Z', and whether that X' or Z' came from A, B, C or D. This is why paths are often quite long after a while, because they encode a lot of history.
• Paths. A Path is nothing more than a list of integers. It's denoted using curly brackets and semi-colons: {A;B;...;Z}. A Path should never be empty {} or have negative integers {0;-1}, but it is certainly possible to create a path like this and it probably won't even crash Grasshopper. Paths are 'grown' by components that (potentially) create more than one output value for a single input value. For example Divide Curve. It creates N points for every single input curve. In cases like this a new integer is appended to the end of the path. 

 

In the next release the Path logic in Grasshopper is somewhat different. I fixed a number of obscure bugs (hopefully without introducing new fresh bugs) and special cased certain operations to somewhat reduce the speed at which paths grow. This may well break files that rely on a specific tree layout, but I hope the temporary sacrifice will be worth the long-term benefits.

 

--

David Rutten

david@mcneel.com

Poprad, Slovakia

• ▶ Reply

Permalink Reply by Jason Wheeler on June 21, 2012 at 1:48am

David... Thanks so much for the quick reply, I'm sure your a busy person! This deff helps out a lot understanding the differences and how they are used.... but to clarify a bit

1) If a List can contain everything that a Set can what is the benefit from using the Set?

2) As Strings are simple text, is this just as simple as it sounds or is/can it be representational?  Ex. A=5 or A=(5,3,9) like in mathematics and programming

Lastly, 3) Can List components be used to control branches or just the specific items in the whole list?  I have a group of data (from the SBox component) that I divided, using the subset length n component from Pieter http://www.grasshopper3d.com/xn/detail/2985220:Comment:444534, into separate branches and I am trying to use the Cull Pattern component to cull the different branches to apply a different geometry to this new group(list?) of data.... I have attached the files and a screenshot.

As always, thank you so much for your help!!!

• ▶ Reply

Permalink Reply by David Rutten on June 21, 2012 at 2:10am

1) If a List can contain everything that a Set can what is the benefit from using the Set?

There is no benefit. Which is why Sets aren't actually a type of storage in Grasshopper. It's just that some components internally treat Lists as Sets. These components look at the value of the items in the Set rather than their position in memory. This is why the [Item Index] component works differently from the [Member Index] component. Both do something useful although you almost always need [Member Index].

2) As Strings are simple text, is this just as simple as it sounds or is/can it be representational?  Ex. A=5 or A=(5,3,9) like in mathematics and programming

I don't quite follow. Strings are text. There is no meaning to them beyond that. However some components/parameter may choose to interpret strings. For example if you plug a string with value "0.5 * Pi" into a number parameter, it will attempt to evaluate that expression as a number and it will adopt the number value 1.5707963267948966192313216916398. If you plug a string that does not evaluate to a number into a number parameter you will get a conversion error.

3) Can List components be used to control branches or just the specific items in the whole list?

Define "List components". Are you talking about all the components inside the Sets→List panel? If so, then no. Components in the List panel do not really give access to the path bit of trees. For that you need components from the Sets→Tree panel.

--

David Rutten

david@mcneel.com

Poprad, Slovakia

• ▶ Reply

Permalink Reply by Jason Wheeler on June 21, 2012 at 3:28am

I guess what I was getting at with

2) is how can a String obtain the value "0.5 * Pi" if it only contains text?

3) Yes that is exactly what I was asking.. Great this is helps!! But now the question would be which components from the Tree panel should I be looking to use to retrieve a cull pattern of branches from the tree?  Ex. I have a list of 17 branches with 8 items in each and I want to only access every other branch. Such as a "true, false" cull pattern.

Thanks again

• ▶ Reply

Permalink Reply by Danny Boyes on June 21, 2012 at 3:47am

Hi Jason,

I demonstrated a cull Nth Nth approach in this discussion http://www.grasshopper3d.com/forum/topics/2nd-item-from-every-2nd-r...

which sounds similar to your needs

• ▶ Reply

Permalink Reply by David Rutten on June 21, 2012 at 4:12am

A string can contain any collection of characters. "0.5 * Pi" is just one of these possible collections. So is "0;5 * Pi", which is a valid string but cannot be evaluated to a number. "0.5 * iP" is another valid string that does not evaluate to a number.

--

David Rutten

david@mcneel.com

Poprad, Slovakia

• ▶ Reply

Permalink Reply by Jason Wheeler on June 21, 2012 at 4:26am

Danny, thanks... that helped a bunch.

David, I also realized that I was confusing the "cull" components as being in the List pulldown menu, so that is probably where the confusion on my part was... also, thank you for the help with understanding strings... again I had a confusion here thinking "text" meant "alphabet".... been a long day, sorry

Thanks again for the help guys

• ▶ Reply

Permalink Reply by Michael Pryor on June 21, 2012 at 6:23am

can we copy this to the FAQ's

• ▶ Reply

Permalink Reply by Jason Wheeler on June 24, 2012 at 8:02pm

I'm guessing this is directed toward David... I think it would be great to put it in FAQ's as that is where I looked first to find the answer, but I don't know how I would be able to put it there.

• ▶ Reply