# Tracking Selected Objects **URL:** \ **Category:** Examples\ **Created:** [January 3, 2022, 4:51pm UTC](https://cl.desmos.com/t/tracking-selected-objects/3922 "2022-01-03T16:51:24Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Alain\_Mangan](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/alain_mangan/32/3174_2.png) [@Alain\_Mangan](https://cl.desmos.com/u/Alain_Mangan)\ **Post date:** [January 3, 2022, 4:51pm UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/1 "2022-01-03T16:51:24Z")
Hi. Trying to wrap my head around how to make create more involved activities (ex: where students click on objects). Got something working reasonably well now that desmos supports the `→` (assignment?) operator : → [SelectingObjects | Desmos](https://www.desmos.com/calculator/oxu0zx4fjz) ### Interesting programming model Many features of Desmos feels very LISP-y to me. Very interesting. ### Problem I find it very difficult to work with evolving state in Desmos. Although my example works well, it is quite involved to figure out how to do this. I get the impression teachers would create better content if the barrier for saving/manipulating state was lowered somehow. Though I agree that concepts from LISP seem to model problems encountered in math very well, I think most people (including myself) have difficulty thinking in this LISP-like state-less fashion. **edit1:** More accurately: it is difficult to modify state programmatically (There is, of course, state in the graphs variables). **edit2:** Desmos programs seem to be declarative in nature (so maybe Prolog is a better comparison than LISP?). ### More examples? Maybe someone could build up a well catalogued database of examples on how to build up more complex activities? → So I suppose this is my first example — hoping it will make it easier for teachers to create more interesting activities. ---
**Author:** ![Alain\_Mangan](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/alain_mangan/32/3174_2.png) [@Alain\_Mangan](https://cl.desmos.com/u/Alain_Mangan)\ **Post date:** [January 3, 2022, 10:08pm UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/2 "2022-01-03T22:08:41Z")
Just noticed someone else posted an interesting set of examples: > **[Ideas, Tricks, Techniques](https://www.desmos.com/calculator/0bvgiggids)** > > Ideas, Tricks, Techniques ---
**Author:** ![aknauft](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/aknauft/32/3701_2.png) [@aknauft](https://cl.desmos.com/u/aknauft)\ **Post date:** [January 4, 2022, 6:51am UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/3 "2022-01-04T06:51:49Z")
Hi, welcome to the forum! A database of complex examples is very interesting to me – the difficulty there is finding the right way to organize things so that there is a reasonable entry point for everyone. Complex examples can get quite complex! But it’s something I like to think about not infrequently. As for manipulating graph state, actions (-\>) are the only non-hacky way to make this sort of change. I have found lists and list operations to be key for most of my complex builds. I like your example! Here’s my recreation, with a slightly different click update implemented in line 4: [Untitled Graph](https://www.desmos.com/calculator/p8fu7fjsrd) ---
**Author:** ![Alain\_Mangan](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/alain_mangan/32/3174_2.png) [@Alain\_Mangan](https://cl.desmos.com/u/Alain_Mangan)\ **Post date:** [January 5, 2022, 1:04am UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/4 "2022-01-05T01:04:21Z")
Thanks for the example. ### Conditionals for updating state I’ve never thought you could use conditionals this way to toggle elements of “HasBeenClicked”. Kind of nice that you don’t need extra lines to generate a “bit mask” like I’ve had to do. Always looking for good ways to balance number of lines with the overall complexity of the code in order to keep things manageable & readable (even if I’m the only one looking at my code). ### More boolean manipulations I’ve also never seen selection being possible by (multiplying?) a set of points by a vector of booleans (line 4): `(xi,yi)[1=HasBeenClicked]` Actually, I can clearly see it is not a multiplication, because if I do: `(xi,yi)([1=HasBeenClicked]*2)` The calculation fails ### Toggle vs enable One thing to note is that your code toggles, whereas mine simply enables (won’t re-enable). But I can surely adapt this solution to my situation. Thanks. ---
**Author:** ![Alain\_Mangan](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/alain_mangan/32/3174_2.png) [@Alain\_Mangan](https://cl.desmos.com/u/Alain_Mangan)\ **Post date:** [January 5, 2022, 1:24am UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/5 "2022-01-05T01:24:02Z")
Oh yeah… I really like the idea of placing all the points in a table like you did. It seems to better group similar things & de-clutter the calculator code. I just wish there was a way to set common restrictions on the `(x,y)` coordinates (namely step size) so students can deal with integer values to simplify mental calculations, etc. ### Column names I would highly suggest naming the columns to something like x\_i & y\_i instead of the default x\_1 & y\_1, though. x\_1 sort of sounds like the x-coordinate of point 1 - and I find that a bit confusing. ---
**Author:** ![pirsquared](https://avatars.discourse-cdn.com/v4/letter/p/4af34b/32.png) [@pirsquared](https://cl.desmos.com/u/pirsquared)\ **Post date:** [January 5, 2022, 6:04pm UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/6 "2022-01-05T18:04:32Z")
> [@Alain\_Mangan](#): > > I’ve also never seen selection being possible by (multiplying?) a set of points by a vector of booleans (line 4): > `(xi,yi)[1=HasBeenClicked]` Not a multiplication as such (as you discovered) but using the ‘elements of the list that satisfy a condition’ implementation mentioned here: [https://help.desmos.com/hc/en-us/articles/4407889068557-Lists](https://help.desmos.com/hc/en-us/articles/4407889068557-Lists) So `(x_i,y_i)` is a list of points, narrowed to only the elements of that list in the positions where `HasBeenClicked` equals 1. ---
**Author:** ![Alain\_Mangan](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/alain_mangan/32/3174_2.png) [@Alain\_Mangan](https://cl.desmos.com/u/Alain_Mangan)\ **Post date:** [January 6, 2022, 12:39am UTC](https://cl.desmos.com/t/tracking-selected-objects/3922/7 "2022-01-06T00:39:00Z")
Excellent! Thanks for pointing me to the doc – and providing me with a search term (I would probably not have found the relevant snippet otherwise). This is great! I get to use modulo arithmetic with this filtering tool: ` L[mod(L,2)=0]`