Here is a slider made with data structures (not mine):
https://github.com/megalon/pd-gui-examples

@toxonic here is a tutorial: https://puredata.info/community/conventions/convention04/lectures/tk-barknecht/tut.tgz/view
and in this thread are some links related to data structures: https://forum.pdpatchrepo.info/topic/10871/learning-data-structures

@Seán I also think printing is the reason. Just get the array and set it into a new array then you can see if it is working...

@ingox I think the patch is a bit broken since the newer Pure Data versions, because donecanvasdialog puts the patch into edit mode. I tried to replace donecanvasdialog with the coords message, but obviously I was doing something wrong. But I think theoretically it could be fixed with coords implemented in the right way.

coords [x_from]? [y_to]? [x_to]? [y_from]? [width]? [heigth]? [graph_on_parent]?;

@Il-pleut I do not think it is possibly, but perhaps I am wrong and it is worth a try...
But, you can store pointers easily into a list: https://forum.pdpatchrepo.info/topic/11435/access-pointer-with-list-store

I made some progress, now it is possible to load different formats with https://pdspectrum.handmadeproductions.de/. For images at least: jpg, png, bmp.
For audio at least: wav, mp3. I also found a nice interface library for Java Script: https://nexus-js.github.io/ui/
And I had another Idea: If the interface is completely on the Java Script side it could be simpler and more efficient to just use ofxPd for Emscripten and Pure Data (which already works, but without the Java Script interaction).

I think this is one of the best things made with Gem and with a good documentation, it helps perhaps: https://chdh.net/egregore_source_manual.htm
You could also have a look into open GL and shaders, you can use them in Gem too.

The idea is that you put the variable into l2s

@toxonic I do not think that it is possible, but if you have several arrays in one struct you can change one element of all arrays with a list. Here I experimented a bit with it: https://forum.pdpatchrepo.info/topic/12059/array-text-pointer

@raynovich you could use something like list to symbol: https://forum.pdpatchrepo.info/topic/10556/vanilla-list2symbol-abstraction-pd-0-47/13