You can’t currently set only one value of an array with an action, you can only change the whole array.
Could this be changed to the solution included in the graph link above? It’s a lot easier, and may be faster computationally.
Nice thinking with that join formula. It looks like you had some special thinking for when the replace index is the first element — you’ll also want to do something slightly different when the replace index is the last element!
Another pattern that I quite like is L -> {[1...length(L)]=i: v, L}. That piece-wise expression is identical to the list L, except at the place where [1..length(L)] is equal to i, where it uses the value v instead.
Problem with the piecewise function is, it’s a lot slower. When you have to do this with an array with, let’s say 1000 values, it’s about 4x slower than the join one.
Nevermind just tested it, it’s faster lol