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@Ven
VALUE ERROR

⎕←(⍳10)(↑⍨)¨1 2 3

Duh.

@Ven
LENGTH ERROR

⎕←3↑⍳10

@Ven
1 2 3

Soo... I don't think I even understand how each works :-(

@Ven ¨ requires that ⍴ of both sides is equal (or one is ⍬ for scalar)

Ok, but:

⎕←(⍳7)(↑⍨)¨⍳7

@Ven
┌─┬───┬─────┬───────┬─────────┬───────────┬─────────────┐
│1│2 0│3 0 0│4 0 0 0│5 0 0 0 0│6 0 0 0 0 0│7 0 0 0 0 0 0│
└─┴───┴─────┴───────┴─────────┴───────────┴─────────────┘

@Ven what are you expecting/what do you want it to do?

⎕←(1↑v)(2↑v)(3↑v)(4↑v)(5↑v)(6↑v)(7↑v←⍳7)

@Ven
┌─┬───┬─────┬───────┬─────────┬───────────┬─────────────┐
│1│1 2│1 2 3│1 2 3 4│1 2 3 4 5│1 2 3 4 5 6│1 2 3 4 5 6 7│
└─┴───┴─────┴───────┴─────────┴───────────┴─────────────┘

@Ven ah, you want ⍵ of ↑ to stay the same, aka ¨ only the left side. (⍳7)↑¨⊂⍳7 does that

obligatory dzaima/apl builtin

@dzaima Ah, of course I didn't try that without the ⍨...

⎕←{1=⍴⍵:⊂⍵⋄(¯1↓1-⍨⍳⍴⍵)↓¨⊂⍵}¨{(⍳⍴⍵)↑¨⊂⍵}⍳7

@Ven
┌───┬─────┬───────────┬───────────────────┬─────────────────────────────┬─────────────────────────────────────────┬───────────────────────────────────────────────────────┐
│┌─┐│┌───┐│┌─────┬───┐│┌───────┬─────┬───┐│┌─────────┬───────┬─────┬───┐│┌───────────┬─────────┬───────┬─────┬───┐│┌─────────────┬───────────┬─────────┬───────┬─────┬───┐│
││1│││1 2│││1 2 3│2 3│││1 2 3 4│2 3 4│3 4│││1 2 3 4 5│2 3 4 5│3 4 5│4 5│││1 2 3 4 5 6│2 3 4 5 6│3 4 5 6│4 5 6│5 6│││1 2 3 4 5 6 7│2 3 4 5 6 7│3 4 5 6 7│4 5 6 7│5 6 7│6 7││

It's probably the most convoluted way to go about this, but hey, it works

Thanks @dzaima. I like that built-in :D

@Ven assuming that's to get all subarrays, why are you excluding all single item arrays except of 1 (or alternatively, including 1?)

@dzaima ah, that's on purpose

@Ven shorter by using ,\ for prefixes

@dzaima Turns out that was a bug D:

⎕←{1≡≢⍵:⊂,⍵⋄⌽⌽¨1↓,\⌽⍵}¨,\⌽⍳7

@Ven
┌───┬─────┬───────────┬───────────────────┬─────────────────────────────┬─────────────────────────────────────────┬───────────────────────────────────────────────────────┐
│┌─┐│┌───┐│┌─────┬───┐│┌───────┬─────┬───┐│┌─────────┬───────┬─────┬───┐│┌───────────┬─────────┬───────┬─────┬───┐│┌─────────────┬───────────┬─────────┬───────┬─────┬───┐│
││7│││7 6│││7 6 5│6 5│││7 6 5 4│6 5 4│5 4│││7 6 5 4 3│6 5 4 3│5 4 3│4 3│││7 6 5 4 3 2│6 5 4 3 2│5 4 3 2│4 3 2│3 2│││7 6 5 4 3 2 1│6 5 4 3 2 1│5 4 3 2 1│4 3 2 1│3 2 1│2 1││

Edit: oops.

if anyone is looking for a good challenge: the Hadamard transform is a lot of fun in apl

i'm gonna bounty it, it's so awesome and so golfable!

i'll reward the shortest valid dyalog apl (not: extended) answer to the Hadamard transform challenge published within 3 days from now with +50 rep for every byte below 35 (max +500 rep)

bytes-to-rep table. coincidentally, "known to be possible" is the same for this challenge - 27 (as of now)