add rank-sensitivity

Author: kidd

2023-12-xx-rank-sensitivity.md

@@ -0,0 +1,71 @@
+---
+title: Rank Sensitivity in K
+categories: [apljk]
+---
+
+I wanted to try Advent of Code 23, and of course, it all went down an
+interesting rabbit hole even before starting.
+
+One way I found to get data into a program (like __DATA__ in perl) was a snippet in discord:
+
+```
+/
+2,2,2
+1,2,2
+3,2,2
+2,1,2
+\
+
+data:{x:0:`argv 1; 1_(x?,"\\")#x}[]
+data
+```
+
+and when trying to figure out why tf it worked....
+
+
+# Rank sensitivity
+
+The `(x?,"\\")#x` part will do a find (`?`) on the `argv 1`, which in
+[ngn/k site](https://ngn.codeberg.page/k) gets all the lines of the
+program.
+
+I thought `find` was right-atomic, and in oK it is, but in ngnk it is not.
+
+```
+rgrau: I'm trying to understand find ? in ngnk. in some cases it feels right atomic (like oK does),
+but some other cases it takes the full right argument to do the search, or some cases it seems it only goes down "1 level".
+
+    "one"?"two" / 0N 0N 0 . right atomic?
+    ("one";"two")?"two" / 1 . takes full right arg. ok gives 0N 0N 0N
+    ("one";"two")?("two"; "one"; "three") / 1 0 0N . 1 level down?
+
+It's very DWIM but it's indistinguishable from magic.  Any trick to fit this in my brain?
+
+chrispsn: This is ‘rank sensitivity’
+chrispsn: See https://matrix.to/#/!laJBzNwLcAOMAbAEeQ%3Amatrix.org/%242AMsFKfIBmSk2-opKjzgZj6tXLQdyEfQpRZoSCUhTvM
+chrispsn: The difference in rank between x and y determines what find outputs
+rgrau: ...magic... 🙂 . thanks!
+```
+
+And that lead to:
+
+https://code.kx.com/q/ref/find/#type-specific and
+https://news.ycombinator.com/item?id=27850272, which seem to indicate
+that `find` will drill down `y` until it looks like that's what you
+want to search for. Extremely DWIM.
+
+
+Back to the initial code, so, `(x?,"\\")#x` will do a reshape of x
+cutting it at the position where Find finds "\\". At first I didn't
+get why the Enlist of "\\" was needed, but if we look at `argv 1` we
+see that single character lines are `,"a"` rather than just `"a"`,
+because a single char is not really a list (or something).
+
+Anyway, now that it's clear how we read that csv, we could convert it
+to a real 3x3 array with regular k, with
+
+```
+`I$''","\'data
+/ or
+.''","\'data
+```