Forum Discussion
HRECURSE instead of MAKEARRAY, recursing LAMBDA
I just realized that my LAMBDA(a;i;FILTER(a;SEQUENCE(ROWS(a))=i)) reinvents CHOOSEROWS(a;i). Using that new Excel function, the final HRECURSE reads
=LAMBDA(i;n;Prev;fn;LET(This;fn(i;LAMBDA(r;CHOOSEROWS(Prev;r)));IF(i<n;HSTACK(This;HRECURSE(i+1;n;This;fn));This)))
ecovonrein Great code! I've used it in VRECURSE form to stack results of an iterative function that wouldn't spill otherwise.
Is there a way to update it to refer back to a specific row/column index of the previous step? I need more than just single row/column operations. I'd like to be able to specify INDEX(p, r, c) in the calculation.
Thanks! However, since I wrote this great piece of code, I learned that there is one function in the Excel toolkit that solves the problem without the need for recursion. Now, brace yourself. The great jesters in Seattle have dubbed this function, which allows us to augment things, REDUCE.
So, if you write:
=REDUCE(0;SEQUENCE(10);LAMBDA(prev;this;VSTACK(prev;HSTACK(this;INDEX(prev;this-1;1)))))
then you get what you want. You are successively augmenting "prev" by forever VSTACKing it (I generate 2 columns with this formula), and you have access to all of "prev", which is what you asked for. That is, you don't need to use V/HRECURSE 😉PS: Credit for the discovery that REDUCE can be abused in this way goes to lori_m.
ecovonrein Thanks. I'm struggling a bit with the REDUCE code on a few fronts: 1) where my function is applied; and 2) where the original array comes into play.
Is there a post that discusses this approach in more detail (search was unhelpful)?
