Re: exercise 2019/04

[Kacper Gutowski]

On Sat, Apr 18, 2020 at 09:54:13PM +0200, Otto Diesenbacher-Reinmüller wrote:
> 4: Knight Moves
(...)
> *. Given a 2-element vector representing thecurrent square for a 
> knight, return a vector of 2-element vectors representing (in any 
> order) all the squares that the knight can moveto. Hint: The outer 
> product operator ∘. could be useful for generating the coordinates.

>  PM ← (∼a∊⊂⍬)/a←(,b ∘.{((|⍺)≠|⍵)/⍺,⍵} b←(¯2 2 ¯1 1)) ⍝ calculate possible moves 
> from a square

I would shorten it a bit into something more like:
PM← (≠/¨|PM)/ PM←, ∘.,⍨ ¯2 2 ¯1 1

>  z ← (∼{⍵[1]∨⍵[2]}¨{((⍵<1)∨⍵>8)}z)/z       ⍝ remove off-board moves

Selection of those locations that lie within the board could be more 
naturally written as (z∊⍳8 8)/z, but I understand you tried to avoid 
generating the whole board here, unlike in the second attempt.

But more importantly, there is a simple relation between the current 
location and valid moves--a relation elsewhere known as the equation 
of a circle.

-k


My solution would be:
{(5=+/¨(H-⊂⍵)⋆2)/H←,⍳8 8 ;H}