I am attempting to implement conways game in haskell for class and have code that looks like this for checking for neighbors of a cell (x,y) in a square board of size (w,h):
Neighbors :: ((Int,Int), [[Bool]]) -> (Int,Int) -> Int
Neighbors arr@((w,h), cells) (x,y) =
let
j = w -1
k = h - 1
checkcell (x,y) = if (cells !! y) !! x then 1 else 0 in
case (x,y) of
-- Check right, bottom, and bottom right
(0,0) -> checkcell (x,y 1) checkcell (x 1,y) checkcell (x 1,y 1)
-- Check bottom, left, right, bottom left, bottom right
(_,0) -> checkcell (x,y 1) checkcell (x-1,y) checkcell (x 1,y) checkcell (x 1,y 1) checkcell (x-1,y 1)
--check bottom, top, right, top right, bottom right
(0,_) -> checkcell (x,y 1) checkcell (x,y-1) checkcell (x 1,y) checkcell (x 1,y 1) checkcell (x 1,y-1)
-- check top, left, right,top left, top right
(_, k) -> checkcell (x,y-1) checkcell (x-1,y) checkcell (x 1,y) checkcell (x-1,y-1) checkcell (x 1,y-1)
-- check bottom, top, left, top left, bottom left
(j, _) -> checkcell (x,y 1) checkcell (x,y-1) checkcell (x-1,y) checkcell (x-1,y 1) checkcell (x-1,y-1)
--check top, left, top left
(j,k) -> checkcell (x,y-1) checkcell (x-1,y) checkcell (x-1,y-1)
--check bottom, left, bottom left
(j,0) -> checkcell (x,y 1) checkcell (x-1,y) checkcell (x-1,y 1)
-- check top, top right, right
(0,k) -> checkcell (x,y-1) checkcell (x 1,y-1) checkcell (x 1,y)
--check all surrounding
(_,_) -> checkcell (x,y 1) checkcell (x,y-1) checkcell (x 1,y) checkcell (x-1,y) checkcell (x 1,y 1) checkcell (x 1,y-1) checkcell (x-1,y 1) checkcell (x-1,y-1)
I are using j and k to fit for the bounds of the array. I am checking for all edge cases, and corners however the code is failing for array sizes and rows at (_, max_row), (max_column, _), (max_column, max_row) and giving incorrect values. This is the output that ghci is giving me:
> world = ((3,3), [[False, False, False], [False, False, False], [False, True,True]])
> liveNeighbors world (2,2)
*** Exception: Prelude.!!: index too large
> liveNeighbors world (0,2)
*** Exception: Prelude.!!: index too large
> liveNeighbors world (0,1)
1
> liveNeighbors world (1,1)
0
The value for (1,1) should give 2 since the bottom and bottom right are true, however it is only giving a value of one. Any idea why this is happening? I'm not looking for a code solution, if someone could point me in the right direction conceptually on what is going wrong that would be greatly appreciated. This is for a project confining to list processing so no other data structures other than lists or tuples are allowed.
CodePudding user response:
There are 2 problems in this code:
order of patterns: in your example you get error message when test your code for
(2,2)
position. It is because pattern(_, k)
goes earlier than(j, k)
, so you check "top, left, right,top left, top right", not "top, left, top left" as you expectedvariables in patterns: pattern
(_, k)
match successfully with other values of(x, y)
in your code and patterns below it are overlapped. You just assignk
toy
, not compare them
So, you can write code like this (I use MultiWayIf
extension for more similarity with original, but it is just syntax sugar):
{-# LANGUAGE MultiWayIf #-}
neighbors :: ((Int,Int), [[Bool]]) -> (Int,Int) -> Int
neighbors arr@((w,h), cells) (x,y) =
let
j = w -1
k = h - 1
checkcell (x,y) = if (cells !! y) !! x then 1 else 0 in
if
-- Check right, bottom, and bottom right
| (x, y) == (0,0) -> checkcell (x,y 1) checkcell (x 1,y) checkcell (x 1,y 1)
--check top, left, top left
| (x, y) == (j,k) -> checkcell (x,y-1) checkcell (x-1,y) checkcell (x-1,y-1)
--check bottom, left, bottom left
| (x, y) == (j,0) -> checkcell (x,y 1) checkcell (x-1,y) checkcell (x-1,y 1)
-- check top, top right, right
| (x, y) == (0,k) -> checkcell (x,y-1) checkcell (x 1,y-1) checkcell (x 1,y)
-- Check bottom, left, right, bottom left, bottom right
| y == 0 -> checkcell (x,y 1) checkcell (x-1,y) checkcell (x 1,y) checkcell (x 1,y 1) checkcell (x-1,y 1)
--check bottom, top, right, top right, bottom right
| x == 0 -> checkcell (x,y 1) checkcell (x,y-1) checkcell (x 1,y) checkcell (x 1,y 1) checkcell (x 1,y-1)
-- check top, left, right,top left, top right
| y == k -> checkcell (x,y-1) checkcell (x-1,y) checkcell (x 1,y) checkcell (x-1,y-1) checkcell (x 1,y-1)
-- check bottom, top, left, top left, bottom left
| x == j -> checkcell (x,y 1) checkcell (x,y-1) checkcell (x-1,y) checkcell (x-1,y 1) checkcell (x-1,y-1)
--check all surrounding
| otherwise -> checkcell (x,y 1) checkcell (x,y-1) checkcell (x 1,y) checkcell (x-1,y) checkcell (x 1,y 1) checkcell (x 1,y-1) checkcell (x-1,y 1) checkcell (x-1,y-1)