[Haskell-beginners] Mutable grid

[mike h]

Hi,
In the end I used a set to hold tuples of int pairs (row, col) and manipulated them


type By = Int 
type Row = Int 
type Col = Int 

type Pixels = Set (Row, Col)

data Screen = Screen

  { maxX   :: Col,
    maxY   :: Row,
    pixels :: Pixels
  }

Thanks

Mike

> On 24 Dec 2016, at 14:37, Magnus Therning <magnus at therning.org> wrote:
> 
> How did it go?
> 
> When I solved that AoC problem I ended up using the matrix package: http://hackage.haskell.org/package/matrix <http://hackage.haskell.org/package/matrix>
> 
> /M
> 
> 
> On 19 Dec 2016 7:31 pm, "mike h" <mike_k_houghton at yahoo.co.uk <mailto:mike_k_houghton at yahoo.co.uk>> wrote:
> Thanks for the pointers - I’ll take a look.
> 
> The background to this is one of the puzzles on Advent Of Code 2016 Q.8.
> https://adventofcode.com/2016/day/8 <https://adventofcode.com/2016/day/8>
> 
> There are (several hundred) sequential  operations on a grid 50 x 6  - initially all zeroes
> e.g. 
> rotate row y=0 by 4 
> rect 2x1 — sets sub grid from (0,0) to (2,1) to all 1s
> rotate column x=35 by 1
> 
> I’m fine about parsing the input to a data structure and executing them i.e. 
> 
> evalExpr :: Expr -> Screen -> Screen     — screen is essentially [[Int]]
> evalExpr e s =
>     case e of
>         (Rect   r c ) ->     evalRect   r c s
>         (RotRow r by) -> evalRotRow r by s
>         (RotCol c by) -> evalRotCol c by s
>         (NOP        ) -> id s
> 
> rotating a row was simple enough, code to  rotate column a bit untidy and not very nice. The 
> evalRect  - which sets values to one in the rectangle of size r x c starting at (0,0) top left - triggered the original question.
> 
> 
> At this point  my knowledge of Haskell is being pushed (which is good) but I have a feeling that 
> my approach is not ‘correct’ once it gets beyond the parsing. Should each of the evalRect, evalRotRow and evalRotCol be called with a Screen (i.e. the grid at the root of this question)?
> Is the state monad a fit for this problem?
> Should I change my approach or is using vector the way forward?
> 
> Many thanks
> 
> Mike
> 
> 
> 
> 
> 
>> On 19 Dec 2016, at 15:27, Michael Orlitzky <michael at orlitzky.com <mailto:michael at orlitzky.com>> wrote:
>> 
>> On 12/19/2016 08:10 AM, mike h wrote:
>>> Hi,
>>> 
>>> I’m looking a problem where I have an NxN grid of ints. I need a
>>> function like setValue x y newVal
>>> 
>>> I have tried using [[Int]] but it does become messy when splitting ,
>>> dropping and then ++ back together.
>>> 
>>> What other options are available to represent a mutable grid?
>>> 
>> 
>> Mutable vectors (from the vector[1] package) are an obvious choice. When
>> I had to do something similar, I wound up going all the way to repa[2],
>> which magically turns all of your grid operations into parallel ones.
>> 
>> 
>> [1] https://hackage.haskell.org/package/vector <https://hackage.haskell.org/package/vector>
>> [2] https://hackage.haskell.org/package/repa <https://hackage.haskell.org/package/repa>
>> 
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