Re: [Haskell-beginners] Mutable grid

How did it go? When I solved that AoC problem I ended up using the matrix package: http://hackage.haskell.org/package/matrix /M On 19 Dec 2016 7:31 pm, "mike h" 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

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 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 [2] https://hackage.haskell.org/package/repa

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