Re: [Haskell-cafe] Project Euler: request for comments

I just noticed that the 20x20 grid has some "00" entries; thus, time could be saved by not touching any of the grid entries 3 cells away. Same for the "01" entries. The challenge, of course, is in finding these entries in the first place. :) On Sun, Aug 28, 2011 at 1:58 PM, KC wrote:

Try something like the following:

-- Project Euler 11

-- In the 20×20 grid below, four numbers along a diagonal line have been marked in red.

-- <snip>

-- The product of these numbers is 26 × 63 × 78 × 14 = 1788696.

-- What is the greatest product of four adjacent numbers in any direction (up, down, left, right, or diagonally) in the 20×20 grid?

import Data.List

-- Doing the one dimensional case. f011 :: [Int] -> Int f011 (t:u:v:xs) = f011helper t u v xs

f011helper :: Int -> Int -> Int -> [Int] -> Int f011helper t u v (w:ws)   | ws == []  = t*u*v*w   | otherwise = yada nada mada

-- What are yada nada mada?

-- The 20x20 grid case will become: f0112D :: [[Int]] -> Int -- where [[Int]] is a list of lists of rows, columns, major diagonals, & minor diagonals.

On Sun, Aug 28, 2011 at 5:10 AM, Oscar Picasso wrote:

...

No. The answer I posted is not good. It worked, by chance, on a couple of small examples I tried but it could end up comparing sequence of 4 numbers that where not initially adjacent.

On Sun, Aug 28, 2011 at 12:32 AM, Oscar Picasso wrote:

...

Maybe this?

f x@(a:b:c:d:[]) = x f (a:b:c:d:e:ys)  = if e >= a                   then f (b:c:d:e:ys)                   else f (a:b:c:d:ys)

On Sat, Aug 27, 2011 at 8:26 PM, KC wrote:

...

Think of the simplest version of the problem that isn't totally trivial.

e.g. A one dimensional list of numbers.

What would you do?

Note: you only want to touch each element once.

The 2 dimensional case could be handled by putting into lists: rows, columns, major diagonals, and minor diagonals.

This isn't the fastest way of doing the problem but it has the advantage of avoiding "indexitis".

-- -- Regards, KC

-- -- Regards, KC