[Haskell-cafe] Project Euler: request for comments
KC
kc1956 at gmail.com
Sun Aug 28 22:58:57 CEST 2011
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 <oscarpicasso at gmail.com> 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 <oscarpicasso at gmail.com> 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 <kc1956 at gmail.com> 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
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