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

28 Aug 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  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