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

Very interesting. But for my taste I would do a cosmetic change. I tend to find it more readable when function calls are written vertically when they have with a great number of parameters or a lot of parens like here. listof4tuples xs = zip4 xs (tail xs) (tail (tail xs)) (tail (tail (tail xs))) On Tue, Aug 30, 2011 at 1:41 PM, KC wrote:

You might like this zipping & folding version.

Explicit recursion has the disadvantage that one has to read the entire function in order to figure out what's going on; whereas using the higher order functions makes things much easier to grasp.

listof4tuples xs = (zip4 xs (tail xs) (tail (tail xs)) (tail (tail (tail xs))))

prods xs = map prods4 (listof4tuples xs)

prods4 (t,u,v,w) = t*u*v*w

maxprods4 xs = maximum $ prods xs

On Mon, Aug 29, 2011 at 9:40 AM, Oscar Picasso wrote:

...

Got it.

f :: [Int] -> Int f (t:u:v:xs) = helper t u v xs

helper :: Int -> Int -> Int -> [Int] -> Int helper t u v (w:ws)  | ws == []  = t*u*v*w  | otherwise = max (t*u*v*w) (f (u:v:w:ws))

I tend to overlook mutual recursion in my toolbox.

Thanks for the nnlightenment.

On Sun, Aug 28, 2011 at 4:54 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:

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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:

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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".

On Fri, Aug 26, 2011 at 6:15 PM, Oscar Picasso wrote: > Like: > 20*19*21*18 > is bigger than > 100*100*3*2 > ? > > If so I need to think about how to formalize it. > > Thanks for the hint. > > On Fri, Aug 26, 2011 at 8:55 PM, KC wrote: >> Is Problem 11 the 4 consecutive #'s problem? >> >> If so what must be true for 4 #'s to have a large product? >> >> Hint: x * y * z * 2 is that going to be larger? >> >> -- >> -- >> Regards, >> KC >> >

-- -- Regards, KC

-- -- Regards, KC

-- -- Regards, KC