Your solution runs really quick! I'll study it. Thank you

2015-01-27 22:25 GMT-02:00 Lyndon Maydwell <maydwell@gmail.com>:

Ah sorry, I didn't notice that you were doing that. The effectiveness of the trick really only comes into play though if you use an analytic solution for finding the sum of the multiples of 3, etc.

I haven't tested this code in a while, but here's what I wrote some time ago:

sum2 :: Integer -> Integer -> Integer -> Integer

sum2 a b ceiling = aX + bX - abX

where

aX  = sum1 a ceiling

bX  = sum1 b ceiling

abX = sum1 (a * b) ceiling

sum1 :: Integer -> Integer -> Integer

sum1 x ceiling = sum1' (even times) times x

where

times = ceiling `div` x

sum1' :: Bool -> Integer -> Integer -> Integer

sum1' True times x = area

where

area = (times + 1) * (times * x) `div` 2

sum1' False times x = max + area'

where

max   = times * x

area' = sum1' True (times - 1) x

Please excuse the poor Haskell style as it is quite possibly the first Haskell program I ever wrote.

On Wed, Jan 28, 2015 at 11:15 AM, Jean Lopes <hawu.bnu@gmail.com> wrote:

Thats actually what I did...

2015-01-27 22:11 GMT-02:00 Lyndon Maydwell <maydwell@gmail.com>:

I remember that when I had a look at Euler 1 I found that there's a fun solution that should run in "constant" time.

You can find the sum of the multiples of 3, add the multiples of 5, and then subtract the multiples of 3*5.

Is that the kind of thing you're looking for?

 - Lyndon

On Wed, Jan 28, 2015 at 10:57 AM, Jean Lopes <hawu.bnu@gmail.com> wrote:

I'm not really good at math, maybe I am missing something obvious ?

Maybe some pointers as of where to start studying math in order to avoid this brute force attempts, at least to help in this particular problem

2015-01-27 21:38 GMT-02:00 Brandon Allbery <allbery.b@gmail.com>:

On Tue, Jan 27, 2015 at 6:29 PM, Jean Lopes <hawu.bnu@gmail.com> wrote:

The problem to be solved: https://www.hackerrank.com/contests/projecteuler/challenges/euler001

It's worth remembering that the Euler problems are all about math understanding; often they are designed such that brute force solutions will time out or otherwise fail.

--

brandon s allbery kf8nh                               sine nomine associates

allbery.b@gmail.com                                  ballbery@sinenomine.net

 

unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

 

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