Programming performance/Magnus Haskell
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- Language: Haskell
- Skill: Intermediate. I'm self taught in Haskell and use it as a hobbyist but haven't used it much for serious work.
- Time: 25 minutes.
- Notes: 10 minutes spent on a silly bug where I had typed 1 instead of 100. This code is not very pretty but it works. A while after submitting I noticed I used the wrong data column. It should say column 4, not 3 in closingPrice.
Code
import Data.List (partition,foldl')
closingPrice::String->Double
closingPrice line =
read (words line !! 3)
sell assets price = sum (map (\(_,shares)->shares*price) assets)
tradeOnce (cash,assets,lastPrice) todayPrice =
if todayPrice <= lastPrice * 97/100 then
let buyCost = cash*10/100 in
(cash-buyCost, (lastPrice,buyCost/lastPrice):assets, todayPrice)
else let (toSell,toKeep) =
partition (\(buyPrice,_)->todayPrice >= buyPrice*106/100) assets
in (cash + sell toSell todayPrice,
toKeep,
todayPrice)
notComment ('#':rest) = False
notComment _ = True
trade input =
let (firstPrice:restPrices) = reverse $
map closingPrice $
filter notComment $
lines input
in foldl' tradeOnce (10000.0,[],firstPrice) restPrices
main = do input <- readFile "gspc.txt"
let (cash,toSell,lastPrice) = trade input
let result = cash + sell toSell lastPrice
print result