[Haskell-cafe] xkcd #287 "NP-Complete"

Hugh Perkins hughperkins at gmail.com
Tue Jul 10 14:54:43 EDT 2007

```This is a compact solution, but it produces multiple permutations of the
same solution, which increases runtime.  I let it run for 10 seconds, then
ctrl-c'd.

Here's a solution that produces all 2 (or three, if you include Barbecue
Sandwich) solutions instantly:

Output:
=====

*Xkcd287> go
Menu 1
******
Mixed Fruit (\$2.15) x 7
Total: 15.05

Menu 2
******
Hot Wings (\$3.55) x 2
Mixed Fruit (\$2.15) x 1
Sample Plate (\$5.8) x 1
Total: 15.05

Menu 3
******
Barbecue Sandwich (\$6.55) x 1
Mixed Fruit (\$2.15) x 2
Mozzarella Sticks (\$4.2) x 1
Total: 15.05

*Xkcd287>

Sourcecode:
=========

module Xkcd287
where

import Char
import IO
import GHC.Float
import List
import qualified Data.Map as Map
import Control.Monad
import Control.Monad.State
import Control.Monad.Reader
import Control.Monad.Writer

menu :: [(String,Int)]
menu = [("Mixed Fruit", 215),
("French Fries", 275),
("Side Salad", 335),
("Hot Wings", 355),
("Mozzarella Sticks", 420),
("Sample Plate", 580),
("Barbecue Sandwich", 655) ]

cost:: Int
cost = 1505

solutions :: [(String,Int)] -> Int -> [[(String,Int)]]
solutions menu targetcost = [ solution | solution <- solutions' menu []
targetcost ]

solutions' :: [(String,Int)] -> [(String,Int)] -> Int -> [[(String,Int)]]
solutions' menu itemssofar targetcost | targetcost == 0 = [itemssofar]
| otherwise = [ solution | item <-
menu,
(null
itemssofar) || ((snd item) <= snd(head itemssofar)),
(snd item)
<= targetcost,
solution <-
solutions' menu (item:itemssofar) (targetcost - (snd item) ) ]

synthesize :: [[(String,Int)]] -> [[(String,Int,Int)]]
synthesize solutions = [ synthesize' solution | solution <- solutions ]

synthesize' :: [(String,Int)] -> [(String,Int,Int)]
synthesize' solution = [ (name,value,count) | (name,(value,count)) <-
synthesize'' ]
where synthesize'' :: [(String,(Int,Int))]
synthesize'' = Map.toList \$ foldr (\(name,value) thismap ->
(process name value (Map.lookup name thismap) thismap) ) Map.empty solution
process :: String -> Int -> Maybe (Int,Int) -> Map.Map String
(Int,Int) -> Map.Map String (Int,Int)
process name value Nothing thismap = Map.insert name (value,1 )
thismap
process name value (Just(value',count)) thismap =
Map.adjust(\(oldvalue,oldcount) -> (oldvalue,oldcount + 1)) name
thismap

createbilling :: [[(String,Int,Int)]] -> [String]
createbilling solutions = [ line | (solution,i) <- (zip solutions [1..]),
line <- ["Menu " ++ show(i), "******"] ++
createbilling' solution ++
["Total: " ++ show( (int2Double \$
foldr (\(name,value,count) total -> (total + (value * count)) ) 0 solution )
/ 100) ] ++
[""]
]

createbilling' :: [(String,Int,Int)] -> [String]
createbilling' solution = [ name ++ " (\$" ++ show((int2Double value) / 100.0)
++ ") x " ++ show(count) | (name,value,count) <- solution ]

go' :: [[(String,Int,Int)]]
go' = synthesize \$ solutions menu cost

go :: IO ()
go = mapM_ putStrLn (createbilling \$ go' )

On 7/10/07, Henning Thielemann <lemming at henning-thielemann.de> wrote:
>
>
> On Tue, 10 Jul 2007, Donald Bruce Stewart wrote:
>
> > These smaller NP problems really love the list monad. here's roconnor's
> > solution from #haskell:
> >
> >     import Control.Monad
> >
> >     menu = [("Mixed Fruit",215),("French Fries",275)
> >            ,("Side Salad",335),("Hot Wings",355)
> >            ,("Mozzarella Sticks",420),("Sampler Plate",580)]
> >
> >     main = mapM_ print
> >             [ map fst y
> >             | i <- [0..]
> >             , y <- replicateM i menu
> >             , sum (map snd y) == 1505 ]
>
> Shouldn't we stay away from integer indices on lists?
>
> [ map fst y |
>     y <- concat (iterate (liftM2 (:) menu) [[]]),
>     sum (map snd y) == 1505]
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
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