Question

我正在努力实现无限背包问题解决方案。我想使用递归和memoization而不是显式状态。

但问题是我找不到在有效的Haskell中表达思想的方法。这两个版本都给了我

    parse error on input ‘=’
    Perhaps you need a 'let' in a 'do' block?
    e.g. 'let x = 5' instead of 'x = 5'



bestUnbounded :: [Integer] -> Integer -> ([Integer], Integer)
bestUnbounded items capacity = if capacity <= 0
                                  then ([], 0)
                                  else maximumBy (comparing snd) solutions
                                      where solutions = filter (\ s -> snd s <= capacity) $ map solve items
                                            solve i   = (solution, weight + i)
                                                where solution, weight = bestUnbounded items (capacity - i)




bestUnbounded :: [Integer] -> Integer -> ([Integer], Integer)
bestUnbounded items capacity
  | capacity <= 0 = ([], 0)
  | otherwise     = maximumBy (comparing snd) solutions
      where solutions = filter (\ s -> snd s <= capacity)  map solve items
            solve i   = let solution, weight = bestUnbounded items (capacity - i)
                        in (solution, weight + i)

如果有人也可以展示如何使用Data.Func.Memoize，我也会感激不尽。

EDIT1

工作解决方案

import Data.List (maximumBy)
import Data.Ord (comparing)
import Data.Function.Memoize (memoize)

bestUnbounded :: [Integer] -> Integer -> ([Integer], Integer)
bestUnbounded items capacity
  | capacity <= 0 = ([], 0)
  | otherwise     = solver' capacity
  where solver' = memoize solver
        solver capacity = maximumBy (comparing snd) solutions
        solutions = filter (\ s -> snd s <= capacity) $ map solve items
        solve i   = let (solution, weight) = bestUnbounded items (capacity - i)
                    in (i : solution, weight + i)

Answer 1

这在语法上无效：

solution, weight = bestUnbounded items (capacity - i)

我认为您打算在(,)上进行模式匹配，如下所示：

(solution, weight) = bestUnbounded items (capacity - i)

对于构造和模式匹配，元组语法总是需要括号。

如何在函数内部使用模式匹配？

1 个答案: