Haskell - 从一维列表中创建一个2D列表

Question

我必须制作一个2D列表[[Int]].[Int]在Haskell中的一维列表中.

该函数应采用args"r"Int表示行数和1D列表,应将其切成长度为"r"的行.

如果列表的长度超过r*r,则应删除列表的其余部分.Howerver如果列表的长度小于r*r,那么缺失的元素应该作为0插入列表中.

例1:

输入:r = 2

list = [1,2,3,4,5,6]

输出:[[1,2],[3,4]]

例2:

输入:r = 3

list = [1,2,3,4,5,6]

输出:[[1,2,3],[4,5,6],[0,0,0]]

所以我的方法是关于thre函数如下:

zeroList :: Int -> [Int] -> [Int]
zeroList r myList = (take (r*r-(length myList)) (0 : zeroList r myList))

processList  ::  Int  ->  [Int]  ->  [[Int]]
processList  r myList = (if (length myList < r*r)
                        then (myList:(zeroList r myList))
                        else if (length (myList > r*r))
                        then (reverse (drop r (reverse myList)))
                        else 
                                myList)

make2DList  ::  Int  ->  [Int]  ->  [[Int]]                     


make2DList r myList = (if myList == [] 
                        then make2DList
                    else ( ( take r (processList r myList) ):( make2DList r ( drop r (processList r myList) ) )))

zeroList函数正常工作,但其他两个函数不起作用.我编写了一些错误消息:

D:\haskell\task1.hs:6:63:
    Couldn't match expected type `[Int]' with actual type `Int'
    Expected type: [[Int]]
      Actual type: [Int]
    In the return type of a call of `zeroList'
    In the second argument of `(:)', namely `(zeroList r myList)'

D:\haskell\task1.hs:14:54:
    Couldn't match expected type `[[Int]]'
                with actual type `Int -> [Int] -> [[Int]]'
    In the expression: make2DList
    In the expression:
      (if myList == [] then
           make2DList
       else
           ((take r myList) : (make2DList r (drop r myList))))
    In an equation for `make2DList':
        make2DList r myList
          = (if myList == [] then
                 make2DList
             else
                 ((take r myList) : (make2DList r (drop r myList))))
Failed, modules loaded: none.
Prelude>

我无法理解,为什么它不起作用zeroList r myList.返回一个普通的列表.

有人可以帮帮我吗？

Answer 1

我不得不承认我不理解你是怎么做的.所有这些if then else都非常糟糕.:-)另外,由于Haskell的惰性评估和无限列表,没有必要事先计算所需的零的确切数量.

草稿如何可以做到:

make2DList r l = take r . chunks r $ l ++ zeroes
  where
    zeroes = [0,0..]
    chunks r xs = take r xs : chunks r (drop r xs)

说明:

1. 通过无限数量的零扩展列表,因此我们不必再担心填充.
2. 创建一个chunks将任何列表拆分为给定长度的块的函数.
3. 应用于chunks填充列表.
4. 根据需要选择尽可能多的行.