Haskell：删除最大的列表 [英] Haskell: Remove largest list

问题描述

  extract :: [高原]  - >高原
 
 extract（x：xs）
 | x == maximumBy（比较`on`长度）（x：xs）= x 
 |否则= （xs）
 extract [] = [] 
  

现在我需要一个函数采取相同的 [Plateau] 并返回一个新的 [Plateau] >

  prune :: [Plateau]  - > [Plateau] 
 prune（x：xs）
 | x  | x> = maximumBy（比较`on`长度）（x：xs）= [] 
 prune [] = [] 
  

我也用 sortBy 以确保最大的列表是最后一个：

 （extract。prune）（sortBy（compare`on`长度）（平台））
  

我的列表

  plateaus = [01000]，[01010，11010 ，10010] [00101]，[01101，01001]] 
  

  [[01000]，[00101]，[01101，01001 ]，[01010，11010，10010]] 
  

prune正在返回一个



  [[01000]，[00101]] 
  

告诉我，由于某种原因，Haskell认为



  [01101，01001]> = [01010，11010，10010] 
  

当我清楚地看到2> = 3是不正确的。

这是为什么？

解决方案

列表比较是词典编纂，而不是长度。所以你得到了你看到的结果，因为01101> =01010，这又是出于同样的原因 - 两个字符串的前两个字符是相等的，第一个字符的第三个字符大于第三个字符第二个。

I have a list of lists, and a function that returns the list with the most items:

extract ::[Plateau]->Plateau

extract(x:xs) 
  |x==maximumBy(compare `on` length)(x:xs)=x 
  |otherwise = extract(xs)
extract []=[""]

Now I need a function to take the same [Plateau] and return a new [Plateau] that has the previous largest removed:

prune::[Plateau]->[Plateau]
prune(x:xs)
  |x < (maximumBy(compare `on` length)(x:xs)=x : prune (xs)
  |x>=maximumBy(compare `on` length)(x:xs)=[]
prune [] = [] 

I also call prune with a sortBy as to make sure the largest list is last:

(extract . prune) (sortBy(compare `on` length)(plateaus))

This starts out to work correctly. my List plateaus looks like:

plateaus = ["01000"], ["01010", "11010", "10010"] ["00101"], ["01101", "01001"]]

here it is sorted:

 [["01000"], ["00101"], ["01101", "01001"], ["01010", "11010", "10010"]]

Now, my function prune is returning a list of

[["01000"], ["00101"]]

that tells me, for some reason Haskell thinks

["01101", "01001"] >= ["01010", "11010", "10010"]

When I can clearly see that 2 >= 3 is not true.

Why is this?

解决方案

List comparison is lexicographic, not by length. So you get the result you see because "01101" >= "01010", which in turn is for the same reason - the first two characters of the two strings are equal and the third character of the first is greater than the third character of the second.

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