过滤一个haskell列表 [英] Filter a haskell list

问题描述

这是我的代码：

  data Review = Review {artiest :: String，
 score： ：整数，
 tour :: Tour，
 datum :: String，
 plaats :: String，
 soortLocatie :: Locatie，
 topSongs :: [String] 
}派生（Eq，Ord，Show）
 
 getBestLoc [] beste = beste 
 getBestLoc（x：xs）beste 
 | （得分×）> beste = getBestLoc xs（得分x）
 |否则= getBestLoc xs beste 
  

我试图做的是让评论成为最好的分数，但我想要返回Locatie。现在我得到最好的成绩。我怎样才能解决这个问题？

编辑

我试过了

  tester :: [Review]  - > Locatie 
 tester = loc 
其中mxscr = maximumBy（比较分数）
 loc = map soortLocatie mxscr 
  

解决方案

  import Data.List（maximumBy）
 import Data.Function（on）
 
 getBestLoc :: [评论]  - >查看
 getBestLoc = maximumBy（比较`on`分数）
  

该函数将返回查看最高分数。在那之后，获得最终评论的任何领域都是微不足道的;你想要的功能是 soortLocatie。 getBestLoc 。

简要说明正在发生的事情：根据文档< on 如下属性：

  g`on` f = \xy  - > fx`g` fy 
  

so

  compare'on` score == \xy  - >分数x`比较'分数y 
  

换句话说，它比较两个分数，返回 LT，GT，EQ 。然后， maximumBy 取一个比较函数和一个列表，并根据比较函数返回最大值。你可以把它看作 maximum == maximumBy比较。

This is the code i have:

    data Review = Review {  artiest :: String,
                    score :: Integer,
                    tour :: Tour,
                    datum :: String,
                    plaats :: String,
                    soortLocatie :: Locatie,
                    topSongs :: [String]
                   } deriving (Eq, Ord, Show)

    getBestLoc [] beste = beste
    getBestLoc (x:xs) beste
        | (score x) > beste = getBestLoc xs (score x)
        | otherwise = getBestLoc xs beste

What I'm trying to do is to get the review whit the best score, but I want the Locatie to be returned. Now I get the best score returned. How can i solve this?

EDIT

So this is the new function I tried

    tester :: [Review] -> Locatie
    tester = loc
        where mxscr = maximumBy (compare `on` score)
      loc = map soortLocatie mxscr

解决方案

import Data.List (maximumBy)
import Data.Function (on)

getBestLoc :: [Review] -> Review
getBestLoc = maximumBy (compare `on` score)

This function will return the Review with the highest score. After that, getting any field of the resulting review is trivial; your desired function would be soortLocatie . getBestLoc.

A brief explanation of what is going on: according to the docs, on follows the property:

g `on` f = \x y -> f x `g` f y

so

compare `on` score == \x y -> score x `compare` score y

In other words, it compares the two scores, return one of LT, GT, EQ. Then, maximumBy takes a comparison function and a list, and returns the maximum according to the comparison function. You can think of it as maximum == maximumBy compare.

这篇关于过滤一个haskell列表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！