Question

你可以帮我解决这个问题。我想提取导师，这是导演的最后一个元素每个元组并使用它来对整个列表进行排序。

import Data.List

type CourseData = [(String,String,String,String,String)]
l :: CourseData

--list contains name of student, year, programme and personal tutor
l = [("fondi","201202378","2012","Bsc280"," tautology"),
     ("fondi","201202378","2012","Bsc280"," tautology"),
     ("Sylvee","200801245","2008","Bsc209","puma"),
     ("dijeje","201307845","2013","Bsc205","tautology"),
     ("heron","201002567","2010","Bsc280","setlhako"),
     ("slow","201198746","2011","Bsc205"," mampu"),
     ("Sylvee","201198746","2008","bsc209"," puma"),
     ("Sylvee","201198746","2008","bsc209"," puma")]




sortByTutor :: CourseData ->String -> [String]
sortByTutor list  =sort[tutor|(name,id,year,prog,tutor)<-list ]

当我使用上面的方法时，它只返回排序的导师列表。我可以改变什么，以便它返回根据导师姓名排序的整个列表？

Answer 1

您可以尝试sortBy中的Data.List：

sortBy (\(_,_,_,_,t1) (_,_,_,_,t) -> compare t1 t) l

Answer 2

Ankur说，使用sortBy。这是一种略有不同的写作方式：

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

sortByTutor xs = sortBy (compare `on` \(_,_,_,_,t) -> t) xs

如果您具有从元组中提取字段的访问器函数，则此选项非常有用，例如：导师：

tutor (_,_,_,_,t) = t

然后你可以简单地重复使用它们：

sortByTutor xs = sortBy (compare `on` tutor) xs

非常易读。

从列表中提取元素并使用它来对整个列表进行排序

2 个答案: