Question

目标是编写一个函数，给定1到12之间的整数评估相关月份的名称（1-> 1月等）。否则，它可以评估为＆＃34;不正确的月份数＆＃34 ;. 这是我的代码：

list = ["Jan","Feb","Mar","April", "May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
month x = if x<13 && x>0
          then x = list !! (x-1)
          else "Not a month"

这是输入错误msg：解析错误＆＃34; =＆＃34;

非常感谢你的帮助

Answer 1

只需删除x =即可。（你为什么写这个？）

list = ["Jan","Feb","Mar","April","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
month x = if x<13 && x>0
          then list !! (x-1)
          else "Not a month"

Answer 2

使用警卫是最好的策略：

month x | x > 0 && x < 13      = list !! (x - 1)
        |otherwise             = "Error"

您还可以使用Maybe进行此类操作：

mylist = ["Jan","Feb","Mar","April", "May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec"]

exampleFunc :: [String] -> Int -> Maybe String
exampleFunc li int
   | int > 0 && int < 13    = Just (li !! (int - 1))
   | otherwise              = Nothing

使用Maybe可以在Nothing条件下返回标准值（otherwise）。

Answer 3

你好写道：

month x = if x0
          then x = list !! (x-1)
          else "Not a month"

您可能习惯于命令式世界（C ++，Java，...），其中有变量可以设置并设置为其他值。在函数和范围声明性语言中，变量只设置一次。

此外，无需将x设置为某个值，您只需返回list!!(x-1)即可：

month x = if x<13 && x>0
          then list !! (x-1)
          else "Not a month"

应该有用。

An - 在我看来 - 更优雅的方式来写这个但是使用警卫：

month x | x > 0 && x < 13 = list !! (x-1)
        | otherwise = "not a month"

Answer 4

您也可以这样写：

month = ["Jan","Feb","Mar","April", "May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
getMonth k = lookup k $ zip [1..] month

haskell if else语句有多个条件

4 个答案: