GroupBy使用Elm

Question

获取记录列表并执行groupby操作的Elm函数定义是什么样的（参见下面的示例）？使用SQL类比，我基本上试图在Elm中实现select last, sum(age) from table group by last。

[{name= "John", last= "Smith", age= 10}
      ,{name="Jason", last= "Smith", age= 9}
      , {name="Jane", last ="White", age =5}]

 return [{last="Smith", age =19},
         {last ="White", age =5}]

Answer 1

您可以使用Dict.Extra.groupBy执行初始分组，然后通过映射列表并总结年龄来对年龄进行求和：

import Dict.Extra exposing (groupBy)

familySumAges : List Person -> List Family
familySumAges =
    groupBy .last
        >> Dict.map (\_ -> List.map .age >> List.sum)
        >> Dict.toList
        >> List.map (uncurry Family)

Answer 2

以下答案由来自Elm Slack频道的@ holy-meekrob和@ilias提供。 https://ellie-app.com/7tqY9w6gNa1/1

module Main exposing (..)

import Dict exposing (..)
import Html exposing (text)


type alias Person =
    { name : String
    , last : String
    , age : Int
    }


type alias Family =
    { last : String
    , age : Int
    }


people : List Person
people =
    [ { name = "John"
      , last = "Smith"
      , age = 10
      }
    , { name = "Jason"
      , last = "Smith"
      , age = 9
      }
    , { name = "Jane"
      , last = "White"
      , age = 5
      }
    ]


sumAges : Person -> Dict String Family -> Dict String Family
sumAges person families =

        Dict.update
            person.last
            (\family ->
                case family of
                    Nothing ->
                        Just { last = person.last, age = person.age }

                    Just fam ->
                        Just { last = person.last, age = fam.age + person.age }
            )
            families


main =
    text (toString (List.foldl sumAges Dict.empty people |> Dict.values))

Answer 3

我的方法与您接受的解决方案不同。

如果对记录列表进行了排序，那么只要姓氏保持不变，我们就可以简单地聚合年龄。 否则，我们需要为看不见的姓氏创建新记录。

p

我们基本上遍历所有人lst并将它们添加到列表p::lst的第一个元素，除非我们第一次遇到该姓氏，然后我们将其添加到data = [{name= "John", last= "Smith", age= 10},{name="Jason", last= "Smith", age= 9}, {name="Jane", last ="White", age =5}] SumByLast data < / p>

用

测试

.last

显然，如果我们将.age和father = begin pdf_father = p.pdf(father) son = multiNormal(father, mulN_cov).rvs(1) pdf_son = p.pdf(son) 作为参数传递，则此功能可以是通用的