Create a list of custom type in F# and create two sequences of that list

问题

I have created my own type in F# called Accounts and I have then created objects for each account.

type Account() =   

  let AccountNumber = ""
  let mutable Balance:float = 0.0

Every account has two fields, AccountNumber (string) and Balance (float).

I have then created an object for every account that holds the AccountName and the Balance.

let acc1 = new Account()  
acc1.Insert("John",10.0)

let acc2 = new Account()  
acc2.Insert("Mike",50.0)

How do I create a list that holds each account (object)? I have tried the following:

let AccountList : Account list = [acc1;  acc2 ;  acc3; acc4 ; acc5; acc6]
let AccountList : Account obj list = [acc1;  acc2 ;  acc3; acc4 ; acc5; acc6]

I cannot solve the problem using the above method because I have to create two sequences from the list:

Sequence 1: All accounts with a balance greater or equal to zero and less than 50 Sequence 2: All accounts with a balance above 50

How do I create a list of my custom type in F# and how do I create two sequences of that list?

回答1:

It is not clear what exactly are you struggling with. However, the following simple example should illustrate most of the key ideas that you probably need to use. First, here is a small version of your Account class (note that I would normally use an immutable record, but I kept it the way you did it):

type Account(balance:float) =   
  let mutable balance = balance
  member x.Balance = balance
  member x.Add(difference) = 
    balance <- balance + difference

I do not see what issue you have with creating the list. The following works just fine:

let acc1 = Account(100.0)
let acc2 = Account(10.0)

let accountList = [acc1; acc2]

Now, to answer the question about finding accounts with balance over 50, you can use the List.filter function to create a new filtered list:

let above50 = 
  accountList |> List.filter (fun acc ->
    acc.Balance > 50.0)

EDIT If you wanted to use a record instead, then you would define the type as:

type Account = { Balance : float }

And create a value using:

let acc1 = { Balance = 100.0 }

回答2:

So I created this answer for the other one but I was waiting on the comment to see if I would answer. And the homework like aspect of this :)

So if you have criteria that bucket an account and want to do that in a single pass, you might want to look at groupBy. Here I use a boolean because there are only 2 possibilities but numbers or a discriminated union are good candidates.

open System

type Account(accountNumber:string, startingBalance:Int64) =
    let mutable balance = startingBalance

    member _.Balance = balance
    member _.Deposit amount = balance <- balance + amount
    member _.Withdraw amount = balance <- balance - amount
    override _.ToString() = accountNumber

let allAccounts = [Account("ABC1", 10L); Account("ABC2", 50L)]

let grouped = allAccounts |> List.groupBy (fun a -> a.Balance >= 50L) |> Map.ofList

let under50 = grouped |> Map.tryFind false |> Option.defaultValue []
let overIncl50 = grouped |> Map.tryFind true |> Option.defaultValue []

printfn "Under: %A" under50
printfn "Over: %A" overIncl50

来源：https://stackoverflow.com/questions/61368307/create-a-list-of-custom-type-in-f-and-create-two-sequences-of-that-list

标签

arrays

list

functional-programming

f#-interactive