f# list sum with negative numbers

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I thought that i made right solutiuon for my problem but i was wrong.

I must calculate sum of every elements in list after last negative number or if
in the list i have only positive claculate sum for all elements.

code what i have done : Unfortunalety i must do it without recusion using only high order function and list module. Any hints how to do that or even how to make it with this recusrion ?

F#

let rec sumList acc lst =
     match lst with
     | [] -> acc
     | hd::tl -> hd + sumList acc tl
     |hd::_ when hd < 0 ->  sumList acc lst

let sum lst = sumList 0 lst ;;

  sum [1;2;-5] ;;

Posted 11-Nov-14 3:10am

Member 11224598

Updated 11-Nov-14 3:11am

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Enrique A. Casanovas Pedre 11-Dec-14 17:58pm

It looks like a school assignment.

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Enrique A. Casanovas Pedre · Answer 1 · 2014-12-11T11:39:00

The sum of all positive numbers after the last non-negative can be calculated like this:

F#

let lst = [-5; 4; -3; 2; 6; 10]
let afterLastNegative =
    lst
    |> List.rev
    |> Seq.takeWhile (fun x -> x > 0)
    |> Seq.sum
printfn "%A" afterLastNegative

List.rev reverses the list then Seq.takeWhile will take numbers while the function

F#

fun x -> x > 0

is true, and it returns true if x is positive. Then Seq.sum simply sums all the numbers taken. A slighly shorter version would be:

F#

let lst = [-5; 4; -3; 2; 6; 10]
let afterLastNegative =
    lst
    |> List.rev
    |> Seq.takeWhile ((<) 0)
    |> Seq.sum
printfn "%A" afterLastNegative

It does exactly the same. (<) is the name of the function operator lesser than and is partially applied with one operator, leaving the other to be supplied by the Seq.takeWhile function.