Return sum of sublist in a list

Question

I've been trying to figure this out for the last few hours with no success what so ever.

Let's say I have a list of lists of int

let list = [[1;3;4;4];[1;3]]

I have to create a function that will sum the sublists and return one list as below:

[12;4]

I've been told that I should use List.fold.

I've tried the following:

let list = [2;3;5]
let sumList list = List.fold (fun acc elem -> acc + elem) 0 list
sumList list

this is returning only an int and works only for an int list and not for a list of list. What are the next steps from here.

Answer 1

Try:

list 
|> List.map List.sum

So you map the List.sum for each element in the list.

Or with fold:

list 
|> List.map (List.fold (+) 0)

(List.fold (+) 0) is the same as the sum function. It starts with zero and adds in each iteration the value to the accumulator.

list 
|> List.fold (fun acc v -> 
    acc @ [(List.fold (+) 0) v]) []

As you see, you can also replace the map with a fold.

list 
|> List.foldBack (fun v acc -> 
    (List.fold (+) 0 v) :: acc) 
    <| []

With List.foldBack it looks a little bit better in my opinion than with fold. But I prefer the first solution.