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Count The Elements In A "List"
f#

Hi all,

Consider the following method for counting all the elements in a list:

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 let countElems l =
     let rec loop l acc =
         match l with
         |h::t -> loop t (acc+1)
         |[] -> acc
     loop l 0;;

And this works fine

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[1..10] |> countElems;; (* val it: int = 10 *)

But it brings up two questions for me:

1.) How would you do this with a Seq.fold? I'm sure there's an equivalent expression with a Seq.fold but I can't quite seem to figure it out. Or maybe a Seq.iter?

2.) Is there someway this could be generalized to any "list" type? That is, this works fine:

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[1..10] |> countElems;;

but this complains about the wrong type

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[|1..10|] |> countElems;;

I realize there are already functions in the library to get this stuff. I'm just trying to learn the fundamentals of FP and recursion is one of the basic building blocks of FP.

.

Try this:

let len l = l |> Seq.fold (fun acc _ -> 1 + acc) 0

Just start with 0 and add one for every element in the sequence.
Should work for any sequence type...

By on 6/6/2010 6:18 PM ()

Try this:

let len l = l |> Seq.fold (fun acc _ -> 1 + acc) 0

Just start with 0 and add one for every element in the sequence.
Should work for any sequence type...

I was trying something sort of between your answer and Mau's and couldn't quite make that last leap of intuition. :-) Thanks.

By on 6/7/2010 5:07 AM ()

Apply '+', start from 0:

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collection |> Seq.fold (+) 0

:-)

By on 6/7/2010 12:39 AM ()

Apply '+', start from 0:

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collection |> Seq.fold (+) 0

:-)

Hi Mau,

This [i]sums[/i] the elements in the list. I was trying to get a count of how many elements are in the list.

By on 6/7/2010 6:40 AM ()

Ouchi, I'm sorry, I misread.

By on 6/7/2010 7:52 AM ()

minor comments:

FP purists would advice you NOT to use recursion if possible and use the higher order function like fold or unfold instead. Try to watch those Channel 9 video by Eric Meijer and you would get that impression.

'fold' in F# is 'left fold' which is straightly speaking not a true 'loop' equivalent. a lazy 'right fold' as far as I know can do anything a loop can.

That said, I found recursion easier to work with :-)

gary

PS. search 'graham hutton' if you want to learn about fold, his papers in this subject are eye opening.

By on 6/7/2010 2:00 PM ()

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