Question

I am trying to create a sequence lazily by using F#.

The sequence is defined as follows:

The nth term of the sequence of triangle numbers is given by, tn = ½n(n+1); so the first ten triangle numbers are:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, …

Here is what I have so far but it dosn’t seem to work:

let tri_seq = 1.0 |> Seq.unfold (fun x -> match x with                                                                                    | _ -> Some (x, 0.5*x*(x + 1.0))) 

Thank you very much who can help me figure out how unfold works. Thanks

Edit: I marked the first answer as correct but it dosnt work, however I slightly modified it and it worked.

let tri_seq = 1.0 |> Seq.unfold (fun x -> Some (0.5 * x * (x + 1.0),x + 1.0)) 

Answer 1

First off, why do you use match if you’ve got only one case?

let tri_seq = 1.0 |> Seq.unfold (fun x -> Some (x, 0.5 * x * (x + 1.0))) 

Second, what “doesn’t seem to work”? Are you aware that you produce an infinite list?

/Edit: For completeness’ sake, here’s the correct solution, which the OP found himself and posted as a comment:

let tri_seq =      1.0 |> Seq.unfold (fun x -> Some (0.5 * x * (x + 1.0), x + 1.0))