How to combine equal sequence elements (functional programming)?

Question

I want to write a function that takes in a sequence <1,1,2,2,3> and returns the sequence with equal elements grouped like <<1,1>, <2,2>, <3>>.

I\'m u

Answer 1

If F# is your language, simply use the Seq.groupBy function:

input |> Seq.groupBy id |> Seq.map snd

Otherwise,

I assume that your language supports Seq.distinct, Seq.fold, Seq.map, and Seq.init. The F# version of these functions can be found in this document.

Then you can do the steps below:

1) Make a distinct seq of the input seq with Seq.distinct:

input |> Seq.distinct

2) Write a function that counts the number of occurrences of a value in a seq by using Seq.fold:

let count x theSeq =
    theSeq
    |> Seq.fold (fun n e -> if x = e then n+1 else n) 0

3) Use the count function to decorate each element of the distinct seq with the number of its occurrences in the input seq:

Seq.map (fun x -> (x, count x input))

4) Finally, use Seq.init to replicate the equal elements:

Seq.map (fun (x, count) -> Seq.init count (fun i -> x))

The whole code:

let count x theSeq =
    theSeq
    |> Seq.fold (fun n e -> if x = e then n+1 else n) 0

input
|> Seq.distinct
|> Seq.map (fun x -> (x, count x input))
|> Seq.map (fun (x, count) -> Seq.init count (fun i -> x))