Take N elements from sequence with N different indexes in F#

Question

I\'m new to F# and looking for a function which take N*indexes and a sequence and gives me N elements. If I have N indexes it should be equal to concat Seq.nth index0, Seq.n

Answer 1

When you need to scan a sequence and accumulate results in O(n), you can always fall back to Seq.fold:

let takeIndices ind sq =
    let selector (idxLeft, currIdx, results) elem =
        match idxLeft with
            | []                               -> (idxLeft, currIdx, results)
            | idx::moreIdx when idx =  currIdx -> (moreIdx, currIdx+1, elem::results)
            | idx::_       when idx <> currIdx -> (idxLeft, currIdx+1, results)
            | idx::_                           -> invalidOp "Can't get here."
    let (_, _, results) = sq |> Seq.fold selector (ind, 0, [])
    results |> List.rev

seq [10 .. 20] |> takeIndices [0;5;10]

The drawback of this solution is that it will enumerate the sequence to the end, even if it has accumulated all the desired elements already.