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joining sequences of tuples
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Is there a way to join multiple sequences of tuples that results in one sequence of joined tuples?

I have one seq(string*int) and many more instances of seq(float). I'd like a clean way to get back seq(string*int*float*float*float...)

Seq.zip and Seq.map2 seem to come close, but not quite. Here's what I've gotten to work, but this just gets ugly as the number of seq(floats) to join grows.

let seqa = [("raw1",1);("raw2",2)]

let seqb = [1.0;2.0]

let seqc = [11.0;22.0]

let pass1 = Seq.map2 (fun x y -> (fst x, snd x, y)) seqa seqb

let pass2 = Seq.map2 (fun x y ->

let a, b, c = x

(a, b, c, y)) pass1 seqc

.

For your particular example, you can use:

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Seq.zip3 seqa seqb seqc |> Seq.map (fun ((x,y),z,w) -> (x,y,z,w))

If you need to combine more than three sequences together, it may be worth it to define your own higher arity zip variants, such as:

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let zip4 xs ys zs ws = Seq.map (fun (x,(y,z,w)) -> x,y,z,w) (Seq.zip xs (Seq.zip3 ys zs ws))
By on 4/22/2009 9:14 AM ()

Hi brett,

You could use a sequence expression

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let seqa = [("raw1", 1); ("raw2", 2)]
let seqb = [1.0; 2.0]
let seqc = [11.0; 22.0]

let joined = seq { for (i, j) in seqa do
                     for x in seqb do
                       for y in seqc do
                         yield (i, j, x, y) }

If the number of seq<float>s is very large i would first put the floats in an array.

By on 4/22/2009 12:55 AM ()

Hi Felix,

I guess that is technically a join, but its giving me the Cartesian product. I want to join based on position. My Seq.map2 code does work, i just think its ugly.

Here's another way:

let joined =

seq { for i in 0..seqa.Length-1 do

yield( seqa |> Seq.nth i |> fst

, seqa |> Seq.nth i |> snd

, seqb |> Seq.nth i

, seqc |> Seq.nth i )

}

A little better, but doesn't feel so 'functional' to me...

By on 4/22/2009 8:56 AM ()

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