Applying partial functions where defined and a different function where not

Question

This is a motivational example, Given:

List((1,2), (2,1), (3,1))

I'd like to return:

List((1,2),(3,1))

I've tried to do this in several ways. First:

List((1,2), (2,1), (3,1)) map { case (a,b) => if (a > b) (a,b) else (b,a) } 
distinct

Then I tried to use a tuple:

List((1,2), (3,4), (2,1)) map { t => if (t._1 <= t._2) t else t.swap }

then defining the partial function a little differently:

val pf: PartialFunction[(Int,Int), (Int,Int)] = {
  case (i, j) if i >= j => (j, i)
}

List((1,2), (3,4), (2,1)) map pf distinct

Is there a way that apply the PartialFunction only to the elementes that is defined for? or compound the PF with Identity in some how.

possible duplicate of [Scala filter tuples (x, y) == (y, x)](http://stackoverflow.com/questions/26916751/scala-filter-tuples-x-y-y-x) — The Archetypal Paul, Dec 09 '14 at 17:14
to answer your other question: "Is there a way that apply the PartialFunction only to the elementes that is defined for? " - `collect` does exactly that. But I think you mean "and leave elements for which it is not defined unchanged"? — The Archetypal Paul, Dec 09 '14 at 17:18
In general, a partial function will not return the same type as its parameter. So there really isn't a place for a generic apply-this-partial-func-if-defined-else-identity method. Given that, I think your previous version with a default case is clear enough. — The Archetypal Paul, Dec 09 '14 at 17:23
Retracted the close vote since you changed the point of the question. — The Archetypal Paul, Dec 09 '14 at 17:24
yes, that's what `collect` does but it is an answer to your question as stated. You don't want that, though - you also want "if not defined use identity" — The Archetypal Paul, Dec 09 '14 at 17:27
Feel free to erease those comments, makes a lot of sense what you say in your third comment. — Joselo, Dec 09 '14 at 17:30

James Iry · Accepted Answer · 2014-12-09T17:29:59.740

1

Here's another form for the sake of completeness

List((1,2), (2,1), (3,1)) map { case x@(i,j) => if (i >= j) (j,i) else x } distinct

Which is basically the same several of your other forms.

Is there a way that apply the PartialFunction only to the elementes that is defined for? or compound the PF with Identity in some how.

Yes. applyOrElse.

In this case there's very little reason to check where your partial function is and isn't defined because the amount of work required to check definedness is going to be the same as the work the total function has to do before figuring out what to do (i.e. comparing the two elements).

But if you really want to

List((1,2), (3,4), (2,1)) map {x => pf.applyOrElse(x, identity[(Int, Int)])} distinct

edited Dec 09 '14 at 17:29

answered Dec 09 '14 at 17:24

James Iry

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"Your PartialFunction is a total function". Not after the most recent edit, it's not :( – The Archetypal Paul Dec 09 '14 at 17:25
Yes! :( i try to reflex what i relay want to ask :) and thanks you i did it! :P – Joselo Dec 09 '14 at 17:31
Yeah, damn edits screwing up my perfectly good answers. I've updated my answer to reflect the new code. – James Iry Dec 09 '14 at 17:32
Yeap :( sorry about that – Joselo Dec 09 '14 at 17:37
Ah, too slow. I just came up with the same `applyOrElse`! – The Archetypal Paul Dec 09 '14 at 17:38

Applying partial functions where defined and a different function where not

1 Answers1