Filter List based on values from another List in scala

Question

Say I have

val l1 = List(1,2,3,4)
val l2 = List(True,False,False,True)

I want to filter elements of l1 that corresponds to True elements in l2

I have done the following:

type Predicate[T] = T => Boolean
def filterXbasedOnY[A, B](X: List[A], Y: List[B], p: Predicate[B]): List[A] = {
    {
      for {i <- 0 until Y.size if p(Y(i))} yield X(i)
    }.toList
}

which is working fine by calling:

val result = filterXbasedOnY(l1, l2, (b:Boolean) => b)

but is this the best way to accomplish this?

l1.zip(l2).filter(_._2).map(_._1)

Andrey Tyukin
– Andrey Tyukin

2018-02-23 13:15:45 +00:00
Commented Feb 23, 2018 at 13:15 — Andrey Tyukin
– Andrey Tyukin, Commented Feb 23, 2018 at 13:15
l1.zip(l2).collect{case (x,true) => x}

jwvh
– jwvh

2018-02-24 00:42:45 +00:00
Commented Feb 24, 2018 at 0:42 — jwvh
– jwvh, Commented Feb 24, 2018 at 0:42

Xavier Guihot · Accepted Answer · 2018-02-23 14:11:13Z

2

List(1, 2, 3, 4).zip(List(true, false, false, true)).filter(_._2).map(_._1)

As suggested by @C4stor, if the idea is to broaden the usage of the filtering to different types, I think we can reduce the complexity (I use toMatch: B in the following to simplify, but the Predicate can still be used):

def filter[A, B](l1: List[A], l2: List[B], toMatch: B): List[A] = {
  l1.zip(l2).filter(_._2 == toMatch).map(_._1)
}

The pipeline would thus be O(n) while keeping working with List objects. Otherwise, calling p(Y(i)) on a List n times would be O(n2).

As specified by @C4stor, and as it's already the case with your solution, it handles l1 being shorter than l2.

filter(List("1", "2", "3"), List("True", "False", "True", "False"), "True") // List(1, 3)

edited Feb 23, 2018 at 14:11

answered Feb 23, 2018 at 12:59

Xavier Guihot

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Comments

C4stor · Accepted Answer · 2018-02-23 13:21:21Z

0

I don't know if it's the best way, but it's at least a good way :

It's easy to read, and explicit
It handles l2 not being booleans, it can easily be fixed to handle correctly X being a smaller size than Y
It's well contained and easy to test

None of the proposed one liners have those desirable properties, so, kudos to you, well done ! You're solution is actually, in my opinion, on the way to be the prod ready version of the proposed one liners.

Btw, may I suggest to you the codereview stackexchange for codereview style questions ?

answered Feb 23, 2018 at 13:21

C4stor

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2 Comments

Andrey Tyukin Over a year ago

In defence of one-liners: 1.1) upper case letters for variable names? 1.2) it's roughly three times as long as the zip-map. 1.3) Being three times as long as zip-map, it invokes the impression as if it does something more sophisticated that a simple zip-map, which is even more confusing. 2) zip works fine if one of the arguments is shorter, the code in Q would crash if X.size < Y.size 3.1) If the code doesn't have to be written, there is also no need to test it. Both zip+map have been thorougly tested. 3.2) It takes O(n^2) time for a blatantly linear operation = not well done.

C4stor Over a year ago

I didn't want to enter a code review style, which isn't appropriate for SO (I verged on closing the question tbh because of that). I just wanted to point that having this kind of function is in fact not a bad thing, and has multiple advantages. I would still not have wrote it this exact way tbh, but I can see the usefulness. But once again, it's a code review question at this point, so I don't wanna err on this side on this site ^^

ConorR · Accepted Answer · 2018-02-23 22:35:15Z

0

For syntax might make it more readable, and the use of a filterFunction means that l2 can be whatever type it needs to be.

for( (l1,l2) <- (list1 zip list2) if (filterFunction(l2))) yield l1

answered Feb 23, 2018 at 22:35

ConorR

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