scala set empty rows in list, set empty value

Question

I have a question about setting the result to Empty (i.e. nothing), I've researched in past questions and did not find a good solution.

The question is quite simple, say I have a List of Int and List of Bool

val a = List(1,2,3,4,5)
val b = List(F,T,T,F,F)

and I want to zip them and do some mapping:

val result = (a,b).zipped.map((x,y)=>(if(b) a else ())

I assume I am doing the right thing above which takes each element of a and b and does the operation where if b is true, return a, else return nothing. I expect the result to only have the numbers (2,3). However, my Eclipse seems to indicate that the result generated is List[AnyVal] instead of List[Int].

I have tested the same setting, but using Lists, and when I set b to List(), the process works and Eclipse is understanding that I want to set an empty List, so I am lost where I when wrong..

Thanks in advance

score 3 · Accepted Answer · 2015-03-10 12:20:52Z

3

You want val result = (a zip b).filter(_._2).map(_._1). map never filters, so don't attempt to return () from its argument and hope it will filter.

edited Mar 10, 2015 at 12:20

answered Mar 10, 2015 at 12:15

user1804599

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Sergii Lagutin · Accepted Answer · 2015-03-10 12:17:22Z

1

val a = List(1, 2, 3, 4, 5)
val b = List(false, true, true, false, false)

a.zip(b)         // zip two lists
  .filter(_._2)  // filter if second element is true
  .map(_._1)     // grab first element of tuple

// List[Int] = List(2, 3)

answered Mar 10, 2015 at 12:17

Sergii Lagutin

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Michael Zajac · Accepted Answer · 2015-03-10 13:05:06Z

0

You could also use collect, which will keep only the elements that match a pattern. You can think of it like a map that discards anything it's not defined for.

val a = List(1, 2, 3, 4, 5)
val b = List(false, true, true, false, false)

(a zip b) collect { case (x, true) => x }

answered Mar 10, 2015 at 13:05

Michael Zajac

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