Scala. Arguments of nested lambdas with short syntax

Question

Please, sorry for my English :( Let's explain my question by examples. We have an array a:

    var a = Array(1,1,1,1,2)

We can:

filter a:
```
a.filter( _ < 2 )
```
Count some elements in a:
```
a.count (_ < 2)
```

Getting unique elements in the collection:

a.filter { i =>
    a.count(_ == i) == 1
}

The question is: how to do third clause but without introducing variable i? Is it possible to write something like:

    a.filter ( a.count ( _ == __) == 1 )

I understand that I can write this (and it's still short):

    a.filter { i => a.count(_ == i) == 1 }

But I'm just interested in the answer.

colevk · Accepted Answer · 2013-11-11 23:02:52Z

5

You can't do this without introducing the variable i because it requires mixing variables from two different scopes (what is known as a closure). If we write this without using the _ placeholder variable, this becomes clearer.

a.filter( outer =>
  a.count( inner =>
    outer == inner
  ) == 1
)

For every value of outer in a, we are creating a new function to pass to count, in which outer is constant and inner is passed in as a parameter. _ can only act as a placeholder for the innermost function it is used in, and so can't be used to replace outer. This is why

a.filter( outer =>
  a.count(
    outer == _
  ) == 1
)

compiles but

a.filter(
  a.count( inner =>
    _ == inner
  ) == 1
)

does not.

answered Nov 11, 2013 at 23:02

colevk

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Comments

Yann Moisan · Accepted Answer · 2013-11-11 23:24:31Z

0

Just to mention an alternate solution (not simpler, but faster I suppose)

a.groupBy(identity).filter{ case (_, v) => v.size == 1}.keys

edited Nov 11, 2013 at 23:24

answered Nov 11, 2013 at 23:19

Yann Moisan

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2 Comments

kiritsuku Over a year ago

or xs groupBy identity collect { case (k, v) if v.size == 1 => k }

colevk Over a year ago

Or (even faster and more complex): a.foldLeft(collection.mutable.Map[Any, Int]().withDefaultValue(0))((m, x) => { m(x) += 1; m }).filter(_._2 == 1).keys. Really only faster for lists of 70 or more elements, though.

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