missing parameter type for expanded function (Scala)

Question

Here's my function:

def sumOfSquaresOfOdd(in: Seq[Int]): Int = {
  in.filter(_%2==1).map(_*_).reduce(_+_)
}

Why am I getting the error "missing parameter type for expanded function"?

relevant: docs.scala-lang.org/tutorials/FAQ/…

Seth Tisue
– Seth Tisue

2022-03-01 18:30:49 +00:00
Commented Mar 1, 2022 at 18:30 — Seth Tisue
– Seth Tisue, Commented Mar 1, 2022 at 18:30

Guru Stron · Accepted Answer · 2022-03-02 06:55:57Z

4

map is accepting function with one parameter ((A) => B) while every _ placeholder represents a separate parameter of anonymous function (i.e. _ * _ is function with two parameters). You can use i => i * i lambda for your map function:

def sumOfSquaresOfOdd(in: Seq[Int]): Int = {
  in.filter(_%2==1)
    .map(i => i * i)
    .reduce(_ + _)
}

Also you can use sum instead of reduce(_ + _) (note that sum will not throw for empty sequences while reduce will):

def sumOfSquaresOfOdd(in: Seq[Int]): Int = {
  in.filter(_%2==1)
    .map(i => i * i)
    .sum
}

edited Mar 2, 2022 at 6:55

answered Mar 1, 2022 at 11:50

Guru Stron

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1 Comment

Ellis Over a year ago

Oh wow I had already tried that earlier and it didn't work, but now I tried it, closed the IDE and opened it again, and it works. Why must programming be this way. Thanks for your help.

chrnybo · Accepted Answer · 2022-03-01 11:45:02Z

2

I guess because map wants a function of a single argument, and you ask it to call * with two arguments. Replace _ * _ with arg => arg * arg and retry.

answered Mar 1, 2022 at 11:45

chrnybo

1455 bronze badges

Comments

Patrick DeLano · Accepted Answer · 2022-03-03 17:59:25Z

2

"map" was called with a function with two parameters when it is expecting a function of one parameter. There's another small bug due to the use of "reduce" - it throws an exception if there aren't any odd Ints in the input Seq.

A better solution would be:

  def sumOfSquaresOfOdd(in: Seq[Int]): Int =
    in.filter(_ % 2 == 1) . map(x => x * x) . sum

You must be using Scala2. Scala3 gives a much better error message:

Error:
2 |  in.filter(_%2==1).map(_*_).reduce(_+_)
  |                        ^^^
  |                        Wrong number of parameters, expected: 1

(Edited to reflect changes in the other answers to this question.)

edited Mar 3, 2022 at 17:59

answered Mar 1, 2022 at 17:00

Patrick DeLano

616 bronze badges

4 Comments

chrnybo Over a year ago

Thanks, Patrick. Care to also explain the wording of OPs error message; "missing parameter type for expanded function", what tricks Scala into reporting it this way?

Patrick DeLano Over a year ago

Sorry, I don't have any idea. But see my edits to my original answer.

Guru Stron Over a year ago

or just sum instead of fold or reduce

Patrick DeLano Over a year ago

Very good point @Guru Stron. I edited my answer to include your improvement.

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