missing parameter type for expanded function (Scala)-CodePudding

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"?

CodePudding user response：

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.

CodePudding user response：

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(_   _)
}

CodePudding user response：

The solutions by @chrnybo and @Guru Stron are fairly good ones and they do solve the original problem - that "map" was called a function with two arguments when it is expecting a function of one argument. But both have a small bug due to the use of "reduce" - they throw an exception if there aren't any odd Ints in the input Seq.

A little better solution might be:

  def sumOfSquaresOfOdd(in: Seq[Int]): Int =
    in.filter(_ % 2 == 1) . map(x => x * x) . foldLeft(0)(_ _)