Function taking an array and a function on an element array

Question

I'm starting to learn Scala and while reading Scala for the Impatient, got to the following solution to one of the exercises:

//No function
def positivesThenZerosAndNegatives(values: Array[Int]) = {
    Array.concat(for (value <- values if value > 0) yield value,
        for (value <- values if value == 0) yield value,
        for (value <- values if value < 0) yield value)
}

But now I was trying to pass as param the function that applies the filter on each comprehensive for:

//Trying to use a function (filter)
def positivesThenZerosAndNegatives2(values: Array[Int]) = {
    Array.concat(filter(values, _ > 0), filter(values, _ == 0), filter(values, _ < 0))
}

def filter[T: Int](values: Array[T], f: (T) => Boolean) = {
    for (value <- values if f(value)) yield value
}

I haven't found the right way to refer to an element array.

Eastsun · Accepted Answer · 2013-03-11 00:25:25Z

3

You can either writer your filter method as following:

import scala.reflect.ClassTag

def filter[T: ClassTag](values: Array[T], f: T => Boolean): Array[T] = {
  for(value <- values; if f(value)) yield value
}

or as this:

def filter(values: Array[Int], f: Int => Boolean): Array[Int] = {
  for(value <- values; if f(value)) yield value
}

Anyway, you can simply re-write your method positivesThenZerosAndNegatives like this:

scala> def positivesThenZerosAndNegatives(values: Array[Int]) = {
     |   values.filter(0 <) ++ values.filter(0 ==) ++ values.filter(0 >)
     | }

answered Mar 11, 2013 at 0:25

Eastsun

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