How do I write a function in scala

问题:

I have the following code:

val ls = List(0, -1, 2, -2)
var removeNegative = List[Int]()
def removeNegative(ls: List[Int]): Int = ls match { 


  case Nil => 0
  case l::for(ls <- ls){
    var removeNegative = List[Int]()
        if(ls >= 0){
                removeNegative = removeNegative :+ ls
        }
    }


    return removeNegative
}


println(removeNegative(ls)

and I used the code in the function body as a standalone and it works, however I have had to add it into a function and I get the following errors:

ScalaFiddle.scala:7: error: illegal start of simple pattern
    case l::for(ls <- ls){
            ^
ScalaFiddle.scala:16: error: '=>' expected but '}' found.
  }
  ^

What have I done wrong here?

回答1:

Not a valid pattern match when deconstructing the list.

See the code snippet below for a more idiomatic way of doing this.

val ls = List(0, -1, 2, -2)

def removeNegative(ls: List[Int]):List[Int] = ls match { 
  case Nil => ls
  case l::tail => 
    if (l < 0) l :: removeNegative(tail) else removeNegative(tail)  
}

回答2:

The simplest way to realize the function you want is to use filter

def removeNegative(xs: List[Int]): List[Int] = xs.filter(_ >= 0)

val ls = List(0, -1, 2, -2)
removeNegative(ls) // List(0, 2)

If you want recursive version:

def removeNegative(xs: List[Int], ans: List[Int] = List.empty[Int]): List[Int] = xs match {

    // case when there is no elements left in xs
    case Nil => ans

    // we get first element of xs and
    // if it is non-negative, append it to ans
    case x::tail => removeNegative(tail, if (x < 0) ans else ans:+x)
}

val ls = List(0, -1, 2, -2)
removeNegative(ls) // List(0, 2)

It is tail recursive, which means it doesn't consume stack for each recursive call.

If you want to know more about tail recursion here is a good start explanation.