Why is this list empty

Question

I want to make a function in scala that uses "match ... case" to double the values of a list.

For example:

doubleList(List(2,1,4,5)) //> res0: List[Int] = List(4, 2, 8, 10)

I wrote this function:

def doubleList(xs: List[Int]): List[Int] = xs match { case y :: ys => y * 2; doubleList(ys); case Nil => xs; }

But I get an empty list as the result:

//> res0: List[Int] = List()

Can anyone teel me what am I doing wrong?

Any reason, why you don't just use map(_ * 2)?

Luka Jacobowitz
– Luka Jacobowitz

2016-04-17 22:43:33 +00:00
Commented Apr 17, 2016 at 22:43 — Luka Jacobowitz
– Luka Jacobowitz, Commented Apr 17, 2016 at 22:43
Yes, I am required to use specificly "match ... case".

undisp
– undisp

2016-04-17 22:44:15 +00:00
Commented Apr 17, 2016 at 22:44 — undisp
– undisp, Commented Apr 17, 2016 at 22:44

Pim Verkerk · Accepted Answer · 2016-04-17 22:47:17Z

The ; closes the statement and effectively throws away the result, use :: instead to create a new List with the result of y * 2 and doubleList(ys)

def doubleList(xs: List[Int]): List[Int] = xs match { case y :: ys => y * 2 :: doubleList(ys) case Nil => xs }

P.S. you don't have to put a ; at the end of a line in scala.

Oh, I didn't know about that ; closes the statement. Thanks for your answer, worked just great.

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