Question

I’m a newbie to scala, I’m just writing a simple function to reverse a given string:

def reverse(s: String) : String
  for(i <- s.length - 1 to 0) yield s(i)

the yield gives back a scala.collection.immutable.IndexedSeq[Char], and can not convert it to a String. (or is it something else?)

how do i write this function ?

Answer 1

Note that there is already defined function:

scala> val x = "scala is awesome"
x: java.lang.String = scala is awesome

scala> x.reverse
res1: String = emosewa si alacs

But if you want to do that by yourself:

def reverse(s: String) : String =
(for(i <- s.length - 1 to 0 by -1) yield s(i)).mkString

or (sometimes it is better to use until, but probably not in that case)

def reverse(s: String) : String =
(for(i <- s.length until 0 by -1) yield s(i-1)).mkString

Also, note that if you use reversed counting (from bigger one to less one value) you should specify negative step or you will get an empty set:

scala> for(i <- x.length until 0) yield i
res2: scala.collection.immutable.IndexedSeq[Int] = Vector()

scala> for(i <- x.length until 0 by -1) yield i
res3: scala.collection.immutable.IndexedSeq[Int] = Vector(16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)