For comprehension with two generators

Question

I want to display an arbitrary number of strings in a GUI-application in a vertical list.

Consider the following example:

val names = Seq[String]("Boris", "Anna", "Markus", "Simone")

for(a <- 1 to names.size + 1; name <- names) yield new ListItem(
  name,
  x = 20
  y = 128 * a
  size = 128
)

Now of course this would create a "cross-product" and print:

Boris Anna Markus Simone
Boris Anna Markus Simone
Boris Anna Markus Simone
Boris Anna Markus Simone

while the wanted result would be

Boris
Anna
Markus
Simone

But how else can I bind the index to the actual variable? I mean if I'd do a counter like:

for(a <- 0 to 1; b <- 0 to 1)
yield(a,b)

I'd get

Vector((0,0), (0,1), (1,0), (1,1))

but if I'd want to I could to (for a <- 0 to 1; b <- 0 to a) yield(a,b) and get

Vector((0,0), (1,0), (1,1))

I have no idea how to do that with my example tho.

Answer 1

You want zipWithIndex, which will create a new Seq containing each element from the original Seq, tupled with it's index within the Seq.

scala> names.zipWithIndex
res2: Seq[(String, Int)] = List((Boris,0), (Anna,1), (Markus,2), (Simone,3))

Which you can then use:

val names = Seq[String]("Boris", "Anna", "Markus", "Simone")
class ListItem(name: String, x: Int, y: Int, size: Int)

scala> for ((name, index) <- names.zipWithIndex) 
           yield new ListItem(name, 20, 128 * index, 128)
res0: Seq[ListItem] = List(ListItem@5956b37d, ListItem@4b22015d, ListItem@2587a734, ListItem@6cf25a2b)

Which can also be written as a map:

names.zipWithIndex.map { case (name, index) =>
    new ListItem(name, 20, 128 * index, 128)
}