Scala forall to compare two lists?

Question

If I wan't to see if each element in a list corresponds correctly to an element of the same index in another list, could I use forall to do this? For example something like

val p=List(2,4,6)
val q=List(1,2,3)
p.forall(x=>x==q(x)/2)

I understand that the x isn't an index of of q, and thats the problem I'm having, is there any way to make this work?

Answer 1

The most idiomatic way to handle this situation would be to zip the two lists:

scala> p.zip(q).forall { case (x, y) => x == y * 2 }
res0: Boolean = true

You could also use zipped, which can be slightly more efficient in some situations, as well as letting you be a bit more concise (or maybe just obfuscated):

scala> (p, q).zipped.forall(_ == _ * 2)
res1: Boolean = true

Note that both of these solutions will silently ignore extra elements if the lists don't have the same length, which may or may not be what you want.

Answer 2

Your best bet is probably to use zip

p.zip(q).forall{case (fst, snd) => fst == snd * 2}

Answer 3

Sequences from scala collection library have corresponds method which does exactly what you need:

p.corresponds(q)(_ == _ * 2)

It will return false if p and q are of different length.