How to define case class with a list of tuples and access the tuples in scala

Question

I have a case class with a parameter a which is a list of int tuple. I want to iterate over a and define operations on a.

I have tried the following:

case class XType (a: List[(Int, Int)]) {
 for (x <- a) {
  assert(x._2 >= 0)
 }
 def op(): XType = {
  for ( x <- XType(a))
    yield (x._1, x._2)
  }
}

However, I am getting the error:

"Value map is not a member of XType."

How can I access the integers of tuples and define operations on them?

Answer 1

You're running into an issue with for comprehensions, which are really another way of expressing things like foreach and map (and flatMap and withFilter / filter ). See here and here for more explanation.

Your first for comprehension (the one with asserts) is equivalent to

a.foreach(x => assert(x._2 >= 0))

a is a List , x is an (Int, Int) , everything's good.

However, the second on (in op ) translates to

XType(a).map(x => x)

which doesn't make sense-- XType doesn't know what to do with map , like the error said.

An instance of XType refers to its a as simply a (or this.a ), so a.map(x => x) would be just fine in op (and then turn the result into a new XType ).

As a general rule, for comprehensions are handy for nested map s (or flatMap s or whatever), rather than as a 1-1 equivalent for for loops in other languages--just use map instead.

Answer 2

You can access to the tuple list by:

def op(): XType = {
  XType(a.map(...))
}