Question

for (i <- Range(1,7); j <- Range(1,7))
  yield (i,j) // want to yield only if there is no such a pair

Is it possible to have access to the list, formed by yield, inside the loop?
For example, if I don’t want to add duplicates.

P.S. The main question is not how to do it in this particular case. But how to avoid duplicates in more complex case, where I want to check, what was already yielded.

Answer 1

You cannot access members of an immutable collection being built from within the for comprehension.

First of all, I’m going to modify your example so it actually produces duplicates:

for (i <- Range(1,3); j <- Range(1,3)) yield (i min j, i max j)
//scala.collection.immutable.IndexedSeq[(Int, Int)] = Vector((1,1), (1,2), (1,2), (2,2))

For comprehensions are just syntactic sugar, so here’s the equivalent using map and flatMap that produces the exact same result

Range(1,3).flatMap{i => Range(1,3).map{ j => (i min j, i max j)}}

As you can see, you’re not actually building a single collection, but rather a collection of collections, and then merging them together as they’re created. The inner map is taking each j in the range and mapping it to a pair, then the outer flatMap is mapping each i to a sequence of pairs and merging them together. If I change the flatMap to just map, the result is this:

Range(1,3).map{i => Range(1,3).map{ j => (i min j, i max j)}}
//Vector(Vector((1,1), (1,2)), Vector((1,2), (2,2)))

So only after the whole operation is finished can you access the result as a single collection. The result is a Vector which extends IndexedSeq[(Int, Int)], so you can use any of that trait’s methods on the result, one of them is distinct:

(for (i <- Range(1,3); j <- Range(1,3)) yield (i min j, i max j)).distinct
//Vector((1,1), (1,2), (2,2))