Scala - 大小写操作（x，y）=> x ++ y

Question

I am new to scala. I was reading one code not able to understand . Can someone please help me to understand below code ?

 def intersectByKey[K: ClassTag, V: ClassTag](rdd1: RDD[(K, V)], rdd2: RDD[(K, V)]): RDD[(K, V)] = {
    rdd1.cogroup(rdd2).flatMapValues{
      case (Nil, _) => None
      case (_, Nil) => None
      case (x, y) => x++y
    }
  }

What does below line means ? How it will be evaluated ?

case (x, y) => x++y

Thanks

Answer 1

rdd1.cogroup(rdd2) returns a value of type RDD[(K, (Iterable[V], Iterable[V]))] . So - in case (x, y) both x and y are Iterable[V] . Iterable overloads the ++ operator, with an implementation that simply means union - returning an iterable with all of x 's values followed by all of y 's values.

Answer 2

The function cogroup returns a RDD[(K, (Seq[V], Seq[W]))] .

So the value is of type Tuple2 . When you use flatMapValues it will flatmap over the values, which are of Type Seq . ++ for Seqs mean concatenating them. Resulting in a combined Seq.

case (x, y) means that you're using pattern matching. In your case if none of the values of your Tuple are Nil, the function will return x ++ y .

The advantage of using flatMapValues in this case is that it will flatten the result, therefore losing all the None values.

You can check out the documentation here. Also if you're not sure what exactly Pattern matching or flatmaps are check this for pattern matching and this for flatmap