How to assign unique contiguous numbers to elements in a Spark RDD

Question

Starting with Spark 1.0 there are two methods you can use to solve this easily:

RDD.zipWithIndex is just like Seq.zipWithIndex, it adds contiguous (Long) numbers. This needs to count the elements in each partition first, so your input will be evaluated twice. Cache your input RDD if you want to use this.
RDD.zipWithUniqueId also gives you unique Long IDs, but they are not guaranteed to be contiguous. (They will only be contiguous if each partition has the same number of elements.) The upside is that this does not need to know anything about the input, so it will not cause double-evaluation.