what is does it mean println(_)?

Question

I have this piece of code in scala

val wordCounts = logData.flatMap(line => line.split(" ")).map(word => (word, 1)).reduceByKey((a, b) => a + b)
wordCounts.foreach(println(_))

So what does println(_) mean and what should it print?

Answer 1

As explained in the section "Placeholder Syntax for Anonymous Functions" of the Spec,

println(_)

is a shortcut for the anonymous function literal

w => println(w)

which in turn is a shortcut for something like

(w: (String, Int)) => println(w)

in this particular case.

Therefore,

wordCounts.foreach(println(_))

simply prints every element of wordCounts.

Note that it can also be written even shorter:

wordCounts foreach println