Apache Spark usage of yield

Question

Can someone tell me what is the use of yield? I know it will print the loops iterating but can I know about it on the conceptual level?

Answer 1

Yield is used in sequence comprehensions. It is applied in combination with for-loop and writes a new element into the resulting sequence. Scala's for-loop comprehensions are syntactic for the composition of multiple operations with map, flatMap, and filter. Or foreach. Scala actually translates a for-expression into calls to those methods, so any class providing them, or a subset of them, can be used with for comprehensions

This : 

for(x <- c; if cond) yield {...} 

is translated on Scala 2.7 into

c.filter(x => cond).map(x => {...}) 

or, on Scala 2.8, into 

c.withFilter(x => cond).map(x => {...}) 

Simple example (from scala-lang)

/** Turn command line arguments to uppercase */

object Main {

  def main(args: Array[String]) {

    val res = for (a <- args) yield a.toUpperCase

    println("Arguments: " + res.toString)

  }

}