Question

Are parallel collections intended to do operations with side effects? If so, how can you avoid race conditions?
For example:

var sum=0
(1 to 10000).foreach(n=>sum+=n); println(sum)

50005000

no problem with this.
But if try to parallelize, race conditions happen:

var sum=0
(1 to 10000).par.foreach(n=>sum+=n);println(sum)

49980037

Answer 1

Quick answer: don’t do that. Parallel code should be parallel, not concurrent.

Better answer:

val sum = (1 to 10000).par.reduce(_+_) // depends on commutativity and associativity

See also aggregate.