map doesn't work with random numbers

[samuela]

This may be specific to the Scala Adapter or a general issue.. I'm not sure. It may also be simply disallowed somewhere in the documentation but I couldn't find anything mentioning it.

val o = Observable.interval(250 millis)
        .map(x => math.random)
        .take(10)
var id = o map(x => x)

o.subscribe(n => println("n = " + n))
id.subscribe(n => println("id = " + n))

produces

n = 0.16257918600676124
id = 0.015701253635574397
n = 0.65811522451999
id = 0.6745847570884935
n = 0.08592558068181289
id = 0.6761688548540207
n = 0.1511738754239501
id = 0.37751043543314455
n = 0.3335425497156287
id = 0.2433811198187099
...

Of course, we expect the values of the two Observables to be equal but they aren't related whatsoever.

[samuelgruetter]

This behavior is the expected behavior. Remove the line var id = ... and replace the line id.subscribe(...) by a second o.subscribe(...), and you will see that the two outputs are still different. The reason is that everywhere in Rx, each subscriber gets its own version of the Observable it subscribes to. So here, each subscriber gets its own "Observable.interval", so x => math.random is executed seperately for each subscriber. If you want several observers to share the same Observable, you should use the publish operator.

[headinthebox]

You have discovered the distinction between "hot" and "cold" observables and the evil that is side-effects.

http://www.introtorx.com/content/v1.0.10621.0/14_HotAndColdObservables.html

[chrisgrimm]

following up on a previous comment, your code should read something like this:

val (start, o) = Observable.interval(250 millis) map {x => math.random} take 10 publish
val id = o map {x => x}
start()
o subscribe {x => println(s"n = $x")}
id subscribe {x => println(s"id = $x")}

[headinthebox]

Note that we will be changing the Scala bindings to use ConnectableObservable so while this code work now, in the future you would write something like so:

val observable = Observable.interval(250 millis).map(x => math.random).take(10).publish
val id = observable.map(x => x)

observable.subscribe(x => println(s"n = $x"))
id.subscribe(x => println(s"id = $x"))

observable.connect

[samuela]

Ah, gotcha.. thanks for the clarification!