Nested loops in scala

Question

I noticed that scala loops differs from java loops:

//java: for(int i = 0; i < 2; i ++){ System.out.println("1loop"); for(int i = 0; i < 2; i ++){ System.out.println("2loop"); } } // output: 1loop 2loop 2loop ...

And in Scala:

ranges.foreach{ range_line => print("1") transactions.foreach{ transaction_line => print("2") } } //output 111...22

Why it happening so? How to rearrange this nested loop to more scala-like style?

val ranges = scala.io.Source.fromInputStream(getClass.getResourceAsStream("/ranges.tsv")).getLines val transactions = scala.io.Source.fromInputStream(getClass.getResourceAsStream("/transactions.tsv")).getLines

Can you show what ranges and transactions are (their types)? — Zoltán
– Zoltán, Commented Feb 16, 2016 at 19:01
Ok, I got it. I just add toArray to the end of ranges and transactions initialization. But my be you can suggest me more sophisticated solution? — Rudziankoŭ
– Rudziankoŭ, Commented Feb 17, 2016 at 9:29

Dylan · Accepted Answer · 2016-02-16 19:23:36Z

My guess is that transactions is not a regular collection like a List or Array - it is probably a representation of some future computation, e.g. a Future or some kind of stream.

If so, the function you pass to transactions.foreach is likely to be running asynchronously (i.e. later on, sometimes on a different thread, typically not blocking the calling thread).

Following this assumption, your call to transactions.foreach is causing some work to happen "in the background", but the ranges.foreach is continuing on, uninterrupted. ranges.foreach could potentially finish, then the work triggered by transactions.foreach finally gets around to running, so you get all of the 2's at the end.

That said, this appears to be an abnormal case. If ranges and transactions were regular collections like a List, you'd see the same looping behavior as you would with a regular Java collection.

Thanks, Dylan! You are right! This is streams. I fixed it by add asArray. Are there any other solutions?

4lex1v · Accepted Answer · 2016-02-16 19:16:05Z

There's definitely some issue with your definitions for ranges and transactions, please do not that foreach call is not equivalent to Java's for-loop, foreach intends to iterate over the collection. If you try to following snippet, which in some sense equivalent to the for-loop of your, then you'll see the desired result:

 scala> (1 to 2).foreach { i1 => println("1loop") (1 to 2).foreach { i2 => println("2loop") } } 1loop 2loop 2loop 1loop 2loop 2loop

Hope it helps

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