Is there a way to make a for loop with two parallel iterators in Scala?

Question

I am trying to do a for loop on a HashMap in Scala, but I also need a count of my iteration, like an index of sorts. Here is what I want the functionality to be:

    val map = HashMap()
    val i = 0;

    for ((k, v) <- map) {
        // do something
        i += 1
    }

But I want it to look something like this, where i is updated inside the for loop syntax. But, this seems to act as a nested loop rather than a parallel iterator.

    for ((k, v) <- map; i <- 0 until map.size) {
        // do something
    }

I would avoid for loops in Scala. They're not very functional, and Scala provides map, flatMap, and foreach methods on most collections so that you don't need to write the boilerplate of a for loop. It's much easier to write collection.foreach(println) than a for loop that does that same thing. — csjacobs24
– csjacobs24, Commented Feb 27, 2020 at 17:16
@csjacobs24Scala doesn't have for loops! Although for comprehensions look similar on the surface, it all desugars into map, flatMap, and filter. — Ethan
– Ethan, Commented Feb 27, 2020 at 20:04

Tim · Accepted Answer · 2020-02-27 06:52:06Z

4

The zipWithIndex method is designed specifically for this purpose:

for {((k, v), i) <- map.zipWithIndex} yield {???}

or

map.zipWithIndex.collect { case ((k, v), i) => ??? }

Personally I would use the second version in this case and only use the for syntax when iterating over multiple collections.

answered Feb 27, 2020 at 6:52

Tim

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