Provide a collectIndexed terminal operator

[jcornaz]

Hi,

Like with collections and sequences we occasionally need to iterate and maintain an index like this:

var index = 0
values.collect {
  // do something with the value, knowing its index.
  ++index
}

Since forEachIndexed already exists for Iterable and Sequence and consumeEachIndexed exists for ReceiveChannel it would seem natural to me that flows provide a collectIndexed terminal operator.

[fvasco]

Hi @jcornaz
what do you think about something like fun <T> Flow<T>.toIndexed(): Flow<IndexedValue<T>>?

[jcornaz]

Hi @fvasco

what do you think about something like fun Flow.toIndexed(): Flow<IndexedValue>?

Why not. except I'd call it withIndex() for consistency with the collection API.

But I think I prefer collectIndexed. I find it more natural to find, and it does not need to wrap each value (which may create unnecessary GC pressure when dealing with a lot of values).

[fvasco]

@jcornaz

I'd call it withIndex()

I agree.

it does not need to wrap each value

In escape analisys we hope

[elizarov]

@jcornaz When you say "we occasionally need" can you provide a slightly more concrete example where this might be needed when you work with flows, please. E.g. if you had recently run into this need in your project, you can explain what was the problem you were solving.

[jcornaz]

can you provide a slightly more concrete example where this might be needed when you work with flows, please.

Last time I felt the need for it, it was to monitor progress. Here is how it'd look like:

lines.collectIndexed { index, line ->
  processLine(line)
  println("${index + 1} lines processed")
}