Getting a Ratio Using a Single Iteration

So, recently I came across the question of how to calculate the ratio of elements of a stream to the total number of elements in said stream. The naïve solution would be:

collection.stream().filter(it -> it.completed()).count() / collection.size()

However, depending on where collection came from this might iterate it twice, producing unnecessary load while doing so. Also, we would need to cast one of the two values to a double otherwise your result would be 0 pretty much all the time.

This is very ugly.

One possible solution is to use a Collector to do the ratio calculation for us. For this we would need a Collector that keeps track of the total number of elements and the number of elements that are completed. Lucky for us there is such a Collector already: the averaging collector. If we map all completed elements to a 1 and all not-completed elements to a 0, the result of the average will match the ratio we are expecting:

collection.stream()
    .collect(Collectors.averagingInt(it -> it.completed() ? 1 : 0));

In Kotlin, there is an average function defined on Iterable<Int> so you can do something very similar:

collection.map { if (it.completed()) 1 else 0 }.average()

You could even combine that with an extension method and turn it into:

collection.map(Foo::toCompletion).average()
…
private fun Foo.toCompletion() = if (completed()) 1 else 0

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