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#announcements
Title
# announcements
q

Quincy

04/27/2021, 7:34 PM
In Java, I can create a map using the toMap collector, like this:
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Stream.of(...)
    .toMap(
        keySelector,
        valueTransform,
        (x, y) -> {
            if (x.getTimestamp().isBefore(y.getTimestamp())) return x;
            else return y;
        });
If there would be duplicate values then the third argument determines what to do about it. In Kotlin, the
associateBy
function always keeps the most recent value. Is there a way to create a map the way Java does, where I get to decide what to do with duplicates?
n

Nir

04/27/2021, 7:55 PM
Yeah, you can use a groupingBy
associate (and friends) is intended to be the simple function for the common use case, the equivalent of
map
basically
r

Roukanken

04/27/2021, 8:01 PM
for example
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listOf(1, 2, 3, 4, 5)
    .groupingBy { it % 2 }
    .reduce { key, accumulator, element -> accumulator * element }
gets you
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{1=15, 0=8}
(in
reduce
,
accumulator
is the value currently associated with
key
and
element
is the one that is trying to be new one) there is also
fold
when you want to also do some type changing mapping, not just selecting or simple operations like I did
q

Quincy

04/27/2021, 8:02 PM
Thank you. I'd used groupingBy previously, but not much, and didn't think about it here. My example becomes:
Copy code
listOf(...)
    .groupingBy(keySelector)
    .reduce { _, x, y -> if (x.timestamp.isBefore(y.timestamp)) x else y }
2
n

Nir

04/27/2021, 8:05 PM
yeah, groupingBy is very nice, I miss it in other languages
if you don't care too much about the efficiency/laziness of it all then it's also possible to do
.associate
followed by
.mapValues
, probably would be a little more readable arguably
since you'd just be able to call
min
minBy
rather
maybe groupBy followed by mapValues, actually. anyhow, doesn't matter.
q

Quincy

04/27/2021, 8:12 PM
I'll play with those suggestions and see if anything strikes as preferable. Thanks again!
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