Developing lambda expression

Question

var t = 0
fun sumIntMap(hm: HashMap<Int,Int>) = hm.forEach{ x -> t+=x.value 
}.let { t }

Trying to sum a Hashmap Int values here with a function, I cannot seem to optimize it so that it requires not capturing and needing the temp var (t).

How can I use temp vars in lambdas and return them? Is how I wrote it the only real way (using a let to return the temp var)?

Thanks!

zsmb13 · Accepted Answer · 2017-11-28 18:47:41Z

4

You can map your HashMap entries to their values to get a List of Ints, and then sum the values in that list:

hm.map { it.value }.sum()

If you have a List instead of a Map, you can also use fold or reduce to do similar things to your original code:

hm.toList().fold(0, { acc, pair -> acc + pair.second })

edited Nov 28, 2017 at 18:47

answered Nov 28, 2017 at 18:06

zsmb13

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5 Comments

Hi Lo Over a year ago

Hi there, really I was looking at the general concept of returning a specific value from a lambda when the lambda handles multiple vars (in this case x & t) using let allows me to do this but can it be done within the lambda itself without chaining?

Qaz Over a year ago

@HiLo, I'm not 100% sure what you're asking, but maybe you're looking for reduce, which is really a more general sum.

zsmb13 Over a year ago

You'll either have to chain calls or use an operator like fold or reduce - those are only available on a List though, not a Map. I've edited my answer with a solution using fold which looks more like your code, that operator might be what you're looking for.

Salem Over a year ago

I believe you could even destructure pair into (_, value), perhaps this fits with Kotlin's functional features :)

Hai Over a year ago

@HiLo the concept you are talking about is fold. Read the second part of @zsmb13 's answer, he's right. The 0 replaces your t = 0 and acc is effectively your t. Also note that you can just do 'hm.values' to get the values: hm.values.fold(0) { t, v -> t + v}

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