`input.map { it.sum() }`?

# announcements

[diesieben07]

input.map { it.sum() }?

[arve]

that would give me the "horizontal sum", no?

[diesieben07]

Oh, I see. I misunderstood your problem. I can't think of a good way to do what you want without many list copies.

[marstran]

So you kinda want this, right?
[1 + 4 + 7, 2 + 5 + 8, 3 + 6 + 9] = [12, 15, 18]

[arve]

yes, correct

[marstran]

And all lists are of the same length?

[arve]

Sure

[bloder]

maybe this?

val input: List<List<Int>> = listOf(listOf(1, 2, 3), listOf(4, 5, 6))

        input.reduce { acc, list -> acc.map { it + list[acc.indexOf(it)] } }

[bloder]

result - [5, 7, 9]

[marstran]

This works, but it creates a lot of intermediate lists:

input.fold(List(sublistLength) { 0 }) { acc, list ->
    acc.zip(list).map { (a, b) -> a + b }
}

[marstran]

Another way would be to write a transpose-function (google it), and then sum each line in the resulting list. input.transpose().map { it.sum() }

[arve]

This is what I have at the moment (supports variable length rows) but return buffer and for loop leave a sour taste

fun sumColumns(data: List<List<Double>>): List<Double> {
    val longest = max(data.map { it.size }) - 1
    
    val result = mutableListOf<Double>()
    
    for (index in 0..longest) {
        val t = data.map { it -> it.getOrElse(index) { 0.0 } }
        result.add(t.sum())
    }
    return result
}

[orangy]

Try sumBy instead of mapping and then summing

[orangy]

also maxBy for the longest

[arve]

sumBy certainly cleaned up some noise 👍