Kotlin array without value initially

Question

Assume I have an existing IntArray: values = [1,2,3,4,5]. Now I need to create another array which holds the sum of all numbers in the original array up to the current index. For example:

sum = [1,3,6,10,15]

because

sum[0] = values[0] (= 1)
sum[1] = values[0] + values[1] (= 1 + 2 = 3)
...

Currently I create sum by:

val sum = IntArray(values.size) {0}
sum[0] = values[0]
for(index in 1 until sum.size) {
  sum[index] = sum[index-1] + values[index]
}

Problem with this code is it needs to provide the initial value of 0 just to create the array. I feel it is quite redundant.

I understand that Kotlin allows a lambda to be pass to an array's constructor. However code like below wouldn't compile because it references sum while sum hasn't been initialised.

val sum = IntArray(values.size) { index ->
   if(index == 0) values[index]
   else sum[index-1] + values[index]
}

I was wondering if there is any nicer way of doing this?

Sergio · Accepted Answer · 2020-07-18 14:35:32Z

1

You can use map function for that:

val values = arrayOf(1,2,3,4,5)

var sum = 0
val sumList = values.map {
    sum += it
    sum
}

Or more concise way is to use scanReduce function:

val values = arrayOf(1,2,3,4,5)
val sumList = values.scanReduce { acc, v ->
    acc + v
}

Note: at the time of writing, scanReduce function is an experimental, so all functions in call hierarchy should be marked with @ExperimentalStdlibApi annotation.

If you need exactly IntArray type:

val sumArray = sumList.toIntArray()

edited Jul 18, 2020 at 14:35

answered Jul 18, 2020 at 14:03

Sergio

31k22 gold badges143 silver badges159 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

IR42 Over a year ago

map returns List<Int>, val sumArray = IntArray(values.size) { sum+=values[it]; sum }

H.Nguyen Over a year ago

Thanks for your answer. It's a great use of map and scanReduce but I would want to avoid transforming back and forth between array and list. So I guess @IR42's comment makes more sense for this scenario.

Collectives™ on Stack Overflow

Kotlin array without value initially

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related