How do I get the sums for all elements of two lists in Kotlin?

Question

In Kotlin I have two lists:

val x: List<Int> = listOf(1,2,3,4,5,6)
val y: List<Int> = listOf(2,3,4,5,6,7)

How do I get:

val z: List<Int> = 3,5,7,9,11,13

without using loops?

Answer 1

Assuming both list have the same size:

1. Using zip

   val sum = x.zip(y) { xv, yv -> xv + yv }
   

2. Using simple map and mapIndexed

   val sum = (0 until x.size).map { x[it] + y[it] }
   // or
   val sum = x.mapIndexed { index, xv -> xv + y[index] }
   

When the size can be different and you would assume 0 for out of range entries:

1. Using an array

   val sum = IntArray(maxOf(x.size, y.size)) { 
       x.getOrElse(it, {0}) + y.getOrElse(it, {0}) 
   }.toList()
   

2. Using range:

   val sum = (0 until maxOf(x.size, y.size)).map { 
       x.getOrElse(it, {0}) + y.getOrElse(it, {0})
   }
   

3. Extending the lists to same size

   val xExtended = x + Array(maxOf(0, y.size - x.size), { 0 })
   val yExtended = y + Array(maxOf(0, x.size - y.size), { 0 })
   val sum = xExtended.zip(yExtended) { xv, yv -> xv + yv }
   

Answer 2

I'd go with a range and map:

val sums = (0 until x.size).map { x[it] + y[it] }

It's probably less overhead than zip.