Hi guys, I am getting difficult to understand this piece of code. I don't understand how `let` is go...

# codereview

[KotlinLeaner]

Hi guys, I am getting difficult to understand this piece of code. I don't understand how let is going inside the lambda block if value is null. Can someone guide me on this. Thanks

[KotlinLeaner]

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.
Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

[KotlinLeaner]

I am adding piece of code.

fun main() {
//    val nums = intArrayOf(2, 7, 11, 15)
//    val target = 9
    val nums = intArrayOf(3, 2, 4)
    val target = 6
//    val nums = intArrayOf(3, 3)
//    val target = 6
    twoSum(nums, target).forEach {
        print(" $it")
    }
}

fun twoSum(nums: IntArray, target: Int): IntArray {
    val map = mutableMapOf<Int, Int>()
    nums.forEachIndexed { index, i ->
        map[i]?.let {
            return intArrayOf(it, index)
        }
        map[target - i] = index
    }
    return intArrayOf()
}

My

youtube▾

link is described that I am debugging the code. My question is how

map[i]?.let {
                return intArrayOf(it, index)
            }

is going inside the 1st and 2nd iteration of return statment and it not going in 3rd iteration. Can anyone help me on this. Thanks

[Thomas Urbanitsch]

Hi, i didn’t check your entire sample, but it sounds like you missing the understanding for the safe call operator (?.)
https://kotlinlang.org/docs/null-safety.html#safe-calls
there is also an example on that page using let to println only values that are not null, which should be similar to your case.

-> it?.let is therefor often used as a if(it != null) replacement

[KotlinLeaner]

Hi @Thomas Urbanitsch my let statment is null so how it going inside me lambda function.

[KotlinLeaner]

My question is that if map[i]? is null in 1st iteration then how should it be going inside the lambda and call intArrayOf(it, index) ?

[KotlinLeaner]

Thanks for your time.

[Thomas Urbanitsch]

https://pl.kotl.in/Hu2kAP1Ck
it is only entering the let in case map[i] != null (as expected)

[KotlinLeaner]

yes in your code it working fine. But In my intellj it going inside the blog. You can clearly see in the video link

https://www.youtube.com/watch?v=tCG4i0q3Zf4&feature=youtu.be▾

[Thomas Urbanitsch]

i just don’t think intellij is showing you the correct lines highlighted there while debugging, otherwise you would also expect your return statement leaving your method, right? :)

[KotlinLeaner]

yes intellj is not showing me correct lines highlighting there while debugging. Yes answer is return the statement