Can't iterate over input string [SOLVED]

Question

So I'm learning Kotlin for fun but I got stuck.

I want to write code that takes an input word and prints its letters on separate lines.

I tried

fun main(args: Array<String>) {
var word = readLine()
for (letter in word) {
println(letter)
}
}

but I get

error: not nullable value required to call an 'iterator()' method on for-loop range

I can print an input word, and I can print a word letter by letter, just not an input word letter by letter.

I figured out a workaround by creating a new string by adding "" to the input string, but that shouldn't be necessary (should it?) See these 4 codes.
Any ideas?

Yes I used the search bar :\

https://code.sololearn.com/c82A163A15A2
https://code.sololearn.com/cPdWINY0Bkx1
https://code.sololearn.com/cA21a100a228
https://code.sololearn.com/cA8a4a8A0a99

Answer 1

The readLine() function returns the type `String?`. The type `T?` signifies that `T` is a nullable type, that is, null can be assigned to that type. Read more here
https://kotlinlang.org/docs/null-safety.html

This means that the readLine() function may return either a valid string or null. Check out the following link for when it returns null:
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.io/read-line.html:

When you try to iterate on a nullable value, as you are doing in your code, the compiler gives error saying that a not nullable value is required for calling the iterator() method.

So to convert a nullable value to a not nullable value, you can use the !! operator. Using !! on type `T?` returns `T` if the value is not null, else it throws the NullPointerException. Read about it here
https://kotlinlang.org/docs/null-safety.html#the-operator

Your fixed code
https://code.sololearn.com/cb9S5h33xvKn/?ref=app

Answer 2

David Ashton

We have to use !! after readLine() when dealing with user input because there maybe possibility to have null value in user inputs. Maybe I am wrong but I found this solution.

https://code.sololearn.com/clYzaq5mos3F/?ref=app

Answer 3

David Ashton

I found other ways to iterate string if you don't want to use !!

https://code.sololearn.com/cLBfwbOE7Fy3/?ref=app

Answer 4

Thanks I Am AJ ! and XXX !
Interesting that you can print but not iterate without !! after readLine().

Answer 5

Dynamite! Thanks I Am AJ ! I can see I'm only scratching the surface.