The for loop

The for loop in Java, which prints each character of a string on a new line, may look something like this:

final String word = "Word";
for (int i = 0; i < word.length; i++) {
    
}

The same loop in Kotlin is:

val word = "Word";
for (i in 0..(word.length-1)) {
    println(word[i])
}

Note that while the usual for loop in Java is exclusive (it excludes the last index by definition, unless specified otherwise), the for loop over ranges in Kotlin is inclusive. That's the reason we have to subtract one from the length to prevent overflow (string index out of range): (word.length-1).

If you want to avoid that, you can use the until function:

val word = "Word";
for (i in 0 until word.length) {
    println(word[i])
}

Unlike some other languages, reversing the range indexes won't work:

val word = "Word";
for (i in (word.length-1)..0) {
    println(word[i])
} // Doesn't print anything

If your intention is to print the word in reverse order, for example, use the downTo function:

val word = "Word";
for (i in (word.length-1) downTo 0) {
    println(word[i])
}

It will print the following output:

d
r
o
W

It may seem confusing that until and downTo are called functions, although they look more like operators. This is another interesting Kotlin feature called infix call, which will be discussed later on.