I don’t understand why `lastEndTokenIndex` is `-1` :neutral_face: If I use `lastIndexOf` without `fr...

# announcements

[Stephan Schroeder]

I don’t understand why lastEndTokenIndex is -1 😐 If I use lastIndexOf without fromIndex than the result is the expected 40. But 40 is bigger than 0, so this fromIndex shouldn’t interfere with the result, should it??

fun main() {
    val sb = StringBuilder("<products><product>2</product><product>4</product></products>")
    val endToken = "</product>"
    val fromIndex = 0
    val lastEndTokenIndex = sb.lastIndexOf(endToken, fromIndex)
    println("lastEndTokenIndex: $lastEndTokenIndex")
}

Kotlin Playground: https://pl.kotl.in/SlHzdIq1j

[karelpeeters]

It starts looking back from fromIndex.

[Stephan Schroeder]

so how do I tell it that it can stop looking backwards, because it’s guaranteed not to be at the start of the string (because i already checked the start of the stringbuilder in an earlier iteration)?

[karelpeeters]

I don't understand what you actually want to find. All indices? The first one? The last one?

[Stephan Schroeder]

i’m looking for the last index that is bigger than fromIndex.

[karelpeeters]

Then just do lastIndexOf and check if the result is indeed larger than fromIndex.

[Stephan Schroeder]

yes, that works, but it’s inefficient, because in the real code the stringbuilder is created in a loop, so the start (everything before fromIndex) is already guaranteed to not contain endToken.

[Ruckus]

Then write your own function

[Stephan Schroeder]

done 😎

fun StringBuilder.lastIndexOfAfter(str: String, startIndexOrAfter: Int): Int {
    tailrec fun lastIndexOfAfter(lastIndexFound: Int, sb: StringBuilder, str: String, fromIndex: Int): Int {
        val foundIndex = sb.indexOf(str, fromIndex)
        if(foundIndex==-1) {
            return lastIndexFound
        }else{
            return lastIndexOfAfter(foundIndex, sb, str, foundIndex+1)
        }
    }
    return lastIndexOfAfter(-1, this, str, startIndexOrAfter)
}

[karelpeeters]

And can't you just look in the newly appended string instead?

[karelpeeters]

Or what are you doing with the buffer?

[Stephan Schroeder]

nope, because the string i’m looking for might have been created when the new chunk was appended (it was seperated before)

[Stephan Schroeder]

But my extension function seems to work and I even used tailrec in it, so I’m feeling pretty good right now 😎

[Stephan Schroeder]

in case somebody is interested in checking it out in the playground: https://pl.kotl.in/HMmaz6g3F

[Stephan Schroeder]

i guess this way less cooler implementation (no tailrec 😔) is superior in terms of readability and performance

fun StringBuilder.lastIndexOfAfter2(str: String, startIndexOrAfter: Int): Int {
    for(index in (this.length-str.length downTo startIndexOrAfter)) {
        if(this.startsWith(str, index)) {
            return index
        }
    }
    return -1
}