I’d like to walk in 2 loops over 0 to half of string and then end of string back to half, this doesn...

# getting-started

[oday]

I’d like to walk in 2 loops over 0 to half of string and then end of string back to half, this doesn’t look like the way to do it

for (i in 0 until halfIndex) {
                for (j in s.length downTo halfIndex) {
                    println("i: ${s[i]} j: ${s[j]}")
                }
            }

[oday]

halfIndex is s.length / 2 let’s say 5 for example, so the first loop from 0 to 5 and the second from 11 to 5

[Sourabh Rawat]

for j in s.lastIndex downTo halfIndex

[Sourabh Rawat]

Also, do you want to nest them or run the loops after one another

[oday]

I want to be testing forwards and backwards at the same time with halfIndex being the stopper

[oday]

yea not nesting

[Sourabh Rawat]

Then you should move the second for loop outside of the first for loop

[ephemient]

well, you immediately have a OOBE on s[j] because j = s.length initially

[ephemient]

if you want to traverse front-to-back and back-to-front in lockstep, consider

for (i in 0 until halfIndex) {
    val j = s.lastIndex - i
    println("${s[i]} ${s[j]}")
}

or maybe

val forward = s.listIterator()
val backward = s.listIterator(s.size)
while (forward.nextIndex() < backward.previousIndex()) {
    println("${forward.next()} ${backward.previous()}")
}

[ephemient]

listIterator only works on List (as the name implies), so that second one is not useful on String

[Sourabh Rawat]

@ephemient I believe your solution won't work for a list/string with odd number of items.

[ephemient]

@Sourabh Rawat depends on what you want to happen with the middle element. what I wrote will skip it, just as @oday's original 0 until halfIndex would, but it's a simple tweak to include it

[Sourabh Rawat]

The original code won't skip it. The second loop will take it. Although it's start condition should be s.lastIndex

[ephemient]

the code "as written" won't take it because it crashes first. the code "as described" ("testing forwards and backwards at the same time") won't take it either, if it's using a 0 until halfIndex loop.

[Sourabh Rawat]

the code is not what OP wants. That is true and already agreed upon. If you were to fix the s.length thing, then the second loop (correct ver. - s.lastIndex downTo halfIndex ) will take the middle element.

[ephemient]

not if it's also using 0 until halfIndex and stepping both in parallel: (0 until halfIndex).zip(lastIndex downTo lastIndex) will exclude the middle element if odd

[oday]

this is what I wanted

var j = final.lastIndex

            for (i in final.indices) {
                if (final[i] != final[j]) {
                    return false
                } else {
                    j--
                }
            }

comparing first and last, second and last - 1, etc. no need for halfIndex turns out, wanted to check for palindrome in a certain way

[Sourabh Rawat]

typical example of https://xyproblem.info/, it seems, for future references.

[oday]

except the solution is in the understanding of the problem

[ephemient]

in that case you don't need to keep track of `j`; that would be equivalent to a tweak to what I wrote with just minor adjustments,

for (i in final.indices) {
    if (final[i] != final[final.lastIndex - i]) {
        return false
    }
}

[ephemient]

if performance isn't a concern, that could be more succinctly represented with

final.zip(final.reversed()) { a, b ->
    if (a != b) {
        return false
    }
}

[oday]

thanks again

[Klitos Kyriacou]

Note that your solution does twice the work that would be necessary to check that a string is a palindrome, because i and j cross each other and then continue to the end, instead of stopping and returning true when they meet half way.

[oday]

true