Hi :smile:, Here’s a programming puzzle I solved ...

# codereview

[skendrick]

Hi 😄,

Here’s a programming puzzle I solved with Kotlin. Please pick it apart, if ya like! I’ve been doing Kotlin for like 5 days. But also lemme know if the approach was off or unclear!

tldr problem description: print a diamond start from A to someLetter

··A··
·B·B·
C···C
·B·B·
··A··

The problem description & the solution: https://gist.github.com/adnauseum/4a7c9762d57f3b82b1dd18b1204af3cb

If you want to review it, 1) thanks, 2) leave a comment in a thread or on the gist—whatever is easiest for you. 🙇 THANK YOU! Actually, the fact that anybody would leave a comment is mind-blowing, so thanks for even reading and putting in the effort!

[Timmy]

Looks good!
I did the challenge before looking at your code and came up with the same algorithm and mostly the same code. Here is what I did differently:
- Since no OOP is involved a top level function is enough, no need for the DiamondPrinter class
- My return type is the complete string (non-nullable). Your list is fine but should be non-nullable
- I used mapIndexed, which does the same as .withIndex().map {}
- I used dropLast(1) instead of slice() since it has a more readable meaning
- In the when statement multiple conditions have the same effect, so combine them like so: leftOfMiddleIndex, rightOfMiddleIndex -> letter
- Skipped some variable definitions (leftOfMiddleIndex and rightOfMiddleIndex) and just inlined them to where they are used (not that important)
- I used an imperative approach to constructing the lines with stringBuilder (repeat(numLettersPerRow) {append(...)}). Might be more performant than constructing a list then joining it, but at this scale it doesn't matter. Actually I like your approach better.
- Used a + instead of .plus()
- I didn't use a separate function makeRow and just had that in the lambda. If you want to keep your makeRow function, use a method reference (lettersUsedInDiamond.mapIndexed(::makeRow))

[skendrick]

Ahh, this is perfect! The class and return type were part of the website’s requirement I found the problem on exercism.io (hate the name of that site) 😄.

These are great insights! THANK YOU!!! 😄

[skendrick]

Wow, @Timmy, just implemented your suggestions. These were exactly the things I was looking for. Tytyty 🙇

[skendrick]

Now, how can I get this whole thing to be one line of code 🤔 😆

[bjonnh]

fun diamond(size: Int): List<String> {
require(size in 1..26)
return (0 until size).map {
var str = ""
repeat (size-it-1) { str += "." }
str += (it+65).toChar()
repeat (2*it-1) { str += "." }
if (it!=0) str += (it+65).toChar()
repeat (size-it-1) { str+="." }
str += "\n"
str
}.let { it + it.dropLast(1).reversed() }
}

[bjonnh]

is one way

[bjonnh]

fun diamond(size: Int): List<String> {
require(size in 1..26)
return (0 until size).map {
var str = ""
repeat (size-it-1) { str += "." }
str += (it+65).toChar()
repeat (2*it-1) { str += "." }
str
}.mapIndexed { index, it -> it+it.dropLast(max(2*index-1,1)).reversed()+"\n" }
.let { it + it.dropLast(1).reversed() }
}

[bjonnh]

that's another way that use symmetry

[bjonnh]

And this is an even more functional way:

[bjonnh]

fun diamond(size: Int): List<String> {
require(size in 1..26)

return (0 until size).map { i ->
(i + 65).toChar().let { c ->
Array(size * 2 - 1) { '.' }.also {
it[size - i - 1] = c
it[size + i - 1] = c
}.joinToString("") + "\n"
}
}.let { it + it.dropLast(1).reversed() }
}