Been practicing with Uncle Bob's Prime Factors kat...

# codereview

[ianbrandt]

Been practicing with Uncle Bob's Prime Factors kata (http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata). Any improvements here, given the absence of C-style for loops in Kotlin? Perhaps a functional approach?

fun primeFactorsOf(integer: Int): List<Int> {
	val primeFactors = mutableListOf<Int>()
	var n = integer
	var divisor = 2
	while (n > 1) {
		while (n % divisor == 0) {
			primeFactors.add(divisor)
			n /= divisor
		}
		divisor++
	}
	return primeFactors
}

For comparison, a Java implementation with C-style loops:

public static List<Integer> primeFactorsOf(int integer) {
	List<Integer> primeFactors = new ArrayList<>();
	for (int divisor = 2; integer > 1; divisor++)
		for (; integer % divisor == 0; integer /= divisor)
			primeFactors.add(divisor);
	return primeFactors;
}

[gildor]

I'm really prefer version with while for much-much better readability and simplicity
And It’s good example why C-style for is dropped 🙈

[dalexander]

Here's a tail recursive implementation as another option. Someone might be able to do this better than I though, since I'm mostly unfamiliar with the patterns. I also engage in a little unnecessary code golf with the pair construction for the result of the if statement.

tailrec fun primeFactorization(number: Int, divisor: Int = 2, res: MutableList<Int> = mutableListOf()): List<Int> {
    if(number <= 1) {
        return res
    }

    val (nextNumber, nextDivisor) = if(number % divisor == 0) {
        res.add(divisor)
        (number / divisor) to divisor
    } else {
        number to (divisor + 1)
    }

    return primeFactorization(nextNumber, nextDivisor, res)
}