Question

I need to write a recursive function to print the prime factoring elements of an integer, ascending.

void printPrimeFactors(int num)
{
int div;

if (isPrime(num) == true)
    cout << num << " ";

else
{
    for (div = 2; div < num; div++)
    {
        if (isPrime(div) == true && num%div == 0)
            printPrimeFactors(num/div);
    }
}

What am I doing wrong? My output, for 20 is:

5 2 5 2 2

My smallest input is a prime number and a smaller input for the recursive function is num div (smallest prime divider of num).

Answer 1

I believe the following will work:

void printPrimeFactors(int num, int div = 2)
{
        if (num % div == 0) {
                std::cout << div << " ";
                printPrimeFactors(num / div, div);
        } else if (div <= num) {
                printPrimeFactors(num, div + 1);
        }
}

As requested, it is recursive, even though recursion isn’t necessary and can be trivially converted into iteration.

The reason your original version doesn’t quite work is twofold:

1. the printing is misplaced, which results in the factors being printed in descending order;
2. after the recursive call you keep trying further divisors, which results in the duplicate factors appearing on the output.