storing values in string during recursion

Question

I want to store the values produced by a recursive function in a string, but I am not sure how to keep after each iteration of the loop. Im not necessarily looking for you to solve it particular to the attached code, but I figured it would give it some context. Simply commenting resources where I can learn this is, of course, welcome. Thanks

int HailstoneNumbers(int N)
{
    vector <char> sequence;
    static int c;

    cout << N << " ";

    if (N == 1 && c == 0) {

        // N is initially 1.
        return c;
    }
    else if (N == 1 && c != 0) {

        // N is reduced to 1.
        c++;
        return c;
    }
    else if (N % 2 == 0) {

        // If N is Even.
        c++;
        HailstoneNumbers(N / 2);
    }
    else if (N % 2 != 0) {

        // N is Odd.
        c++;
        HailstoneNumbers(3 * N + 1);
    }
}

Hint: You don't want the sequence to be automatically destructed every time this function returns. — Mooing Duck
– Mooing Duck, Commented Jan 15, 2020 at 22:58
Huh. I've never heard the Collatz Conjecture sequence called "Hailstone" before. I'd advise looking up C++ collatz examples. In 99% of cases, if you have a static variable in your recursion, it means you're trying to force the recursion the wrong way. — scohe001
– scohe001, Commented Jan 15, 2020 at 22:58
first it seems your recursive calls need to be return HailstoneNumbers... second pass the vector by reference — GreatAndPowerfulOz
– GreatAndPowerfulOz, Commented Jan 15, 2020 at 23:01

asmmo · Accepted Answer · 2020-01-15 23:23:43Z

2

If you want to store N values, you can store them in a vector as follows

int HailstoneNumbers(int N, vector<int>& sequence)
{
    int c;

    sequence.push_back(N);

    if (N == 1 && c == 0) {

        // N is initially 1.
        return c;
    }
    else if (N == 1 && c != 0) {

        // N is reduced to 1.
        c++;
        return c;
    }
    else if (N % 2 == 0) {

        // If N is Even.
        c++;
        HailstoneNumbers(N / 2, sequence);
    }
    else if (N % 2 != 0) {

        // N is Odd.
        c++;
        HailstoneNumbers(3 * N + 1, sequence);
    }
}

declare a vector before calling your function as vector<int> sequence; then call your function using your N and this vector

answered Jan 15, 2020 at 23:23

asmmo

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