Newtons Method finding cube root, answer comes out as 0 every time

Question

I am using a program with C++ that will calculate the cube root of a given float point number using Newton Methods. My program compiles, but the answer always comes out to zero. Here's the program:

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main()
{
    const double epsilon = 0.00000001;
    double A = 0;
    double x1 = A / 10;
    double x2 = 0;
    int iter = 0;

    cout << "Please enter a number to square. " << endl;
    cin >> A;

    while ((fabs(x1 - x2) > epsilon) && (iter < 100)) {
        x1 = x2;
        x2 = ((2 / 3 * x1) + (A / (3 * x1 * x1)));
        ++iter;
    }

    cout << "The answer is : " << x2 << endl;
}

`2/3` is done as integer math. And so is 0. Try making it `2.0/3.0` — Mike Vine, Feb 09 '17 at 17:49
*My program compiles* -- A program compiling ok only means that the syntax is correct. It has no bearing on whether the program will run correctly or give the correct results. — PaulMcKenzie, Feb 09 '17 at 17:51
Mike: thanks for catching the integer math, fixed it but still didnt work. — Blake B., Feb 09 '17 at 17:58
Jarvis, I dont mean to come off as rude, but how do I not enter it? Can you elaborate more? — Blake B., Feb 09 '17 at 18:02

score 3 · Accepted Answer · answered Feb 09 '17 at 18:02

You were assigning variables to be zero, so you weren't going into the loop and you were also dividing by zero because you set x1=x2 and along with what was said in the comments to your post. So I moved some assigning and declarations and everything worked out fine

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main()
{
    const double epsilon = 0.00000001;
    double A = 0;
    double x1 = 0;
    double x2 = 1;
    int iter = 0;

    cout << "Please enter a number to square. " << endl;
    cin >> A;
    x1 = A / 10.0;
    while ((fabs(x1 - x2) > epsilon) && (iter < 100)) {
        x1 = x2;
        x2 = ((2.0 / 3.0 * x1) + (A / (3.0 * x1 * x1)));
        ++iter;
    }

    cout << "The answer is : " << x2 << endl;
}

Thank you! That worked. It was just that small difference with setting x1 = x2 and all. Thanks again! — Blake B., Feb 09 '17 at 18:11

Newtons Method finding cube root, answer comes out as 0 every time

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