CODEWITHSUNDEEP
c++newtons-method
Blake B.
Blake B.

Reputation: 13

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

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;
}

Upvotes: 1

Views: 984

Answers (1)

Adam M.
Adam M.

Reputation: 1193

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;
}

Upvotes: 3

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