DoubleOseven - 1 year ago 99

C++ Question

Hi I have checked numerous other questions but can't seem to figure out why my implementation will not converge. I keep getting the "Error, no convergence" part of my program even when I enter the root.

The function is y = x^2 - 1

Here is the code:

`// Newton sqaure root finder function`

#include <iostream>

#include <cmath>

int main()

{

using namespace std;

// Enter an initial guess x

cout << "Enter an initial guess: ";

double x;

cin >> x;

// Define & initialize the error, tolerance and iteration variables

double tol = 1e-12;

cout << 1e-12;

double error = tol + 1;

int it = 0;

int max_it = 100;

// Define the x1 variable to hold the latest result (root approximation)

double x1;

// Start while loop with guess x to find the root

while (error > tol && it < max_it)

{

x1 = x + (x*x-1) / (2*x);

error = fabs(x1 - x);

x = x1;

it++;

cout << error << endl;

}

if (error <= tol)

{

cout << "The root is " << x << endl;

}

else

{

cout << "Error, no convergence" << endl;

}

cin.get();

cin.get();

return 0;

}

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Answer Source

You have a typo in the formula

```
x1 = x + (x*x-1) / (2*x);
```

it should be

```
x1 = x - (x*x-1) / (2*x);
```

You may see it here: https://en.wikipedia.org/wiki/Methods_of_computing_square_roots

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