1

I'm trying to implement the algorithm described here under 'definition', http://en.wikipedia.org/wiki/Laguerre%27s_method

To summarise it takes an initial guess 0, and then iterates to find the root.

In lines 68 to 71 I was hoping this would exit the function, i.e. when the root is found...but it's still carrying on.

Can anyone please explain why?

Thanks in advance!

```
#include <iostream>
#include <cmath>
#include <vector>
#include <iomanip>
using namespace std;
//Declare functions
void poly(vector<double> A, double x, double& p, double& px, double& pxx, double a, double G, double H, double denom1, double denom2, int degree, int max);
//---------------------------------------------------------------------------------------
int main()
{
vector<double> A; // vector of coefficients a^degree...a^0
int degree; // highest degree
int max = 0; // maximum number of iterations
double x; // value of x, initial guess
double p; // p(x)
double px; // p'(x)
double pxx; // p''(x)
double a;
double G;
double H;
double denom1;
double denom2;
poly(A, x, p, px, pxx, a, G, H, denom1, denom2, degree, max);
return 0;
}
//---------------------------------------------------------------------------------------
void poly(vector<double> A, double x, double& p, double& px, double& pxx, double a, double G, double H, double denom1, double denom2, int degree, int max)
{
cout << "For polynomial p(x) = a0 + a1x + a2x^2 + ... + anx^n" << endl;
cout << "Enter the polynomial degree, n" << endl;
cout << "Example: 2 for a quadratic, 3 for a cubic..." << endl;
cout << "Degree: ";
cin >> degree;
A.resize (degree + 1);
for (int i = degree; i >= 0; i--)
{
cout << "Enter coefficient a" << i << ": ";
cin >> A[i];
}
cout << "Enter the initial guess x: ";
cin >> x;
cout << "Enter the maximum number of iterations allowed: ";
cin >> max;
for (int i = 1; i <= max; i++)
{
//Calculations for p(x)
p = A[degree];
for (int i = (degree - 1); i >= 0; i--)
{
p = p*x;
p = p+A[i];
}
if (p==0)
{
return;
}
//First differentiation on the coefficients
for (int i = 1; i <= degree; i++)
{
A[i - 1] = i * A[i];
}
//Horner's Method - calculations for p'(x)
px = A[degree - 1];
for (int i = (degree - 2); i >= 0; i--)
{
px = px*x;
px = px+A[i];
}
//Second differentiation on the coefficients
for (int i = 1; i <= degree-1 ; i++)
{
A[i - 1] = i * A[i];
}
//Horner's Method - calculations for p''(x)
pxx = A[degree - 2];
for (int i = (degree - 3); i >= 0; i--)
{
pxx = pxx*x;
pxx = pxx+A[i];
}
//Laguerre's Method
G = px/p;
H = G*G - pxx/p;
denom1 = (G + sqrt((degree-1)*(degree*H - G*G)));
denom2 = (G - sqrt((degree-1)*(degree*H - G*G)));
if ( abs(denom1) >= abs(denom2) )
{
a = degree/denom1;
}
else
{
a = degree/denom2;
}
cout << "The root approximation for x = " <<fixed<<setprecision(7)<< x << " is " <<fixed<<setprecision(7)<< a << endl;
x = x - a;
}
return;
}
```

*Edited 7 Years Ago by sexyzebra19*: n/a