Christian - 6 months ago 19

C++ Question

I've been working for a couple hours trying to figure out what I'm doing wrong. All I need to do is find one root from a polynomial represented by an array using Newton's method. The two functions (polyval and polyder) seem to be giving me the right answers, and I feel that the main code is correctly doing Newton's method. I was hoping someone experienced could give me some advice.

`#include <iostream>`

#include <cmath>

using namespace std;

float polyval(float*, int, float);

void polyder(float*, int, float*);

int main(void) {

int n;

float x=-1,f;

float tol=pow(10,-5);

cout << "Enter polynomial order:" << endl;

cin >> n;

float* p=new float[n+1];

float* dp=new float[n];

cout << "Enter coefficients, starting with the highest power:" << endl;

for (int k=0;k<n+1;k++) {

cin >> p[k];

}

polyder(p,n,dp);

f=polyval(p,n,x);

while (fabs(f)>tol) {

x=x-f/polyval(dp,n,x);

f=polyval(p,n,x);

cout << x << endl;

cout << f << endl;

}

cout << "A real root is at x= " << x << endl;

cout << "To verify: p(" << x << ") = " << polyval(p,n,x) << endl;

return 0;

}

float polyval(float* p, int n, float x) {

float px;

px=0;

for (int k=0;k<n+1;k++) {

px=px+p[k]*pow(x,n-k);

}

return px;

}

void polyder(float* p, int n, float* dp) {

for(int k=0;k<n;k++) {

dp[k] = p[k+1] * (k+1);

}

}

Answer

Your call to `polyval(dp,n,x)`

will access beyond the allocated space for `dp`

, which has `n`

entries and not the requred `n+1`

.