Matlab's polyval and polyder in C++

[redsox595]

Hello, I am trying to write functions that behave exactly as the polyval and polymer functions do in Matlab. My code is outputting the wrong solution and I am not sure where my errors are. Thank you

#include <iostream>
#include <cmath>
using namespace std;
 
float polyval(float*, int, float);
void polyder(float*, int, float*);
 
int main() {
 
    int n;
    float x, dp;
 
    cout << "Enter polynomial order: ";
    cin >> n;
    float* p = new float[n+1];
 
    cout << "Enter coefficients, starting with the highest power: ";
    for (int i=0; i<n+1; i++) cin >> p[i];
 
    cout << "Enter x = ";
    cin >> x;
 
    cout << "Value = " << polyval(p, n, x) << endl;
 
    cout << "Derivative = " << dp << endl;
 
    return 0;
}
 
 
float polyval(float* p, int n, float x) {
    float px=0;
 
    for (int j=n; j>0; j--)
    {
        for (int i=0; i<n+1; i++)
        {
            px = p[i]*pow(x,j);
        }
    }
    return px;
}
 
 
void polyder(float* p, int n, float* dp) {
    dp={0};
    for(int i=0; (i=n); i++) {
        //dp += (n - i) * pow(p[i], ((n - i) - 1));
    }
}
 

[weaknessforcats]

This code:

float polyval(float* p, int n, float x) {
    float px=0;
 
    for (int j=n; j>0; j--)
    {
        for (int i=0; i<n+1; i++)
        {
            px = p[i]*pow(x,j); <<<<<????
         }
    }
    return px; <<<<????
 }
 

appears to calculate a float px over and over and overstores the previous calculated value. It looks like the last calculation is returned.

There may be other stuff but this code looks odd.

[redsox595]

How could I fix it? I am new to C++ and I want px to be the value of the polynomial when evaluated at x.

[weaknessforcats]

You means like a summation?

If so, you can code:

px = px + p[i]*pow(x,j); 

or you can use a combination operator that does the same thing:

px += p[i]*pow(x,j); 

[redsox595]

I have figured out the polyval function. Do you have any ideas as to how I could write a function to differentiate a polynomial of the form
p(x) = p(1) * x^(n) + p(2) * x^(n-1) + p(3)*x^(n-2) + ... + p(n)*x + p(n-1)

[weaknessforcats]

I'm not a math major but how does this look? I may not have it correctly.

double polyval(double* p, int n, double x) {
    double px = 0.0;
 
    for (int i = 0; i < n - 1; ++i)
    {
    px += p[i] * pow(x, n - i);    //
    }
 
 
    return px;
}