Thread: Pointer help

Originally Posted by Hodor

Looking at it again I was mistaken. I didn't notice that the "coefficients" were being added backwards and the implementation is using methods described here: Horner's method - Wikipedia

I don't know which of the below implementations below I prefer, but I do know that comments should be added (if there was comments in the initial code I'd not have initially thought it was implemented incorrectly, for example )

Code:

#include <stdio.h>
#include <math.h>

double calc_abv(double abw);
double calc_abv2(double abw);
double calc_abv3(double abw);

int main()
{
    for (double abw = 0.0; abw <= 1.0; abw += 0.1) {
        printf("%lf, %lf, %lf, %lf\n", abw, calc_abv(abw), calc_abv2(abw), calc_abv3(abw));
    }
    return 0;
}


double calc_abv(double abw)
{
    const double a = -0.000039705486746795932;
    const double b =  1.2709666849144778;
    const double c = -0.40926819348115739;
    const double d =  2.0463351302912738;
    const double f = -7.8964816507513707;
    const double g =  15.009692673927390;
    const double h = -15.765836469736477;
    const double i = 8.8142267038252680;
    const double j = -2.0695760421183493;


    double temp = j;
    temp = temp * abw +i;
    temp = temp * abw +h;
    temp = temp * abw +g;
    temp = temp * abw +f;
    temp = temp * abw +d;
    temp = temp * abw +c;
    temp = temp * abw +b;
    double abv = temp * abw +a;

    return abv;
}

double calc_abv2(double abw)
{
    const double a = -0.000039705486746795932;
    const double b =  1.2709666849144778;
    const double c = -0.40926819348115739;
    const double d =  2.0463351302912738;
    const double f = -7.8964816507513707;
    const double g =  15.009692673927390;
    const double h = -15.765836469736477;
    const double i = 8.8142267038252680;
    const double j = -2.0695760421183493;

    return  a +
            b * abw +
            c * pow(abw, 2) +
            d * pow(abw, 3) +
            f * pow(abw, 4) +
            g * pow(abw, 5) +
            h * pow(abw, 6) +
            i * pow(abw, 7) +
            j * pow(abw, 8)
    ;
}

double calc_abv3(double abw)
{
    static const double coefficients[] = {
        -0.000039705486746795932,
        1.2709666849144778,
        -0.40926819348115739,
        2.0463351302912738,
        -7.8964816507513707,
        15.009692673927390,
        -15.765836469736477,
        8.8142267038252680,
        -2.0695760421183493
    };

    double abv = coefficients[0];
    double abw_mult = abw;
    for (size_t i = 1; i < sizeof(coefficients) / sizeof(coefficients[0]); ++i)
    {
        abv = abv + coefficients[i] * abw_mult;
        abw_mult *= abw;
    }
    return abv;
}

Edit: or laserlight's implementation, of course