Unexpected loss of precision when dividing doubles

Question

I have a function getSlope which takes as parameters 4 doubles and returns another double calculated using this given parameters in the following way:

double QSw         


        

Answer 1

Patrick seems to be right about (c-a) being the main cause:

d-b = -1,72219 - (-1,64161) = -0,08058

c-a = 2,70413 - 2,71156 = -0,00743

S = (d-b)/(c-a)= -0,08058 / -0,00743 = 10,845222

You start out with six digits precision, through the subtraction you get a reduction to 3 and four digits. My best guess is that you loose additonal precision because the number -0,00743 can not be represented exaclty in a double. Try using intermediate variables with a bigger precision, like this:

double QSweep::getSlope(double a, double b, double c, double d)
{
    double slope;
    long double temp1, temp2;

    temp1 = (d-b);
    temp2 = (c-a);
    slope = temp1/temp2;

    return slope;
}