Line x and y n-intercept point / C++

Can someone please help me. I understand the equation of a line and how to solve for the zero intercept on paper, but I have trouble converting it to code. More specifically, I need to calculate the point at which a line intercepts any given X or Y coordinate with two different functions...

double CalcLineYIntercept(LINE *line, double yintercept) { }
double CalcLineXIntercept(LINE *line, double xintercept) { }

So, CalcLineYIntercept would return the X coordinate of the point where the line intercepts yintercept (not necessarily zero). I have trouble converting algebraic equations to code (and yes I understand that C++ is an algebraic language, but the code itself does not simply and isolate variables). Is there a simple way to accomplish this?

Thank you ve开发者_C百科ry much

double CalcLineYIntercept(LINE *line, double yintercept) 
{ 
    dx = line->x2 - line->x1;
    dy = line->y2 - line->y1;

    deltay = yintercept - line->y2;
    if (dy != 0) { 
        //dy very close to 0 will be numerically unstable, account for that
        intercept = line->x2 + (dx/dy) * deltay;
    }
    else {
        //line is parrallel to x-axis, will never reach yintercept
        intercept = NaN;
    }
}

Reverse x and y to get the other function.

Subtract yintercept from the line's y-coordinates, then use the math you already know to solve for x when y = 0. Mutatis Mutandis for xintercept.