How to further define a function?

Hey guys, I'm writing a program and I have a section there where I need to define a function that looks like half a U letter (cutting it vertically). Now, what I have is the part where I define the curve and the part going up. It's like this:

function y = CreateFullSection(obj, x)
            % Full Section curves
            % ******* UNDER DEVELOPMENT ****

              xFS=obj.sacXAtMaxArea;
              R = ((obj.Hs(xFS)*obj.B/2 - obj.SAC(xFS))/(1.0 - pi/4.0))^0.5;

              if ((x >= 0) && (x < R))
                  y = obj.B/2 - R + (2*R*x - x^2)^0.5;
              elseif (x >= R)
                  y = obj.B/2;
              end;

The value B is a constant and R is the curvature radius tht is calculated using functions that are done and working.

I'm plotting this using the following code:

 function fig = PlotFullSect(obj, hull)
            % Plot Full Sections
            % [-1, +1]

            fig = figure;
            set(fig,'units','points','position',[0,0,600,300]);

            numPts = 50;

            x = linspace(0,obj.D,numPts);
            y = zeros(1,numPts);
            for i=1:numPts
                y(i) = obj.CreateFullSection(x(i));
            end;

            plot(y,x);
            title('Full Section');

plot(y,x) because the axes need to be switched, it is not a mistake. this gives me a line that starts at around 46 (= obj.B/2 - R) on the horizontal axis and then ha the radius and the straight part going up. And they seem to be correct.

what I need help with is to define this function to include the part where the fucntion = zero from 0 to 46.

Any ideas?

Thank you