problem with For loop

Hi, i have written this code, but not sure why I don't get anything in my plot. I was wondering if someone could point out what I doing wrong. I am pretty much trying to plot values for certain v_dot values.

global x1lco x2lco

alpha12 = -2.086211466368665;
alpha22 = 1.378057393962953;
alpha11 = -0.082454051332553;
alpha21 = 0.395565864578256;
alpha0 = 0.641764119934185;

g = (x1lco.^2 + x2lco.^2).^2 + alpha12.*x1lco.^2 + alpha22.*x2lco.^2 + alpha11.*x1lco + alpha21.*x2lco + alpha0;

i = 0
for phi = 0:.01:2*pi;
    flag = 0;
    i = i + 1;
    j = 0;
    for r = .01:0.01:3;
        j = j + 1;
        v_dot = -1.*r.^2.*(g + 2.*(alpha22 - alpha12).*sin(phi).*cos(phi)) -1.*r.*(alpha21.*cos(phi)- alpha11.*sin(phi)); 
        if v_dot >=0 & flag == 0
            r_b(i) = r;
            phi_b(i) = phi; 
            flag = 1
        end
    end

end

x1 = r.*cos(phi);
x2= r.*sin(phi);

plot (x2,x1)

Answer 1

[Kye Taylor]

You are only plotting a single point... That is, the variables r and phi are only scalars (1x1 doubles) when you define x1 and x2. Therefore x1 and x2 are also simply scalars.