Problem with while loop

Hello,

The goal of my code is to plot the function for a Tsai-Wu surface which is supposed to be a ellipsoide, the problem arrises with my while loop which does not seem to be doing anything. I've simplified the equation, but as you can see the value for my variable G can never excede 1. Any help or a pointer in the right direction would be really appreciated, thanks!

sig1c = -1.5; %always set compression as negative
sig1t = 1; %always set tension as positve
sig2c = -1.5;
sig2t = 1;
sig3c = -2;
sig3t = 0.5;

F1 = ((1/sig1t)-(1/sig1c));
F2 = ((1/sig2t)-(1/sig2c));
F3 = ((1/sig3t)-(1/sig3c));
F11 = (1/(sig1c*sig1t));
F22 = (1/(sig2c*sig2t));
F33 = (1/(sig3c*sig3t));

syms sig1 sig2 sig3 G
% sig1 = 0;
% sig2 = 0;
% sig3 = 0;
%[sig1,sig2,sig3] = meshgrid(linspace(-5,5,25));

%G = F1.*sig1 + F11.*sig1.^2 + F2*sig2 + F22.*sig2.^2 + F3.*sig3 + F33.*sig3.^2 - 1;

    [sig1,sig2,sig3] = meshgrid(linspace(-5,5,25));
while G < 1
    G = F1.*sig1 + F11.*sig1.^2 + F2*sig2 + F22.*sig2.^2 + F3.*sig3 + F33.*sig3.^2 - 1;
end

    fv = isosurface(sig1,sig2,sig3,G,0);
    fv = isosurface(G,0);
    fvc = isosurface(G,'red');
    p = patch(isosurface(sig1,sig2,sig3,G,0));
    %isonormals(sig1,sig2,sig3,G,p)
    %isonormals(sig1,sig2,sig3,G,p)    

    view(3);

[DGM]

What is the intent of the loop? The value of G isn't different on subsequent passes. It will either loop once or it will loop infinitely.

[Kevin Hanekom]

Im new to programing so alot of this is blury to me but the goal is to use different values of sig1, sig2, and sig3, up to a point where g is <=1, and then plot those points forming a ellipsoid

Answer 1

[Chunru]

Change

while G < 1
    G = F1.*sig1 + F11.*sig1.^2 + F2*sig2 + F22.*sig2.^2 + F3.*sig3 + F33.*sig3.^2 - 1;
end

to:

G = F1.*sig1 + F11.*sig1.^2 + F2*sig2 + F22.*sig2.^2 + F3.*sig3 + F33.*sig3.^2 - 1;

[Kevin Hanekom]

Thanks! But wouldnt that still allow G to be greater then 1 when we plot?

[Chunru]

Since you are ploting isosurface of G==0, it does not matter if some values of G is greater than 1 or not. Howver if you do really want to clip the G to be some value (say 1), then you can add this:

G(G>1) = 1;

[Kevin Hanekom]

Great thank you for helping me! You guys are what make this community so great.