plotting this program is causing a problem how to fix it?

clear
k = 1E-3;
[y(4),y(5),y(6)] = meshgrid(-4:0.25:4,-4:0.25:4,-4:0.25:4);

Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

dy(1) = (P - y(1).*((abs(y(4)))^2 +1))./tf;
dy(2) = (P - y(2).*((abs(y(5)))^2 +1))./tf;
dy(3) = (P - y(3).*((abs(y(6)))^2 +1))./tf;


dy(4)= (y(1)-a).*((y(4))./tc) + (k./tc).*(y(5)).*cos(y(7));
dy(5)= (y(2)-a).*((y(5))./tc) + (k./tc).*(y(4)).*cos(y(7)) + (k./tc).*(y(6))*cos(y(8));
dy(6)= (y(3)-a).*((y(6))./tc) + (k./tc).*(y(5)).*cos(y(8));


dy(7) = o(1,1) - (k./tc).*((y(4)./y(5)) + (y(5)./y(4))).*sin(y(7)) + (k./tc).*(y(6)/y(5)).*sin(y(8));
dy(8) = o(1,2) - (k./tc).*((y(5)./y(6)) + (y(6)./y(5))).*sin(y(8)) + (k./tc).*(y(4)/y(5)).*sin(y(7));

r = sqrt((dy(4)).^2 + (dy(5)).^2 + (dy(6)).^2 );
quiver(y(4),y(5),y(6), (dy(4))./r, (dy(5))./r, (dy(6))./r,1/2,'linewidth',1);
hold on;
axisa equal
set(gca,'Fontsize',20)
axis([-4 4 -4 4 -4 4])

while (true)
    y0 = ginput(1);
    ti = 0;
    tf = 1E-2;
    tspan=[ti tf];
    [T,Y]= ode45(@(t,y) rate_eq(t,y,k),tspan,y0);
    plot3(Y(:,4),Y(:,5),Y(:,6));
    xlabel("A1");
    ylabel("A2");
    zlabel("A3");
end


function dy = rate_eq(t,y,k)

dy = zeros(8,1);

P = 0.2;
a = 0.1;
tf = 230E-6;
tc = 30E-9;
o(1,1) = 3E4;
o(1,2) = 4E4;


dy(1) = (P - y(1).*((abs(y(4)))^2 +1))./tf;
dy(2) = (P - y(2).*((abs(y(5)))^2 +1))./tf;
dy(3) = (P - y(3).*((abs(y(6)))^2 +1))./tf;


dy(4)= (y(1)-a).*((y(4))./tc) + (k./tc).*(y(5)).*cos(y(7));
dy(5)= (y(2)-a).*((y(5))./tc) + (k./tc).*(y(4)).*cos(y(7)) + (k./tc).*(y(6))*cos(y(8));
dy(6)= (y(3)-a).*((y(6))./tc) + (k./tc).*(y(5)).*cos(y(8));


dy(7) = o(1,1) - (k./tc).*((y(4)./y(5)) + (y(5)./y(4))).*sin(y(7)) + (k./tc).*(y(6)/y(5)).*sin(y(8));
dy(8) = o(1,2) - (k./tc).*((y(5)./y(6)) + (y(6)./y(5))).*sin(y(8)) + (k./tc).*(y(4)/y(5)).*sin(y(7));
end
% is my algorithm is correct in this? please modify if thereis any possible scenario.

Answer 1

[Walter Roberson]

[y(4),y(5),y(6)] = meshgrid(-4:0.25:4,-4:0.25:4,-4:0.25:4);

You want y(4), y(5), y(6) to each represent a matrix of values. You cannot do that with () indexing, but you could do

[y{4},y{5},y{6}] = meshgrid(-4:0.25:4,-4:0.25:4,-4:0.25:4);

Then you have

dy(1) = (P - y(1).*((abs(y(4)))^2 +1))./tf;

ON the right you would have to change that y(4) to y{4}, like

(P - y(1).*((abs(y{4}))^2 +1))./tf;

and then you would have to adjust the ^2 for the fact that you are working with matrices

(P - y(1).*((abs(y{4})).^2 + 1))./tf;

but you have not defined y(1) at this point. If you did use [y{4},y{5},y{6}] = meshgrid(etc) above then y(1) would have been created implicity, but it would be an empty cell and so

dy(1) = (P - y{1}.*((abs(y{4})).^2 + 1))./tf;

would be calculating with empty y{1} and so the calculation is going to return an empty array as a result, so you would further need

dy{1} = (P - y{1}.*((abs(y{4})).^2 + 1))./tf;

but that is a complicated way to calculate what is going to be dy(1) = {[]};

Well, except for the fact that P is not defined at that point, and tf is not defined either.

dy(4)= (y(1)-a).*((y(4))./tc) + (k./tc).*(y(5)).*cos(y(7));

a and tc and k are not defined either... and you have not assigned anything to y(7) ...

dy(7) = o(1,1) - (k./tc).*((y(4)./y(5)) + (y(5)./y(4))).*sin(y(7)) + (k./tc).*(y(6)/y(5)).*sin(y(8));

o is not defined either...

It looks to me as if you are trying to create a bunch of formulas for differential equations. If you were to syms various variables into existence and use syms y [1 7] then you could create the formuals. But you would not be able to plot the formulas until you substituted in numeric values for the symbolic variables.

I recommend that you have a look at the Symbolic Toolbox, in particular at the work-flow used in the first example for odeFunction

[SAHIL SAHOO]

I tried and there was lot of chaos I will leave it here though and thanks for help.