In an assignment A(I) = B, the number of elements in B and I must be the same.

Good day,

I am trying to numerically solve two differential equations and run into the error "In an assignment A(I) = B, the number of elements in B and I must be the same." at d_dt(2)=2*w.*r.*thetap+((w^2)+(thetap.^2)).*r;

Script:

R=1000000;

w=0.0031413613;     %0.18 degrees in radians

Tr=2000;

Ac=(w^2)*R;

vp=w*R;

m=1;

d=0.1;

c1=(0.00155)*d;

[t,xmat]=ode45(@funcfin, [0 300], [0 vp 0 0]);

h=R-xmat(:,1);

figure(1)
plot(h,xmat(:,3))
xlabel('height')
ylabel('phi')
title('height vs phi')

Function file:

function d_dt=funcfin(t,g)

global w

r=g(1);
rp=g(2);
theta=g(3);
thetap=g(4);

d_dt=zeros(4,1);

d_dt(1)=rp;
d_dt(2)=2*w.*r.*thetap+((w^2)+(thetap.^2)).*r;
d_dt(3)=thetap;
d_dt(4)=((-2.*rp)./r).*(w+thetap);

Thank you for your time.

Answer 1

[Star Strider]

I could not replicate the error you got. I suspect the reason was that you have to declare the global variables in your main script and your function, and you did not declare it in your main script. This made in an empty variable in your ODE function.

I avoid global variables unless absolutely necessary, so I began by adding ‘w’ as a third argument to your function, and made appropriate changes in the ode45 call. I also changed your initial conditions from zero to eps because the values that began with zero remained at zero, including the ones you want to plot.

The changed ode45 call:

[t,xmat]=ode45(@(t,g) funcfin(t,g,w), [0 300], [0 vp 0 0]+eps);

Only the first line in the ‘funcfin’ function changes:

function d_dt=funcfin(t,g,w)

Other than these changes, your code remains the same, and runs without error.

[Nicholas Zamora]

Thank you so much.

[Star Strider]

My pleasure!

Answer 2

[Brendan Hamm]

It appears in this code that all of the values on the RHS are scalars, so this should not be an issue. That being said something is obviously happening, so you should use the debugging tools in MATLAB to diagnose this. In particular set a breakpoint at the line that causes the error and execute the code again. This will allow you to look into the workspace of the function and see if any of the variables in the line are not scalar values (possibly complex values which are stored as a 2 element vector).

Other thoughts on this code:

1. Why do you use .* if you expect these variables (r,*w*,*thetap*) to be scalars instead of just using *?

2. Why use a global variable?

In general it is not recommended to use global variables if you can avoid them. Here is how you avoid it here:

function d_dt = funcfin(t,g,w)
   r=g(1);
   rp=g(2);
   theta=g(3);
   thetap=g(4);
   d_dt= [ rp; 2*w.*r.*thetap+((w^2)+(thetap.^2)).*r; thetap; ((-2.*rp)./r).*(w+thetap)];
end

Now your function knows about w, but many "function-functions" like ode solvers cannot deal with these extra inputs. For this reason we have anonymous functions:

w = toRadians('degrees',0.18); % Get the full precision of a double value
myFuncFin = @(t,g) funcfin(t,g,w);

This will behave exactly like your previous function, except there is no need to have a global variable, we can embed the value of w directly into the function's workspace. Now you can pass this function to the ode solver:

[t,xmat] = ode45(@myFuncFin, [0 300], [0 vp 0 0]);

Global variables can make debugging very difficult, so I recommend changing your code to the above before attempting the debugger.

Good Luck!