"Index exceeds the number of array elements"

Hello,

I am recieving this error:

Index exceeds the number of array elements (1).

Error in sym/subsref (line 890)
            R_tilde = builtin('subsref',L_tilde,Idx);

Error in E_field (line 7)
Ey = matlabFunction(E(2));

Here is the function file the error is referencing:

function [Ex, Ey, Ez] = E_field()

syms x y z
R_s = 0.02;
V = 0;
epnaut = 8.854187E-12;
k = 1/(4*pi*epnaut);
Q = (V*R_s)/k;
r = [x, y, z];
E = (k*Q/norm(r)^3)*r;
Ex = matlabFunction(E(1));
Ey = matlabFunction(E(2));
Ez = matlabFunction(E(3));

end

[Walter Roberson]

The posted code works for me. However you should using the 'vars' option of matlabFunction or else your function handle is not going to accept any arguments because your R values are all constants.

Answer 1

[Matt J]

Perhaps this is what you are looking for,

function [Ex, Ey, Ez] = E_field(x,y,z)

    R_s = 0.02;
    V = 0;
    epnaut = 8.854187E-12;
    k = 1/(4*pi*epnaut);
    Q = (V*R_s)/k;
    
    r = [x, y, z];
    E = (k*Q/norm(r)^3)*r;
    Ex = E(1);
    Ey = E(2);
    Ez = E(3);
    
end

[Matt J]

Or perhaps this,

function [Ex, Ey, Ez] = E_field()

    R_s = 0.02;
    V = 0;
    epnaut = 8.854187E-12;
    k = 1/(4*pi*epnaut);
    Q = (V*R_s)/k;

    Ex = @(x,y,z)  x*(k*Q/norm([x,y,z])^3) ;
    Ey = @(x,y,z)  y*(k*Q/norm([x,y,z])^3) ;
    Ez = @(x,y,z)  z*(k*Q/norm([x,y,z])^3) ;
    
end

[Tom Keaton]

Why would using matlabFunction() not work here?

[Tom Keaton]

Also I am currently referencing this in another function as such. If I used this syntax, how would that change in the reference?

function bdip = bdipuniodefun(t,s)

%Using SI units
q = -1.60217662E-19;
m_e = 9.11E-31;

global Bx By Bz Ex Ey Ez

if isempty(Bx)
    [Bx, By, Bz] = B_test();
end

if isempty(Ex)
    [Ex, Ey, Ez] = E_test();
end

bdip = [s(4); s(5); s(6); (q/m_e)*(Ex(s(1),s(2),s(3)) + s(5)*Bz(s(1),s(2),s(3)) - s(6)*By(s(1),s(2),s(3))); (q/m_e)*(Ey(s(1),s(2),s(3)) + s(6)*Bx(s(1),s(2),s(3)) - s(4)*Bz(s(1),s(2),s(3))); (q/m_e)*(Ez(s(1),s(2),s(3)) + s(4)*By(s(1),s(2),s(3)) - s(5)*Bx(s(1),s(2),s(3)))];

end

[Walter Roberson]

Your norm is 0. Multiply by x y z and you get 0 0 0. When you matlabFunction that without a vars option then matlab sees that the symvar([0 0 0]) is empty and decides that what is wanted is a function handle that takes no parameters and returns a 0 of constant size. If you provided a vars option you at least would get back a function handle that permitted parameters when it returned the scalar 0.0

[Tom Keaton]

@Matt J I get a new but the same error when I try using either of your recommendations:

Index exceeds the number of array elements (1).

Error in sym/subsref (line 890)
            R_tilde = builtin('subsref',L_tilde,Idx);

Error in E_field (line 7)
Ey = matlabFunction(E(2));

Error in bdipuniodefun

Error in odearguments (line 90)
f0 = feval(ode,t0,y0,args{:});   % ODE15I sets args{1} to yp0.

Error in ode15s (line 150)
    odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in collisions7 (line 216)
    	[T,S] = ode15s(@bdipuniodefun, tspan, icv); %Matlab's ODE solver

[Walter Roberson]

Could you remind us which MATLAB version are you using?

[Tom Keaton]

MATLAB Version: 9.6.0.1099231 (R2019a) Update 1
Operating System: Microsoft Windows 10 Pro Version 10.0 (Build 18363)
Java Version: Java 1.8.0_181-b13 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode

[Walter Roberson]

Is there a reason you are using global instead of persistent ? Is there some other function that will use the created Bx By Bz Ex Ey Ez ?

[Tom Keaton]

Sorry I missed this question. The answer is yes which is why I am using it. I will show the code that causes this issue in another post.

Answer 2

[Walter Roberson]

V = 0;

V is 0.

Q = (V*R_s)/k;

V is 0, so Q will be 0.

E = (k*Q/norm(r)^3)*r;

Provided that norm(r ) is not 0 and k is not infinity, then because Q is 0, E is going to be

zeros(size(r))

symvar() of a vector of 0 is empty. matlabFunction() of a constant, when you do not use any 'vars' option, is a function handle that accepts no inputs and returns a scalar copy of the constant.

The original code that was posted does not give me an index exceeded matrix dimension when I test it in R2019a.

[Tom Keaton]

Under this specific circumstance this does work for me now.