"Index exceeds the number of array elements"

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Tom Keaton 2020-1-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/502486-index-exceeds-the-number-of-array-elements

评论： Tom Keaton 2020-2-4

采纳的回答： Matt J

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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

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Walter Roberson 2020-1-29

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.

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采纳的回答

Matt J 2020-1-29

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编辑：Matt J 2020-1-29

在 MATLAB Online 中打开

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

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Walter Roberson 2020-2-1

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 2020-2-4

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.

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Walter Roberson 2020-2-1

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https://ww2.mathworks.cn/matlabcentral/answers/502486-index-exceeds-the-number-of-array-elements#answer_413288

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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.