Index in position 2 exceeds array bounds (must not exceed 3).

Can anyone please explain me the mistake here, I am not able to understand the error.

Theta_n=(theta:(phi/n):(theta+phi));
nNN=size(Theta_n,2);
x=zeros(n+1,1);
y=zeros(n+1,1);
for j=1:m+1
    for i=((j-1)*nNN+1):j*nNN
        x(i,1)=R*cos(Theta_n(1,i));
        y(i,1)=R*sin(Theta_n(1,i));
    end

Answer 1

[Walter Roberson]

We are not told what n is, but we can deduce from evidence that n = 2

syms theta phi
n = 2

n = 2

Theta_n=(theta:(phi/n):(theta+phi));
size(Theta_n)

ans = 1×2

1 3

nNN=size(Theta_n,2)

nNN = 3

and we see that nNN is n+1.

We do not know what m is, so let us say m = 1 for the moment. We do not know what R is

m = 1

m = 1

syms R

x=zeros(n+1,1,'like',R);
y=zeros(n+1,1,'like',R);
for j=1:m+1
    for i=((j-1)*nNN+1):j*nNN
        [j,i]
        x(i,1)=R*cos(Theta_n(1,i));
        y(i,1)=R*sin(Theta_n(1,i));
    end
end

ans = 1×2

1 1

ans = 1×2

1 2

ans = 1×2

1 3

ans = 1×2

2 4

Error using sub2ind (line 55)
Out of range subscript.

Error in sym/subsref (line 904)
R_tilde = sub2ind(size(L), Idx.subs{:});

Your code is effectively trying to access Theta_n as if it were a vector intended to represent an array with nNN = (n+1) {=3 here} rows, and m+1 columns. But that is a problem because you can be sure that Theta_n only has n+1 elements, so it cannot be used to emulate an array with more than one column. If your m is greater than 0, then j can be greater than 1, and you will have problems.

And really it is a waste to hae those loops. Just do

x = reshape(R * cos(Theta_n), nNN, []);
y = reshape(R * sin(Theta_n), nNN, []);

with no looping. You will still only end up with one column in x and y, but you will do so without an error message.

If you are relying upon there ending up with m+1 columns in x and y, then your definition of Theta_n would have to change, such as if you were adding multiples of phi.

[S Priya]

Thank you for the help sir.

I have used

nNN=size(Theta_n,2);

x = reshape(R * cos(Theta_n), nNN, []);

y = reshape(R * sin(Theta_n), nNN, []);

for x and y co-ordinates. Can I use it for z- co-ordinate too? Can you please elaborate how reshape function works?

for z- co-ordinates to be used. I have used

z=zeros(nNN,3,m+1);

for j=1:m+1

z(1:nNN,1,j)=x(:);

z(1:nNN,2,j)=y(:);

z(1:nNN,3,j)=(j-1)*L/m;

end