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

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S Priya on 19 Aug 2021
Commented: S Priya on 19 Aug 2021
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

Walter Roberson on 19 Aug 2021
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 on 19 Aug 2021
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

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