Create array of variables in Matlab

Question

In the M-file "matrixT.m", i wrote the function matrixT to generate a n*3 matrix like that

function T=matrixT(alpha,n)
T=zeros(3,n);
for i=1:n
    T(1,i)=cos(alpha(i));
    T(2,i)=sin(alpha(i));
    T(3,i)=sin(alpha(i)-i*pi/2);
end

Then i use it in my main M-file

alpha=sym('alpha');
V=subs(hessian(det(matrixT(alpha,3)),alpha),alpha,alpha0);

but there are many error. Can you help me to fix this?

aardvarkk · Accepted Answer · 2012-04-25 17:36:20Z

3

I believe your problem lies in the assignment: T(1,i)=cos(alpha(i));. You have assigned alpha to be a symbolic variable, but then you try to assign it to an array of doubles and MATLAB complains. Is it possible to evaluate the value of alpha before attempting to place it inside the array T?

answered Apr 25, 2012 at 17:36

aardvarkk

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6 Comments

minhbsu Over a year ago

How can I assign an array of variables for alpha?

aardvarkk Over a year ago

You'll probably want to use your subs function earlier to substitute a real value for alpha before you try to put it into the T array.

minhbsu Over a year ago

the problem is, i must assign the vector alpha (n-dimensional) and take the hessian? can i do it? Could you make an example for me?

aardvarkk Over a year ago

alpha is not a vector, it's a symbolic variable. If you have to calculate the Hessian multiple times using different values of alpha, I would recommend placing the Hessian calculation inside a for loop, and in each loop iteration set the value of alpha and then calculate what you want.

aardvarkk Over a year ago

If you only have one value for alpha, then don't make it a symbolic variable. Just use the value of alpha to make your T matrix and everything should be fine. However, if you have multiple values for alpha, you'll need to evaluate V in a loop because it will take on different values for different values of alpha.

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