MATLAB: How to position groups of matrices inside a bigger matrix

indexingmatrixmatrix manipulation

I have a program in which I have some matrices of varying size that runs through a piece of code, for the current moment i only have two matrices. What i want to know is how would i place the matrices 'M' in a bigger matrix of zeros in a way that the values always stay in the diagonal and the rest of the matrices are zeros. So, for example if a have two matrices with size 4×4 the matrix would be a 8×8 matrix with [1:4,1:4] and [5:8,5:8] being values and the rest zeros. I want to do it in a way that i can do it for n-matrices. Here is the code i use.

for iter=1:2
    D=Stations{iter};
    n=length(D);
    c=[0;0;0;0];
    rowvector=ones(1,n);
    dt0=rowvector
    dn=sqrt((D(1,:)-m0(2)).^2+(D(2,:)-m0(3)).^2+(D(3,:)-m0(4)).^2);
    dtx = -(D(1,:)-m0(2))./(v.*dn);
    dty = -(D(2,:)-m0(3))./(v.*dn);
    dtz = -(D(3,:)-m0(4))./(v.*dn);
    M=[dt0; dtx; dty; dtz];
end

'Stations' is a cell array i use to store the different matrices

Best Answer

blkdiag function is the tool you are looking for. Help it in MATLAB. Note you can use a loop within which the function is called iteratively to cope with n sub-matrices.

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Use an if statement

if iter == 1
    dn=sqrt((D(1,:)-m0(2)).^2+(D(2,:)-m0(3)).^2+(D(3,:)-m0(4)).^2);
    dtx = -(D(1,:)-m0(2))./(v.*dn);
    dty = -(D(2,:)-m0(3))./(v.*dn);
    dtz = -(D(3,:)-m0(4))./(v.*dn);
else
    dn=sqrt((D(1,:)-m0(6)).^2+(D(2,:)-m0(7)).^2+(D(3,:)-m0(8)).^2);
    dtx = -(D(1,:)-m0(6))./(v.*dn);
    dty = -(D(2,:)-m0(7))./(v.*dn);
    dtz = -(D(3,:)-m0(8))./(v.*dn);
end

However note that you set m0 to all zeros on every iteration so I don't see why you're even wanting to use it.

MATLAB: How do i create a program that, depending on the input value would create more or less variables

Forget about dynamically accessing variable names, simply vectorize your code and you will immediately be on the way to writing simpler, more efficient MATLAB code. Rather than these:

da=((A(1,1)-m0(2))^2+(A(2,1)-m0(3))^2+(A(3,1)-m0(4))^2)^(1/2);
db=((A(1,2)-m0(2))^2+(A(2,2)-m0(3))^2+(A(3,2)-m0(4))^2)^(1/2);
dc=((A(1,3)-m0(2))^2+(A(2,3)-m0(3))^2+(A(3,3)-m0(4))^2)^(1/2);
dd=((A(1,4)-m0(2))^2+(A(2,4)-m0(3))^2+(A(3,4)-m0(4))^2)^(1/2);

Just write this:

dV = sqrt((A(1,:)-m0(2)).^2+(A(2,:)-m0(3)).^2+(A(3,:)-m0(4)).^2);

And rather than these:

dtax0=-(A(1,1)-m0(2))/v*da;
dtbx0=-(A(1,2)-m0(2))/v*db;
dtcx0=-(A(1,3)-m0(2))/v*dc;
...

do something like this (the exact code you need depends on the size of v):

dtx0 = -(A(1,:)-m0(2))./v.*d;

or even do all of x, y and z at once, something like:

dt0 = -bsxfun(@minus,A(1,:),m0(:))./v.*d;

The actual operations you require depend on the sizes and classes of the arrays.

Knowing how to manipulate matrices/arrays is fundamental to using MATLAB. The name MATLAB comes from "MATrix LABoratory" and not from "puts the data into lots of separate variable and make it difficult to work with". The introductory tutorials are a good place to start learning how to use matrices effectively:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

You should also read these:

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

Experiment! The more you practice, the more it will make sense and the better you can use matrices.

Best Answer

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