MATLAB: Inv matrix command error

invmatrix

I am using a code my TA gave us to use for our lab report (had to edit some of his code to make it work so far) and he wants us to use the inv command and I have no experience with this. Any ideas on what to adjust to make it work?

Best Answer

Normally the inv is applied to the left hand side coefficients, since people usually write the "A" matrix on the left and the "b" vector on the right in an Ax=b linear system. So I am guessing you need to change the inv( ) calculation to this:

V = inv(LHS)*RHS;

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MATLAB: Compare inv(X) Vs mex inv (X)

I think it's rather likely with a difference of that magnitude that you have left memory integrity checks on. Try

>> cfg = coder.config('mex');
>> cfg.IntegrityChecks = false;
>> cfg.ResponsivenessChecks = false;
>> codegen test_inv -args {zeros(100)} -config cfg
>> x = randn(100);
tic;
for n = 1: 100
    y = test_inv(x);
end
toc;
tic;
for n = 1: 100
    y = test_inv_mex(x);
end
toc;
Elapsed time is 0.082537 seconds.
Elapsed time is 0.109494 seconds.

The remaining difference is not surprising since there is some overhead and we are essentially just comparing the compiled and optimized MATLAB implementation to the compiled MATLAB Coder implementation. MATLAB Coder is not intended to speed up MATLAB linear algebra. They are mostly compiled and highly optimized routines. If it could do much of that, we'd just speed up the MATLAB implementation. If you want to see it speed something up, you usually need to have a substantial amount of your code in there. Also, in this particular case there are some differences in the algorithms. MATLAB Coder is generating unblocked code, and here the matrix is large enough that I would rather use a blocked algorithm.

MATLAB: Wrong output from eigs function

According to the documentation of eigs(), by default, it returns 6 largest eigenvalues and their corresponding eigenvectors. Matrix V contains the eigen values along the diagonal, whereas matrix A contains six eigen vectors, one in each column.

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MATLAB: Compare inv(X) Vs mex inv (X)

MATLAB: Wrong output from eigs function

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