Which mldivide ('\') function is better

Dear,

I have a 9x9 float matrix M, when I use it to \ function and got pretty much diferent result (P) between Matlab 2008 and 2015.

P = M \ eye(size(M))

My question is - Which one is better ? mldivide() in 2015 or mldivide() in 2008 Matlab version ? (suppose later is better ?)

Thanks,

--Dan

Answer 1

[Bruno Luong]

Use pseudo inverse PINV to get more stable solution. Never use "\" on nearly singular matrix. You are happy with 2008 is just pure luck.

Answer 2

[the cyclist]

I highly doubt that a bug was found in mldivide later than 2008, so I doubt the later function is "more correct" or "more accurate". Instead, I suppose that there was some minor algorithmic change (e.g. some kind of time optimization).

How big is the difference between the two results? I'm guessing it is very small -- the order of normal floating point error -- such that the difference is down around the level of the overall accuracy of the calculation in the first place.

Can you upload the matrix M in a MAT file?

Answer 3

[Dan NG]

I have to type it for you.

the results differences about 1.0+e19 for each (if not zero)

[Walter Roberson]

save -ascii -double and copy/paste that. It will not be bit-for-bit accurate (you need one more digit than is used by -ascii -double for bit-for-bit accuracy), but it should be enough for us to see 1E19 differences easily.

[Walter Roberson]

Differences that big suggest that either you are working with values near 1E-20 or else that your matrix is nearly singular.

[Dan NG]

Yes, my matrix is nearly singular. So far I'm happy with results from MatLab 2008 but it is so different in 2015 MatLab. I try to find a solution without redo my 9x9 matrix.