cov in R2009b vs. R2010a

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D on 29 Jun 2011
I used the cov function in R2009b and in R2010a and it returned different results.
I am aware of the fact that there are multiple functions named cov in Matlab, depending on your toolboxes; both functions are in the directory matlab/datafun
The revision date on both functions is $Revision: $ $Date: 2009/03/16 22:17:40 $.
What could be producing this discrepancy?
edit: R2009b is on a Mac (OS X 10.5.8); R2010a is on a PC (7).
  1 Comment
Laura Proctor
Laura Proctor on 29 Jun 2011
Could you post the code that you ran as well as the results?

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Accepted Answer

Walter Roberson
Walter Roberson on 29 Jun 2011
This is not entirely unexpected. The BLAS toolboxes are optimized for specific processors, so differences in processors can result in differences in calculations. Likewise, differences in the JIT of the different versions of MATLAB can result in the same code producing different results.
If the differences are small relative to the results, the situation is probably normal.

More Answers (1)

D on 29 Jun 2011
Sorry, it seems to be a question of precision. I subtracted the two matrices for comparison, but I missed the factor of 1.0e-15 that multiplied the resulting matrix.
  1 Comment
Sean de Wolski
Sean de Wolski on 29 Jun 2011
"Time for another cup of coffee" moment.

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