MATLAB: Fast conversion of 2 matrices to 1 complex matrix

complexmemory allocation

Hi,
After reading my data from an external binary file, I have a 3 dimensional array representing one complex matrix, like this:
A(1,:,:) = rand(10);    % real part
A(2,:,:) = rand(10);    % imaginary part
I need to convert that into a complex matrix, e.g.
A = complex(A(1,:,:), A(2,:,:));
But this is rather slow. As far as I have been able to find out, Matlab stores complex matrices internally as two matrices, one for the real and one for the imaginary part. This is about the same as my original 3-dimensional matrix — and should be quite fast! My assumption therefore is that Matlab uses a temporary variable which of course needs to be allocated, resulting in slow code.
Is it possible to make Matlab do this conversion without allocating more memory? Maybe just setting the 'complex' attribute to A… It might be possible using mex files, but I never used them before, and I would prefer a 'pure' Matlab solution.
Any comments, hints or keywords I can google for are welcome!
Regards Argon

Best Answer

For your specific example, the A(1,:,:) and A(2,:,:) data blocks are contiguous memory and, more importantly, were both allocated with a single malloc (or similar) call in the background. They cannot be legally separated into separate real and imaginary parts of another variable. You will be required to copy the data as you (and Wayne King) are doing.
Regarding mex routines, there are ways to do stuff like this. I have code (as yet unpublished to the FEX) that can pretty much do what you are asking, but it involves manipulating the mxArray structure behind the scenes (i.e., not using the official API functions). Also, for your particular case, one would have to keep a shared data copy of the new complex variable locked up in the mex routine to prevent MATLAB from trying to free the imaginary part (if that ever happened MATLAB would crash). To clear it you would have to manually reverse the process that you did to create it. So it gets to be very tricky to manage, but technically it can be done. Can you redo the part of your code that creates A and instead put the A(1,:,:) and A(2,:,:) parts into separate variables from the outset? If so, then what you are asking becomes much easier to do in a mex routine (but still requires unofficial behind the scenes techniques).
Related Question