Determine number of bytes per element under program control

Does MATLAB have the equivalent of the C/C++ 'sizeof' function?

I have some code that can run on many numeric types, and I need to be able to calculate the number of bytes per element in a given numeric array, let's call it 'A'

bytesPerElement = sizeof(class(A));

I suppose I could do something like:

w = whos(A);
bytesPerElement = w.bytes / numel(A);

but I would be surprised if there was not a function that calculates the number of bytes per element directly from the variable class. I just can't seem to find that function ... entering the wrong words into the search engine, I suppose.

Answer 1

[dpb]

No sizeof directly that I know of, no. Wouldn't be hard to build one from isa, however.

An outline would be something otoo--

function bytes = sizeof(x)
  % return size in bytes of numeric entity x

    bytes = (isa(x,'double') | isa(x,'int8') | isa(x,'uint8'))*8 + ...
            (isa(x,'single') | isa(x,'int4') | isa(x,'uint4'))*4 + ...
             ...

The rest should be obvious... :)

doc isa  % for the various class choices

Some checking for invalid types would be good, of course...

[Daniel Lyddy]

I ended up going the 'whos' route:

function bytes = bytesPerElement(x)

    w = whos('x');
    bytes = w.bytes / numel(x);

end

Simple enough, I guess, but I'm still perplexed that something like this doesn't exist already. I figured that such a function would have been used to calculate the 'bytes' field of the struct returned by whos, but since that function is mex-ified, I can't see what it's doing under the hood.

[dpb]

I tend to forget about the command form for whos ... good thought; simpler than the isa route, indeed.

[Oliver Woodford]

Uint8 means 8 bits, so only 1 byte. A switch on class(x) is another option.

Answer 2

[James Tursa]

There is a mex function for this, mxGetElementSize, but it doesn't do you much good at the m-file level:

http://www.mathworks.com/help/matlab/apiref/mxgetelementsize.html?searchHighlight=mxgetelementsize

Another way to do this assuming x is not empty (probably not any faster):

numel(typecast(x(1),'uint8'))

[dpb]

Wrap the mex-call in a mex-routine???

[James Tursa]

As requested:

#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    if( nrhs ) {
        plhs[0] = mxCreateDoubleScalar(mxGetElementSize(prhs[0]));
    }
}