Determine number of bytes per element under program control

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Daniel Lyddy
Daniel Lyddy 2014-7-2
评论： Oliver Woodford 2015-11-13
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.

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回答（2 个）

dpb
dpb 2014-7-3
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...
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dpb
dpb 2014-7-4
I tend to forget about the command form for whos ... good thought; simpler than the isa route, indeed.
Oliver Woodford
Oliver Woodford 2015-11-13
Uint8 means 8 bits, so only 1 byte. A switch on class(x) is another option.

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James Tursa
James Tursa 2014-7-3
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'))
  2 个评论
James Tursa
James Tursa 2014-7-5
As requested:
#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
if( nrhs ) {
plhs[0] = mxCreateDoubleScalar(mxGetElementSize(prhs[0]));
}
}

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