ValueError: only a scalar struct can be returned from MATLAB
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I tried to use matlab.engine in python to import a struct from Matlab. my operation system is macOS Sierra, and the error message is :
File "/lib/python2.7/site-packages/matlab/engine/fevalfuture.py", line 82, in result
self._result = pythonengine.getFEvalResult(self._future,self._nargout, None, out=self._out, err=self._err)
ValueError: only a scalar struct can be returned from MATLAB
The values in the fields of the struct are cell, logical, single, unit8, and unit16. I am not sure what scalar struct refers to. Has anyone met the same problem before ?
Thanks!
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Walter Roberson
2017-9-30
A scalar struct is one for which size() of the struct itself says [1 1]. It does not matter what size the fields contained in the struct are, just the size of the struct itself. For example,
test1.this = 1:20;
test1.that = 'hello';
test2(1).this = 1;
test2(1).that = char('what','is','the','result','here?');
test3(1).this = 1:20;
test3(1).that = 'hello';
test3(2).this = 1;
test3(2).that = char('what','is','the','result','here?');
With these lines, size(test1) and size(test2) will both be [1 1] -- they are scalar structures, no matter how big the fields they hold are. Referring to test1.that would give you the single result 'hello' .
However, size(test3) will be [1 2]: it is not a scalar structure, it is a structure array. Referring to test3.that would give you a comma-separated list of two results, the first of which was 'hello' and the second of which was the char array, because of "structure expansion".
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