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colon operator in compiled python package

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Jieqiang Wei
Jieqiang Wei on 9 Nov 2020
Commented: Sean de Wolski on 12 Nov 2020
Hi, I compiled one python package from a matlab one using library compiler. Everything went well until I did a test in Python’s Interpreter. In my Matlab code, I have a function called returnS_test_compiler with a line
f = (fRange(1):fRange(2):fRange(3))';
which generates a list and then "transpose" it.
I assume by compiling the code into python package, I do not need to modify anything in the code. So when I call this returnS_test_compiler in python imported as a python package, it gives the following error:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/local/MATLAB/MATLAB_Runtime/v97/toolbox/compiler_sdk/pysdk_py/matlab_pysdk/runtime/deployablefunc.py", line 80, in __call__
nlhsWasSpecified, stdoutObj, stderrObj).result()
File "/usr/local/MATLAB/MATLAB_Runtime/v97/toolbox/compiler_sdk/pysdk_py/matlab_pysdk/runtime/futureresult.py", line 135, in result
raise e
File "/usr/local/MATLAB/MATLAB_Runtime/v97/toolbox/compiler_sdk/pysdk_py/matlab_pysdk/runtime/futureresult.py", line 123, in result
raise e
File "/usr/local/MATLAB/MATLAB_Runtime/v97/toolbox/compiler_sdk/pysdk_py/matlab_pysdk/runtime/futureresult.py", line 113, in result
self._nlhs, out=self._out, err=self._err)
matlab_pysdk.runtime.MatlabRuntimeError: An error occurred when evaluating the result from a function. Details:
File /root/.mcrCache9.7/return0/returnS_test/returnS_test_compiler.m, line 15, in returnS_test_compiler
Undefined function 'colon' for input arguments of type 'cell'.
It seems that it is complaining ( : : ) is not recognisable as a python sentence. Any suggestions?

Answers (3)

Steven Lord
Steven Lord on 9 Nov 2020
The variable fRange is a cell array. The colon operator doesn't know how to work on cell arrays.
a = {1};
b = {10};
x = a:b % throws error
Operator ':' is not supported for operands of type 'cell'.
To get this to work, pass the contents of the cells in the cell arrays to the colon operator.
a = {1};
b = {10};
x = a{1}:b{1} % does not throw an error.

Sean de Wolski
Sean de Wolski on 9 Nov 2020
You should call the MATLAB module with a numeric input rather than a list. This is the data type conversion table. Use numeric types (probably matlab.double)
https://www.mathworks.com/help/releases/R2020b/matlab/matlab_external/pass-data-to-matlab-from-python.html
  1 Comment
Sean de Wolski
Sean de Wolski on 12 Nov 2020
This page is even more clear in exactly what you need to do:
https://www.mathworks.com/help/releases/R2020b/matlab/matlab_external/matlab-arrays-as-python-variables.html

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Jieqiang Wei
Jieqiang Wei on 10 Nov 2020
I find this one is closest to what I am looking for, it seems the way I define the input variable in python manner directly is not a good option.
https://se.mathworks.com/matlabcentral/answers/437509-python-interface-to-matlab-runtime#comment_656434
  1 Comment
Sean de Wolski
Sean de Wolski on 12 Nov 2020
You seem to be finding the engine so that doesn't seem like the issue. You just need to cast the python data to a matlab double. The doc page I linked shows how to do this.

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