Passing from Python to MATLAB a cell array of structures

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Let's say that we want to pass from Python an object corresponding to a mixed MATLAB cell array of structures, as an example a final MATLAB object like: toto{1}.weapon{2}.Name='fleurs' ...
I tried in Python:
>>> import scipy.io as sio
>>> import numpy as np
>>> toto = np.zeros((2,), dtype=np.object)
>>> toto[0] = {}
>>> toto[1] = {}
>>> toto[0]['weapon'] = np.zeros((2,), dtype=np.object)
>>> toto[0]['weapon'][0] = {}
>>> toto[0]['weapon'][1] = {}
>>> toto[0]['weapon'][1]['Name'] = 'fleurs'
>>> toto
array([{'weapon': array([{}, {'Name': 'fleurs'}], dtype=object)}, {}], dtype=object)
>>> sio.savemat('toto.mat', {'toto':toto})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio.py", line 207, in savemat
MW.put_variables(mdict)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 876, in put_variables
self._matrix_writer.write_top(var, asbytes(name), is_global)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 626, in write_top
self.write(arr)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 655, in write
self.write_cells(narr)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 759, in write_cells
self.write(el)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 653, in write
self.write_struct(narr)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 764, in write_struct
self._write_items(arr)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 782, in _write_items
self.write(el[f])
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 655, in write
self.write_cells(narr)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 759, in write_cells
self.write(el)
File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 647, in write
% (arr, type(arr)))
TypeError: Could not convert {} (type <type 'dict'>) to array
So, is it possible to create in Python and pass to MATLAB a cell array of structures ?
Did I make a mistake ?

Answers (1)

Eric Condamine
Eric Condamine on 14 Mar 2017

Ooops I just understood why the TypeError exception was raised ... In fact, this is the right way to pass a cell array of structures from Python to MATLAB but ... It seems that the scipy.io.savemat does not allow empty dictionary. Indeed If I complet all the dictionnaries all is working fine !:

In Python:

>>> import scipy.io as sio   
>>> import numpy as np
>>> toto = np.zeros((2,), dtype=np.object)
>>> toto[0]={}
>>> toto[1]={}
>>> toto[1]['Site']=['Tataouine']
>>> toto[0]['weapon'] = np.zeros((2,), dtype=np.object)
>>> toto[0]['weapon'][0]={}
>>> toto[0]['weapon'][1]={}
>>> toto[0]['weapon'][1]['Name']='fleurs'
>>> toto[0]['weapon'][0]['Name']='bonbons'
>>> toto
array([ {'weapon': array([{'Name': 'bonbons'}, {'Name': 'fleurs'}], dtype=object)},
       {'Site': ['Tataouine']}], dtype=object)
>>> sio.savemat('toto.mat', {'toto':toto})

In MATLAB:

>> load('toto')
>> toto
toto = 
      [1x1 struct]    [1x1 struct]
>> toto{1}.weapon{2}.Name
ans =
fleurs

Hope this will help you to not waste your time with this silly problem !!

  1 Comment
Ryland Mathews
Ryland Mathews on 17 Jul 2019
I am currently pulling in a pandas dataframe (of strings) into matlab using code like below:
is there a way to then convert the dataframe into usuable form to plot in a matlab table, I tried for hours to convert it and couldnt get it done.
clear classes
mod = py.importlib.import_module('test')
py.importlib.reload(mod)
cP = py.list(py.test.FlowTracker().todays_portfolio(1,10))

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