CODEWITHSUNDEEP
pythonmatlabscipy
Adam G.
Adam G.

Reputation: 75

Scipy IO Loadmat error: ValueError: Mat 4 mopt wrong format

I have seen this question floating around without any definite answers such as here. I have .mat data converted from a different data structure and am trying to load it in python using scipy.io.loadmat. For some files, this approach works perfectly fine, but for others I get this error:

    mat = sio.loadmat(i, verify_compressed_data_integrity=False)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/site-packages/scipy-1.7.1-py3.8-macosx-10.9-x86_64.egg/scipy/io/matlab/mio.py", line 226, in loadmat
    matfile_dict = MR.get_variables(variable_names)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/site-packages/scipy-1.7.1-py3.8-macosx-10.9-x86_64.egg/scipy/io/matlab/mio4.py", line 390, in get_variables
    hdr, next_position = self.read_var_header()
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/site-packages/scipy-1.7.1-py3.8-macosx-10.9-x86_64.egg/scipy/io/matlab/mio4.py", line 346, in read_var_header
    hdr = self._matrix_reader.read_header()
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/site-packages/scipy-1.7.1-py3.8-macosx-10.9-x86_64.egg/scipy/io/matlab/mio4.py", line 108, in read_header
    raise ValueError('Mat 4 mopt wrong format, byteswapping problem?')
ValueError: Mat 4 mopt wrong format, byteswapping problem?

I'm not sure what is causing this issue. I save the .mat files the same way every time so they should all be readable. I also tried h5py and get a similar error. Are there any suggestions on how I can read my data files?

Upvotes: 0

Views: 1103

Answers (1)

Andrew Janke
Andrew Janke

Reputation: 23898

You might be saving the .mat files in different "version" formats without realizing it. If your code calls save(...) without explicitly specifying a format, it uses the default version for your Matlab session, which is a persisted per-user preference that you can set inside the Matlab GUI. And if you don't set a default format in Preferences, the default version that save(...) uses varies with the version of Matlab.

The differences between MAT-file versions are significant. In particular, v7.3 completely changed the format to an HDF5-based one (which I don't think scipy.io.loadmat supports). See https://www.mathworks.com/help/matlab/import_export/mat-file-versions.html.

Check your actual .mat file versions. And if you want your code to be really portable, change the save(...) calls in your code to explicitly specify the MAT-file version using a '-v<whatever>' argument.

Upvotes: 1

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