MATLAB: How to read in a key:value dictionary formatted string into a cell or structured array format

containers.mappython

I would like to read the following pythonic dict string (returned by a system command) into either a struct or cell array. The containers.Map class requires pre-parsed cells of keys and values, but is there a pre-existing code for reading this?

{"xlo": -1.909999966621399, "yhi": 1.0, "xhi": 1.0, "n_triangles": 20676, "n_points": 10300, "flat_area_z": [], "n_edges": 32824, "zhi": 4.0, "ylo": -1.6299999952316284, "zlo": -5.8145337670434996e-18, "n_unmatched_edges": -136}

Thanks in advance!

Best Answer

Assuming you have R2016b or later, this is easily done with jsondecode:

str = '{"xlo": -1.909999966621399, "yhi": 1.0, "xhi": 1.0, "n_triangles": 20676, "n_points": 10300, "flat_area_z": [], "n_edges": 32824, "zhi": 4.0, "ylo": -1.6299999952316284, "zlo": -5.8145337670434996e-18, "n_unmatched_edges": -136}';
s = jsondecode(str)
m = containers.Map(fieldnames(s), struct2cell(s))

Note: in some earlier versions of matlab (at least R2016a) you can use the undocumented matlab.internal.webservices.fromJSON instead of jsondecode.

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MATLAB: How to convert a cell array of structures to array of structures

As long as the structures in the cells have all the same fields and that these structures in the cells are either scalar or arrays with the same number of rows:

nodedata = [nodeinfo.NodeData{:}];
%or
nodedata = cell2mat(nodeinfo.NodeData);

MATLAB: How to convert a Python dictionary to a MATLAB type

While MATLAB structs have strict naming conventions for their data, there are other MATLAB data types which a dictionary can be converted into.

If we already have our Python environment loaded (which can be done manually with the MATLAB "pyenv" function), we can create a Python dictionary directly in MATLAB with the following command:

dict = py.dict(pyargs('2020-1-1 10:00:00.1', 'Value_1', '2020-1-2 12:10:00.5', 'Value_2'))

We can now convert this Python dictionary into a MATLAB map by iterating over the dictionary:

M = containers.Map;
for raw_key = py.list(keys(dict))
    key = raw_key{1};
    value = dict{key};
    M(string(key)) = string(value);
end

Alternatively, we can convert the dictionary into a MATLAB table:

T = table;
for raw_key = py.list(keys(dict))
   key = raw_key{1};
   value = dict{key};
   T2 = table(string(value), 'VariableNames', [string(key)]);
   T = [T T2];
end

Note that this dictionary could not be converted into a MATLAB struct because the keys of the dictionary do not conform to struct naming conventions, since they do not start with a letter.

Best Answer

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