MATLAB: Is it possible to get a Table from an mxArray in a MEX function

MATLABmexmxarraytable

I have a bunch of data in Tables (i.e. using the table data type introduced a few releases ago) and I would like to pass this to a MEX function.

I cannot find anything in the mxArray documentation regarding how to get a table from a mxArray. Does such functionality exist?

If it doesn't exist, what are the alternatives and/or is it planned for the near future?

Best Answer

The best solution I could find is to create a MATLAB function to convert a table to struct of arrays as shown below (and then pass that struct to the MEX function). The function for doing so is attached.

I would very much appreciate hearing about other proposed solutions from those in the community who are more expert than myself.

function [ outStruct ] = table2structofarrays( inTable )
%TABLE2STRUCTOFARRAYS Convert a table to a struct of arrays.
%   Usage:  outStruct = TABLE2STRUCTOFARRAYS( inTable )
%

% Convert a table with M rows and N variables to a struct with N fields, 
% each of which contains a 1-dimensional array of length M and where the
% field names in the struct are the same as the variable names in the
% table.
%
% NOTE: There ia a built-in function TABLE2STRUCT which converts a table to
% an array of structs.  However, there are HUGE performance advantages of
% a struct of arrays over an array of structs.
% Make sure the input really is a table
if ~isa(inTable, 'table')
    error('Error.  Input to function %s must be a table, not a %s', mfilename, class(inTable))
end
% Create an empty struct with no fields
outStruct = struct;
% If the table has explicitly defined row names, then add a field for these
if ~isempty(inTable.Properties.RowNames)
    outStruct = setfield(outStruct, 'RowNames', inTable.Properties.RowNames)
end
% Iterate through all of the variables in the table
for varNum=1:width(inTable)
    % Get the variable name as a cell array with 1 element
    varNameCell = inTable.Properties.VariableNames(varNum);
      % Extract the variable name as a string
      varName = varNameCell{1};
      % Add a new field to the struct containing the data for this variable
      outStruct = setfield(outStruct, varName, inTable.(varNum));
  end
end

Related Solutions

MATLAB: How to use object arrays in MEX

You can do this with a helper m-file:

% CreateClassArray.m helper file for mxCreateClassArray mex function
function class_array = CreateClassArray(dims,classname)
estring = 'class_array(';
for k=1:numel(dims);
    estring = [estring '1:' num2str(dims(k)) ','];
end
estring(end) = ')';
estring = [estring '=' classname ';'];
eval(estring);
end

And then you can use this function in your mex routine:

/* Create a class array with default class element entries */
mxArray *mxCreateClassArray(mwSize ndim, const mwSize *dims, const char *classname)
{
    mxArray *rhs[2];
    mxArray *mx, *mxtrap;
    mwSize i;
    double *pr;
      rhs[0] = mxCreateDoubleMatrix(1, ndim, mxREAL);
      pr = mxGetPr(rhs[0]);
      for( i=0; i<ndim; i++ ) {
          pr[i] = dims[i];
      }
      rhs[1] = mxCreateString(classname);
      if( mxtrap = mexCallMATLABWithTrap(1,&mx,2,rhs,"CreateClassArray") ) {
          mxDestroyArray(mxtrap);
          mx = NULL;
      }
      mxDestroyArray(rhs[1]);
      mxDestroyArray(rhs[0]);
      return mx;
  }

Caution: The above is bare bones with NO ARGUMENT CHECKING. I will clean this up with more comments and put in argument checking and post to the FEX in the near future. Also, the above assumes R2008b or later. For R2008a there is no mexCallMATLABWithTrap function in the API. In that case download the FEX submission noted below for mxGetPropertyPtr and there is a replacement mexCallMATLABWithTrap function included.

You might also be interested in this FEX submission for getting the mxArray pointers to class element properties:

http://www.mathworks.com/matlabcentral/fileexchange/30672-mxgetpropertyptr-c-mex-function

MATLAB: Get a subset of a structure array in mex

This is the code I ended up using. At the end of the day I essentially wanted to have an object "template" (i.e. field names defined) that I then populate with values at the appropriate time.

Prior to the example code below, I've initialized empty "template" objects into a cell array, 1 for each unique type of object that I've encountered.

    //- This is specific to my code, but ref_object is an empty structure
    //  BUT with fields, like in Matlab => s = struct('a',{},'b',{},'c',{})
    int object_id = data.object_ids[object_data_index];
    mxArray *ref_object = mxGetCell(data.objects,object_id);
    //- Create a fresh structure/structure array from the reference
    //- Copies the field names
    mxArray *return_obj = mxDuplicateArray(ref_object);
    //- I know the # of fields, this is a bit of a note to my self
    //  in that I'm not sure which of these will execute faster
    //- "data" is a C structure
    //Not sure which is better ...
    //int n_fields = data.child_count_object[object_data_index];
    int n_fields  = mxGetNumberOfFields(return_obj);
    //- This is the magic that I think I pulled from one of James Tursa's other
    //  posts (and this one too). The important part is that the "data" (in mxGetData, mxSetData)
    //  for a structure or structure array is a linear array of pointers to mxArrays.
    mxArray **object_data = mxCalloc(n_fields*n_objects,sizeof(mxArray*));
    mxSetData(return_obj,object_data);
    mxSetN(return_obj,n_objects);
    return return_obj;

Best Answer

Related Solutions

MATLAB: How to use object arrays in MEX

MATLAB: Get a subset of a structure array in mex

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