How to build a mex return structure?

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Jeff 2016-6-29

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https://ww2.mathworks.cn/matlabcentral/answers/293010-how-to-build-a-mex-return-structure

评论： Jeff 2016-7-22

I am unable to figure out how to build and attach matrices to the structure I have being returned

        mwSize dims[2] = {1,NumChans};
        plhs[0] = mxCreateStructArray(2, dims, NUMBER_OF_FIELDS, field_names);

I want to place matrices in one of the fields where I'll have a different one for each structure element. I can't seem to find any examples how best to do this, would there possibly be such an example?

Thanks

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Answer 1

Adam 2016-6-29

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https://ww2.mathworks.cn/matlabcentral/answers/293010-how-to-build-a-mex-return-structure#answer_227166

编辑：Adam 2016-6-29

在 MATLAB Online 中打开

You can create an mxArray as you would anywhere else, e.g.

mxArray* myArray = mxCreateDoubleMatrix( 3, 4, mxREAL );
double* dataPtr = mxGetPr( myArray );
// Some code here to fill up myArray however you want using dataPtr
mxSetField( plhs[0], 0, field_names{1}, myArray ); // Assign matrix to first field name of 1st structure in array
mxSetField( plhs[1], 1, field_names{1}, myOtherArray ); // Assign matrix to first field name of 2nd structure in array

etc

with myOtherArray created by whatever method you choose also.

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Jeff 2016-6-29

在 MATLAB Online 中打开

a little more background, I'm using a C++ SDK to read a proprietary data file format, where my mex first extracts the number of channels, then loops through them. The channel read section has a single line function which gets the data for the current channel in the loop, where I'm creating and flushing the buffer used for this each loop, since I don't know how many datapoints there are until the channel information is read.

Here's an abbreviated version of the loop

// read the number of channels
if (ts.getChannelCount(&NumChans))
{
mwSize dims[2] = {1,1};
dims[1] = NumChans;
plhs[0] = mxCreateStructArray(2, dims, NUMBER_OF_FIELDS, field_names);
mxArray *tmp; 
      for (ChanIndex=0; ChanIndex<NumChans; ChanIndex++)
      {
          // Get number of data points, x-axis start time and sample rate for this channel
          if ( ts.getPointCount(ChanIndex,&NumPoints) && ts.getXBase(ChanIndex, &XBase) && ts.getSampleRate(ChanIndex, &SampleRate) )
          {
              name_field = mxGetFieldNumber(plhs[0],"NumPoints");
              tmp = mxCreateNumericMatrix((mwSize) 1,(mwSize) 1,mxUINT64_CLASS, mxREAL);
              *mxGetPr(tmp) = (int long long) NumPoints;
              mxSetFieldByNumber(plhs[0],ChanIndex,name_field,tmp);
           }
          // Read the data for this channel
          FTFloat *DataBuf = new FTFloat[NumPoints];
          if (ts.getData(ChanIndex, 0, (FTUnsLong)NumPoints, DataBuf))
          {
              this is where I want to asign the data read into DataBuff 
              into the plhs[0] structure in the "Data" field
              name_field = mxGetFieldNumber(plhs[0],"Data");
              mxSetFieldByNumber(plhs[0],ChanIndex,name_field,????);
              delete[] DataBuf;
              DataBuf = 0;
          }
      }
  }

James Tursa 2016-6-29

编辑：James Tursa 2016-6-29

Can you post your current code or pseudo-code with comments as to what you want at the end of the process? I.e., "I want an X-element structure with fields named AAA, BBB, etc and each field will contain ETC ETC".

I am not really sure where you are getting stuck. Once you know how many elements you want (NumChans?), you loop through them to set the field values. Looks like you are doing that already, so I don't see where you are having problems with this.

Jeff 2016-7-22

在 MATLAB Online 中打开

I neglected to make it an array of pointers, this works fine

mxArray **Data = new mxArray*[ NumChans ];

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How to build a mex return structure?

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