access data read from file

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Trying to read a text file of real numbers into an array, then output the data sequentially as part of a matlab function in Simulink. Main problem is, if I don't initialize the array "RF=zeros(Filesize,1)" the "Output=RF(CountRom/2)" complains "Subscripting into an mxArray is not supported". Setting "Filesize" requires prior knowledge of the length of the file. If "Filesize" is wrong, the fscanf complains that the "read" and "expected" don't match. I'm sure there's a simple solution, just not sure what it is.

function Output = ReadTxt()
    coder.extrinsic('fopen')
    coder.extrinsic('fclose')
    coder.extrinsic('fscanf')
      FileSize = 29866;
      persistent CountRom RF 
      if isempty(CountRom)
          CountRom =  uint32(0);
          RF = zeros(FileSize,1);
          fid = -1;
          fid = fopen('Filename.txt', 'r');
          if (fid > 0)
              RF=fscanf(fid, '%f');
              fclose(fid);
          else
              error('File open failed'); 
          end
      end
      if (CountRom < FileSize)
          CountRom = CountRom + 1; 
          Output = RF(CountRom / 2); % note, RF is base 1
      else
          fclose('all');
          Output = 0;
      end
  end