Saving the parameters returned from mex

The timestamp from my mex file is passed back to mat file as data. I encountered that it saves the first and the last data it receives and the rest all zeros. How do i edit my mat file to ensure i get all the values? I guess the problem lies with the data=zeros(1,1) and the return parameter that is data. how do i change it?

Mat File:

clear;clc;
config;

data = zeros(1,1);

[status, validity, data] = readGPS(OPEN,'gps-2011-10-13-15-15.log');
index = 1;

while(status ~= STOP)
   [status, validity, data] = readGPS(READ);
   if validity == VALID_DATA
       data(:,index) = data;
       index = index + 1;

     end
     save('gpstime.mat','data')
  end

Mex File:

double timestamp;
int status, validity;
FILE *fid; 
void mexFunction(int nlhs, mxArray *plhs[],int nrhs, const mxArray *prhs[])
{
   int Op;
   double *status_out, *validity_out, *data_out;
   char data [17], buffer [256];
   char *str;

     /*check to find out the operation required*/
     if (nrhs == 2)
     {
        Op = OPEN;
     }
     else
     {
        Op = READ;
     }      

     /*Different cases for different operations*/
     switch (Op)
     {
         case OPEN:
             fid=fopen("gps-2011-10-13-15-15.log","r");  //open file as read   
             validity = INVALID;
             timestamp = NO_VALUE;
             if (fid != NULL)
             {
                 status = CONTINUE;
             }
             else
             {
                 status = STOP;
             }              
             break;  

         case READ:
             if(!feof(fid))
             {
                 status = CONTINUE;
                 validity = VALID_DATA;
                 fgets(buffer, 255, fid);          //read in the first line of the file
                 str = strtok(buffer, ",");        //get the timestamp 
                 strcpy(data, str);            
                 timestamp = atof(data);           //change from char to double
                 mexPrintf("Timestamp:%f \n",timestamp);
             }

             break;

         case CLOSE:
             fclose(fid);
             validity = INVALID;
             timestamp = NO_VALUE;
             status = STOP;
             break;
        default:
             mexErrMsgTxt("Incorrect parameter 3.");
             break;
        }       

       /*Passing back results*/
        plhs[0] = mxCreateDoubleMatrix(1,1,mxREAL); //create an mxArray
        status_out = mxGetPr(plhs[0]);              //get a pointer to the data
        status_out[0] = status;     

        plhs[1] = mxCreateDoubleMatrix(1,1,mxREAL);
        validity_out = mxGetPr(plhs[1]);
        validity_out[0] = validity;     

        plhs[2] = mxCreateDoubleMatrix(1,1,mxREAL);
        data_out = mxGetPr(plhs[2]);
        data_out[0] = timestamp;      

}

Answer 1

[Friedrich]

Hi,

the problem is that you use the variable data for two different things. To store the overall data and as returnvalue of that mex file. Try it like this:

clear;clc;
config;

data = zeros(1,1);

[status, validity, data] = readGPS(OPEN,'gps-2011-10-13-15-15.log');
index = 1;

while(status ~= STOP)
   [status, validity, tmp] = readGPS(READ);
   if validity == VALID_DATA
       data(:,index) = tmp;
       index = index + 1;

   end
 end
 save('gpstime.mat','data')

Is there a reason why you are doing this with a mex file? You can do this in MATLAB directly pretty easy.

[Jw]

I have a large file and mex provides a faster processing. And also many files to process

Thanks alot for your help!! =)

[Friedrich]

thanks for the clarification. So your issue is solved now?

[Jw]

yup solved. thanks alot for your help.

[Friedrich]

Could you please mark it as accepted than? Thanks.

[Titus Edelhofer]

Just a comment: for the last lines of your mex file you might also use mxCreateDoubleScalar ...