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about error message "matrix does not represent a grid" in gnuplot

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Hi, all:

     I used fopen in c to read in a data file. I used plot 'data.txt' matrix with image to plot out the image. However, when I used fopen to write in the same data set as a new data file and I used same command line with new data file as: plot 'data1.txt' matrix with image, the image cannot be plotted and error message said:

      matrix does not represent a grid


      My basic codes are:

   
------------------------------------------------------

 
int main()
{
#ifdef WIN32
	FILE *pipe = _popen(GNUPLOT_NAME, "w");
#else
	FILE *pipe = popen(GNUPLOT_NAME, "w");
#endif
	
	int xsize = 20, ysize = 41;
	float **din;

	// alloc memory
	din = (float **)malloc(sizeof(float *) * ysize);


	for (int i = 0; i < ysize; i++) {
		din[i] = (float *)malloc(sizeof(float) * xsize);		
	}


	// read in 2D data
	FILE *fp, *fp1;	
	fp = fopen("wedgegas.txt", "r");
	fp1 = fopen("inversedwedgegas.txt", "w");

	
	// read in data
	if (fp == NULL) {
		printf("fp is NULL!\n");
		exit(1);
	}

	for (int i = 0; i < ysize; i++) {
		printf("i=%d \n ", i);
		for (int j = 0; j < xsize; j++) {
			fscanf(fp, "%f ", &din[i][j]);
			printf("%f ", din[i][j]);
		}
		fscanf(fp, "\n");
		printf("\n ");
	}

	
	/* write final inversed data to din */
	if (fp1 == NULL) {
		printf("fp1 is NULL!\n");
		exit(1);
	}
	for (int i = 0; i < ysize; i++) {
		printf("%d  \n", i);
		for (int j = 0; j < xsize; j++) {			

			fprintf(fp1, "%f ", din[i][j]);
			printf("%f ", din[i][j]);
		}
		fprintf(fp1, "\n");
		printf("\n");
	}


	/* ---------------------------- plot results ------------------------------------------------------- */
plot:
	if (pipe != NULL)
	{
		// plot sinc wave	plot 1	
		fprintf(pipe, "set multiplot layout 1,2 \n");         // set multiple plots																	 


		fprintf(pipe, "plot 'wedgegas.txt' matrix with image \n"); // plot type	  

																

		fprintf(pipe, "%s \n", "e");             // termination character, end a plot
		fflush(pipe);                           // flush the pipe

		
												
		fprintf(pipe, "plot 'inversedwedgegas.txt' matrix with image \n"); // plot type

		fprintf(pipe, "%s\n", "e");             // termination character
		fflush(pipe);                           // flush the pipe
												

												// wait for key press
		std::cin.clear();
		std::cin.ignore(std::cin.rdbuf()->in_avail());
		std::cin.get();

#ifdef WIN32
		_pclose(pipe);
#else 
		pclose(pipe);
#endif
	}
	else
		std::cout << "Could not open pipe" << std::endl;



	fclose(fp);
	fclose(fp1);



	return 0;
}

------------------------------------------------------------

    Any suggestions? Thanks in advance!

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about error message "matrix does not represent a grid" in gnuplot Dawn <duan77084@gmail.com> - 2018-01-04 10:56 -0800

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