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Date: Thu, 4 Jan 2018 10:56:34 -0800 (PST)
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Subject: about error message "matrix does not represent a grid" in gnuplot
From: Dawn <duan77084@gmail.com>
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Xref: csiph.com comp.graphics.apps.gnuplot:3845

Hi, all:

     I used fopen in c to read in a data file. I used plot 'data.txt' matri=
x 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 plott=
ed and error message said:

      matrix does not represent a grid


      My basic codes are:

  =20
------------------------------------------------------

=20
int main()
{
#ifdef WIN32
	FILE *pipe =3D _popen(GNUPLOT_NAME, "w");
#else
	FILE *pipe =3D popen(GNUPLOT_NAME, "w");
#endif
=09
	int xsize =3D 20, ysize =3D 41;
	float **din;

	// alloc memory
	din =3D (float **)malloc(sizeof(float *) * ysize);


	for (int i =3D 0; i < ysize; i++) {
		din[i] =3D (float *)malloc(sizeof(float) * xsize);	=09
	}


	// read in 2D data
	FILE *fp, *fp1;=09
	fp =3D fopen("wedgegas.txt", "r");
	fp1 =3D fopen("inversedwedgegas.txt", "w");

=09
	// read in data
	if (fp =3D=3D NULL) {
		printf("fp is NULL!\n");
		exit(1);
	}

	for (int i =3D 0; i < ysize; i++) {
		printf("i=3D%d \n ", i);
		for (int j =3D 0; j < xsize; j++) {
			fscanf(fp, "%f ", &din[i][j]);
			printf("%f ", din[i][j]);
		}
		fscanf(fp, "\n");
		printf("\n ");
	}

=09
	/* write final inversed data to din */
	if (fp1 =3D=3D NULL) {
		printf("fp1 is NULL!\n");
		exit(1);
	}
	for (int i =3D 0; i < ysize; i++) {
		printf("%d  \n", i);
		for (int j =3D 0; j < xsize; j++) {		=09

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


	/* ---------------------------- plot results -----------------------------=
-------------------------- */
plot:
	if (pipe !=3D NULL)
	{
		// plot sinc wave	plot 1=09
		fprintf(pipe, "set multiplot layout 1,2 \n");         // set multiple plo=
ts																	=20


		fprintf(pipe, "plot 'wedgegas.txt' matrix with image \n"); // plot type	 =
=20

															=09

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

	=09
											=09
		fprintf(pipe, "plot 'inversedwedgegas.txt' matrix with image \n"); // plo=
t type

		fprintf(pipe, "%s\n", "e");             // termination character
		fflush(pipe);                           // flush the pipe
											=09

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

#ifdef WIN32
		_pclose(pipe);
#else=20
		pclose(pipe);
#endif
	}
	else
		std::cout << "Could not open pipe" << std::endl;



	fclose(fp);
	fclose(fp1);



	return 0;
}

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

    Any suggestions? Thanks in advance!