# IBM Visualization Data Explorer Programmer's Reference

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## 8.2 Writing an Import Module

Any external filter, like the one just illustrated, has the disadvantage of running more slowly than an import module because it sends the data to a file or through a socket. A built-in module reads the data into memory, where Data Explorer uses it directly. In this example, the import module SimpleImport reads the same HDF file as the external filter did.

Note: Because the import module is very simple and does not require the traversal of input Fields, the Module Builder is not used in this example. (This C program is also found in `/usr/share/dx/samples/program_guide/simpleimportfilter.c`

    01   #include <dx/dx.h>

`df.h` is a necessary include file for HDF library routines.

    02   #include <df.h>
    03
    04   #define MAXRANK 3
    05
    06   Error m_SimpleImport(Object *in, Object *out)
    07   {
    08     Array a=NULL;
    09     Field f=NULL;
    10     char *filename;
    11     int dims, counts[MAXRANK], numelements, i, j;
    12     float deltas[MAXRANK*MAXRANK], origins[MAXRANK], *data;

Extract the file name from `in[0]`, and check that it is a string.

    13   if (!in[0]) {
    14     DXSetError(ERROR_BAD_PARAMETER,"missing filename");
    15     goto error;
    16   }
    17   else if (!DXExtractString(in[0], &filename)) {
    18     DXSetError(ERROR_BAD_PARAMETER, "filename must be a string");
    19     goto error;
    20   }

The HDF library routine `DFishdf` checks the file for accessibility and for the correct (HDF) format. If the file is not accessible or is not an HDF file, the routine generates an error message.

    21   if (DFishdf(filename) != 0) {
    22      DXSetError(ERROR_BAD_PARAMETER,
    23                 "file \"%s\" is not accessible, or is not an hdf file",
    24                  filename);
    25      goto error;
    26   }
    27

Initialize the HDF library.

    28   DFSDrestart();

The HDF library routine `DFSDgetdims` returns the dimensionality of the grid (1D, 2D, etc.) in `dims`. The number of positions in each dimension is returned in the Array `counts`.

    29   DFSDgetdims(filename, &dims, counts, MAXRANK);

Make a new Array (scalar).

    30   a = DXNewArray(TYPE_FLOAT, CATEGORY_REAL, 0);
    31   if (!a)
    32     goto error;

Determine the number of elements in the data Array.

    33   numelements=1;
    34   for (i=0; i<dims; i++)  {
    35      numelements= numelements * counts[i];
    36   }

Allocate space to the data Array.

    37   if (!DXAddArrayData(a, 0, numelements, NULL))
    38      goto error;

Get a pointer to memory for the data Array.

    39   data = (float *)DXGetArrayData(a);
    40   if (!data)
    41      goto error;

The HDF library routine `DFSDgetdata` reads the data from the HDF file to the data Array.

    42   DFSDgetdata(filename, dims, counts, data);

Create a new Field.

    43   f = DXNewField();
    44   if (!f)
    45     goto error;

Set the dependency of the data to "positions."

    46   if (!DXSetStringAttribute((Object)a, "dep", "positions"))
    47     goto error;

Set the data Array as the data component of `f`.

    48   if (!DXSetComponentValue(f, "data", (Object)a))
    49     goto error;
    50   a=NULL;

Create the connections Array. `DXMakeGridConnections` sets up the element type. Place the connections in the Field.

    51   a = DXMakeGridConnectionsV(dims, counts);
    52   if (!a)
    53      goto error;
    54   if (!DXSetComponentValue(f, "connections", (Object)a))
    55      goto error;
    56   a=NULL;

Now create the position origin and deltas for the position (origin 0 and deltas 1 in each dimension).

    57   for (i=0; i<dims; i++) {
    58      origins[i] = 0.0;
    59      for (j=0; j<dims; j++) {
    60        if (i==j)
    61          deltas[i*dims + j] = 1.0;
    62        else
    63          deltas[i*dims + j] = 0.0;
    64      }
    65   }

Create the positions Array and place it in the Field.

    66
    67   a = DXMakeGridPositionsV(dims, counts, origins, deltas);
    68   if (!a)
    69     goto error;
    70   if (!DXSetComponentValue(f, "positions", (Object)a))
    71     goto error;
    72   a=NULL;

`DXEndField` sets default attributes and creates the bounding box.

    73   if (!DXEndField(f))
    74     goto error;
    75

Set `f` as the first output of the module.

    76   out[0]=f;
    77   return OK;
    78

On error, delete `f` and `a`.

    79  error:
    80   DXDelete((Object)f);
    81   DXDelete((Object)a);
    82   return ERROR;
    83 }

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