Module org.hdfgroup.object
Package hdf.object.h4

Class H4Vdata

Object
hdf.object.HObject
hdf.object.Dataset
hdf.object.CompoundDS
hdf.object.h4.H4Vdata
All Implemented Interfaces:
CompoundDataFormat, DataFormat, MetaDataContainer, Serializable
public class H4Vdata extends CompoundDS implements MetaDataContainer
H4Vdata describes a multi-dimension array of HDF4 vdata, inheriting CompoundDS. A vdata is like a table that consists of a collection of records whose values are stored in fixed-length fields. All records have the same structure and all values in each field have the same data type. Vdatas are uniquely identified by a name, a class, and a series of individual field names. How to Select a Subset Dataset defines APIs for reading, writing and subsetting a dataset. No function is defined to select a subset of a data array. The selection is done in an implicit way. Function calls to dimension information such as getSelectedDims() return an array of dimension values, which is a reference to the array in the dataset object. Changes of the array outside the dataset object directly change the values of the array in the dataset object. It is like pointers in C. The following is an example of how to make a subset. In the example, the dataset is a 4-dimension with size of [200][100][50][10], i.e. dims[0]=200; dims[1]=100; dims[2]=50; dims[3]=10;
We want to select every other data point in dims[1] and dims[2] 
     int rank = dataset.getRank();   // number of dimensions of the dataset
     long[] dims = dataset.getDims(); // the dimension sizes of the dataset
     long[] selected = dataset.getSelectedDims(); // the selected size of the dataet
     long[] start = dataset.getStartDims(); // the offset of the selection
     long[] stride = dataset.getStride(); // the stride of the dataset
     int[]  selectedIndex = dataset.getSelectedIndex(); // the selected dimensions for display

     // select dim1 and dim2 as 2D data for display, and slice through dim0
     selectedIndex[0] = 1;
     selectedIndex[1] = 2;
     selectedIndex[1] = 0;

     // reset the selection arrays
     for (int i=0; i<rank; i++) {
         start[i] = 0;
         selected[i] = 1;
         stride[i] = 1;
    }

    // set stride to 2 on dim1 and dim2 so that every other data point is selected.
    stride[1] = 2;
    stride[2] = 2;

    // set the selection size of dim1 and dim2
    selected[1] = dims[1]/stride[1];
    selected[2] = dims[1]/stride[2];

    // when dataset.read() is called, the selection above will be used since
    // the dimension arrays is passed by reference. Changes of these arrays
    // outside the dataset object directly change the values of these array
    // in the dataset object.
Version:
1.1 9/4/2007
Author:
Peter X. Cao
See Also: