MATLAB Programming

Hierarchical Data Format (HDF4) Files
their names and data types. For example, the HDF4 SD API defines an
attribute, named cordsys, in which you can specify the coordinate system
used by the data set. Possible values of this attribute include the text strings
'cartesian', 'polar', and'spherical'.
Predefined attributes can be useful because they establish conventions that
applications can depend on. The HDF4 SD API supports predefined attributes
for data sets and dimensions only; there arenopredefinedattributesforfiles.
For a complete list of the predefined attributes, see the NCSA documentation.
In the HDF4 SD API, you create predefined attributes the same way you
create user-defined attributes, using the SDsetattr routine. In MATLAB, use
the hdfsd function, specifying setattr as the first argument:
attr_name = 'cordsys';
attr_value = 'polar';
status = hdfsd('setattr',sds_id,attr_name,attr_value);
The HDF4 SD API also includes specialized functions for writing and
reading the predefined attributes. These specialized functions, such as
SDsetdatastrs, are sometimes easier to use, especially when you are reading
or writing multiple related predefined attributes. You must use specialized
functions to read or write the predefined dimension attributes.
You can associate multiple attributes with a single HDF4 object. HDF4
maintains an attribute index for each object. The attribute index is
zero-based. The first attribute has index value 0, the second has index value
1, and so on. You access an attribute by its index value.
Each attribute has the format name=value, wherename (called label in
HDF4 terminology) is a text string up to 256 characters in length and value
contains one or more entries of the same data type. A single attribute can
have multiple values.
Step 5: Closing HDF4 Data Sets. After writing data to a data set in an
HDF4 file, you must close access to the data set. In the HDF4 SD API, you
use the SDendaccess routine to close a data set. In MATLAB, use the hdfsd
function, specifying endaccess as the first argument. As the only other
argument, specify a valid HDF4 SD data set identifier, sds_id in this example:
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