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Export to HDF4 Files

Write MATLAB® Data to HDF4 File

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Write MATLAB arrays to a scientific dataset in an HDF4 file.

Add Namespace to Import List

Add the matlab.io.hdf4.* path to the import list.

import matlab.io.hdf4.*

Prefix subsequent calls to functions in the matlat.io.hdf4.sd namespace with sd, rather than the entire namespace path.

Create HDF4 File

Create a new HDF4 file using the matlab.io.hdf4.sd.start function. This function corresponds to the SD API function, SDstart.

sdID = sd.start('mydata.hdf','create');

sd.start creates the file and returns a file identifier named sdID.

To open an existing file instead of creating a new one, call sd.start with 'write' access instead of 'create'.

Create HDF4 Dataset

Create a dataset in the file for each MATLAB array you want to export. If you are writing to an existing dataset, you can skip ahead to the next step. In this example, create one dataset for the array of sample data, A, using the matlab.io.hdf4.sd.create function. This function corresponds to the SD API function, SDcreate. The ds_type argument is a character vector specifying the MATLAB data type of the dataset. 

A = [1 2 3 4 5 ; 6 7 8 9 10 ; 11 12 13 14 15]; 
ds_name = 'A';
ds_type = 'double';
ds_dims = size(A);
sdsID = sd.create(sdID,ds_name,ds_type,ds_dims);

sd.create returns an HDF4 SD dataset identifier, sdsID.

Write MATLAB Data to HDF4 File

Write data in A to the dataset in the file using the matlab.io.hdf4.sd.writedata function. This function corresponds to the SD API function, SDwritedata. The start argument specifies the zero-based starting index. 

start = [0 0];
sd.writeData(sdsID,start,A);

sd.writeData queues the write operation. Queued operations execute when you close the HDF4 file.

Write MATLAB Data to Portion of Dataset

Replace the second row of the dataset with the vector B. Use a start input value of [1 0] to begin writing at the second row, first column. start uses zero-based indexing.

B = [9 9 9 9 9];
start = [1 0];
sd.writeData(sdsID,start,B);

Write Metadata to HDF4 File

Create a global attribute named creation_date, with a value that is the current date and time. Use the matlab.io.hdf4.sd.setAttr function, which corresponds to the SD API function, SDsetattr. 

sd.setAttr(sdID,'creation_date',string(datetime('now')));

sd.Attr creates a file attribute, also called a global attribute, associated with the HDF4 file identified by sdID.

Associate a predefined attribute, cordsys, to the dataset identified by sdsID. Possible values of this attribute include the text strings 'cartesian', 'polar', and 'spherical'.

attr_name = 'cordsys';
attr_value = 'polar';
sd.setAttr(sdsID,attr_name,attr_value);

Close HDF4 Dataset

Close access to the dataset, using the matlab.io.hdf4.sd.endAccess function. This function corresponds to the SD API function, SDendaccess. You must close access to all the datasets in and HDF4 file before closing the file. 

sd.endAccess(sdsID);

Close HDF4 File

Close the HDF4 file using the matlab.io.hdf4.sd.close function. This function corresponds to the SD API function, SDend. 

sd.close(sdID);

Closing an HDF4 file executes all the write operations that have been queued using SDwritedata.

Manage HDF4 Identifiers

MATLAB® supports utility functions that make it easier to use HDF4 in the MATLAB environment.

View All Open HDF4 Identifiers

Use the gateway function to the MATLAB HDF4 utility API, hdfml, and specify the name of the listinfo function as an argument to view all the currently open HDF4 identifiers. MATLAB updates this list whenever HDF identifiers are created or closed. In this example only two identifiers are open.

hdfml('listinfo')

No open RI identifiers
No open GR identifiers
No open grid identifiers
No open grid file identifiers
No open annotation identifiers
No open AN identifiers
Open scientific dataset identifiers:
	262144
Open scientific data file identifiers:
	393216
No open Vdata identifiers
No open Vgroup identifiers
No open Vfile identifiers
No open point identifiers
No open point file identifiers
No open swath identifiers
No open swath file identifiers
No open access identifiers
No open file identifiers

Close All Open HDF4 Identifiers

Close all the currently open HDF4 identifiers in a single call using the gateway function to the MATLAB HDF4 utility API, hdfml. Specify the name of the closeall function as an argument:

hdfml('closeall')

See Also

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