h5writeatt

Write HDF5 attribute

Syntax

h5writeatt(filename,loc,attr,val)

h5writeatt(filename,loc,attr,val,'TextEncoding',encoding)

Description

h5writeatt(filename,loc,attr,val) writes the attribute named attr with the value val to the specified location in the HDF5 file.

h5writeatt(filename,loc,attr,val,'TextEncoding',encoding) writes attr to the specified location in the HDF5 file using the specified encoding. 'UTF-8' is the default setting for 'TextEncoding', while 'system' uses the system encoding to represent characters. It is usually unnecessary to specify 'system' as the encoding.

Examples

Write Attribute to Root Group

Write an attribute to the root group of examplefile.h5 whose value is the current time.

date = datestr(now);
h5writeatt('examplefile.h5','/','creation_date', date);

Read the attribute from the root group of the HDF5 file.

val1 = h5readatt('examplefile.h5','/','creation_date')

val1 = 
'03-Mar-2023 06:27:42'

Write Attribute to Dataset

Create an array of doubles and write it to the dataset /g4/world.

attData = [0 1 2 3];
h5writeatt('examplefile.h5','/g4/world','val2',attData);

Display the dataset metadata. The attribute val2 is listed one of the attributes belonging to the dataset.

h5disp('examplefile.h5','/g4/world');

HDF5 examplefile.h5 
Dataset 'world' 
    Size:  36x19
    MaxSize:  36x19
    Datatype:   H5T_IEEE_F64LE (double)
    ChunkSize:  []
    Filters:  none
    FillValue:  0.000000
    Attributes:
        'val2':  0.000000 1.000000 2.000000 3.000000 

Input Arguments

filename — File name

character vector | string scalar

File name, specified as a character vector or string scalar containing the name of an existing HDF5 file.

Depending on the location you are writing to, filename can take on one of these forms.

Location	Form
Current folder	To write to the current folder, specify the name of the file in filename. Example: 'myFile.h5'
Other folders	To write to a folder different from the current folder, specify the full or relative path name in filename. Example: 'C:\myFolder\myFile.h5' Example: 'myFolder\myFile.h5'
Remote Location	To write to a remote location, filename must contain the full path of the file specified as a uniform resource locator (URL) of the form: scheme_name://path_to_file/my_file.ext Based on your remote location, scheme_name can be one of the values in this table. Remote Location scheme_name Amazon S3™ s3 Windows Azure® Blob Storage wasb, wasbs For more information, see Work with Remote Data. Example: 's3://bucketname/path_to_file/myFile.h5'

loc — Location in file

character vector | string scalar

Location in file, specified as a character vector or string scalar containing the full path name of an existing group or dataset to which you want to associate the attribute.

attr — Name of attribute

character vector | string scalar

Name of attribute, specified as a character vector or string scalar containing the name of an attribute belonging to a group or dataset. If the attribute does not exist, h5writeatt creates the attribute with the name specified.

If the specified attribute already exists but does not have a datatype or dataspace consistent with val, h5writeatt deletes the attribute and recreates it. String attributes are created with a scalar dataspace, which is a single, zero-dimensional data point.

val — Value of attribute

character vector | string array | numeric value

Value of attribute to be written, specified as a character vector, string scalar, or numeric value.

encoding — Text Encoding

'UTF-8' (default) | 'system'

Text encoding, specified as the comma-separated pair consisting of 'TextEncoding' and one of these values:

• 'UTF-8' — Represents characters using UTF-8 encoding.

• 'system' — Represents characters as bytes using the system encoding (not recommended).

Limitations

• h5writeatt does not support writing to files stored remotely in HDFS™.

Version History

Introduced in R2011a

R2020b: Write attributes to HDF5 files at a remote location

You can write attributes to HDF5 files in remote locations, such as Amazon S3, Windows Azure Blob Storage, and HDFS.

R2020b: Write attributes to HDF5 files with Unicode names

You can write attributes to HDF5 files whose names are encoded as Unicode characters.

R2020a: UTF-8 character encoding is the default setting

UTF-8 is now the default character encoding for h5writeatt so that all Unicode code points can be represented in HDF5 files. Previously, the default encoding was ASCII.