netcdf.putAtt

Write data to netCDF attribute

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    Syntax

    netcdf.putAtt(ncid,varid,attname,attvalue)
    netcdf.putAtt(ncid,varid,attname,attvalue,xtype)

    Description

    netcdf.putAtt(ncid,varid,attname,attvalue) writes the attribute named attname with value attvalue to the netCDF variable identified by varid in the netCDF file or group specified by ncid. The written attribute value is of the netCDF data type that best matches the MATLAB® data type of attvalue. For more information about how MATLAB determines the best match, see MATLAB to NetCDF Data Type Conversion.

    example

    netcdf.putAtt(ncid,varid,attname,attvalue,xtype) writes attvalue as the data type specified by xtype.

    example

    Examples

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    Open Live Script

    Make a copy of the example netCDF file and open the copy for writing. Write an attribute to the avogadros_number variable.

    copyfile("example.nc","myfile.nc")
ncid = netcdf.open("myfile.nc","NC_WRITE");
varid = netcdf.inqVarID(ncid,"avogadros_number");

attname = "significant_figures";
attvalue = 9;
netcdf.putAtt(ncid,varid,attname,attvalue)

    Check that the value was written as expected, using the netcdf.getAtt function.

    netcdf.getAtt(ncid,varid,attname)
    ans = 
9

    Close the netCDF file.

    netcdf.close(ncid)
    Open Live Script

    Create and open a netCDF file.

    ncid = netcdf.create("myfile.nc","NOCLOBBER");

    Write a global attribute. Use netcdf.getConstant("NC_GLOBAL") for varid.

    varid = netcdf.getConstant("NC_GLOBAL");

attname = "creation_time";
attvalue = string(datetime("now"));
netcdf.putAtt(ncid,varid,attname,attvalue)

    Check that the value was written as expected, using the netcdf.getAtt function.

    netcdf.getAtt(ncid,varid,attname)
    ans = 
'09-Aug-2025 11:53:13'

    Close the netCDF file.

    netcdf.close(ncid)
    Open Live Script

    Create and open a netCDF-4 file.

    ncid = netcdf.create("myfile.nc","NETCDF4");

    Write a global attribute using MATLAB string array. Use netcdf.getConstant("NC_GLOBAL") for varid.

    varid = netcdf.getConstant("NC_GLOBAL");

attname = "temperature_units";
attvalue = ["°F" "°C"];
netcdf.putAtt(ncid,varid,attname,attvalue)

    Check that the value was written as expected, using the netcdf.getAtt function.

    netcdf.getAtt(ncid,varid,attname)
    ans = 1×2 string
    "°F"    "°C"

    Close the netCDF file.

    netcdf.close(ncid)
    Open Live Script

    Make a copy of the example netCDF file and open the copy for writing. Write an attribute to the temperature variable. Specify xtype as "NC_STRING".

    copyfile("example.nc","myfile.nc")
ncid = netcdf.open("myfile.nc","NC_WRITE");
varid = netcdf.inqVarID(ncid,"temperature");

attname = "Month";
attvalue = "March";
xtype = "NC_STRING";
netcdf.putAtt(ncid,varid,attname,attvalue,xtype)

    Check that the value was written as expected, using the netcdf.getAtt function.

    netcdf.getAtt(ncid,varid,attname)
    ans = 
"March"

    Close the netCDF file.

    netcdf.close(ncid)

    Input Arguments

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    Identifier of a netCDF source, specified as a nonnegative integer scalar. The netCDF source can be a netCDF file or a netCDF group.

    Data Types: double | single | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

    Identifier of a netCDF variable, specified as a nonnegative integer scalar.

    To write a global attribute, specify varid as netcdf.getConstant("NC_GLOBAL").

    Data Types: double | single | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

    Attribute name to write, specified as a string scalar or character vector.

    Example: "myAtt"

    Attribute value, specified as a numeric array, text, or a cell array.

    Note

    If attvalue has more than one dimension, then the netcdf.putAtt function flattens attvalue in column-major order before writing the attribute value. For example, specifying attvalue as [1 2 3; 4 5 6] and specifying attvalue as [1 4 2 5 3 6] have the same effect.

    Additionally, for attributes of type NC_VLEN, if attvalue contains any entries that have more than one dimension, then the netcdf.putAtt function flattens those entries in column-major order before writing the values. For example, for an attribute of type NC_VLEN, specifying attvalue as

    {[0.5 0.3]; [0 -0.7 5.2; 4.6 2.5 1.8]}

    and specifying attvalue as

    {[0.5; 0.3] [0; 4.6; -0.7; 2.5; 5.2; 1.8]}

    have the same effect.

    Data Types: double | single | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | char | string | cell

    NetCDF data type, specified as a string scalar, character vector, or nonnegative integer scalar.

    • You can specify xtype as one of these string scalars or character vectors.

      Value of xtypeMATLAB Class
      "NC_DOUBLE"double
      "NC_FLOAT"single
      "NC_INT"int32
      "NC_SHORT"int16
      "NC_BYTE"int8
      "NC_CHAR"char
      "NC_INT64" (*)int64
      "NC_UINT64" (*)uint64
      "NC_UINT" (*)uint32
      "NC_USHORT" (*)uint16
      "NC_UBYTE" (*)uint8
      "NC_STRING" (*)string

      (*) These values of xtype are valid only for sources with format netcdf4.

    • You can specify xtype as a numeric value returned by the netcdf.getConstant function.

    • For attributes of the user-defined NC_VLEN types that correspond to MATLAB cell arrays, you can specify xtype as a numeric value returned by the netcdf.defVlen function.

    Data Types: double | single | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | char | string

    More About

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    Tips

    • You cannot use netcdf.putAtt to set the _FillValue attribute of variables in netCDF-4 files. Use the netcdf.defVarFill function to set the fill value for a variable.

    • This function corresponds to several (nc_put_att_*) functions in the netCDF library C API.

      To use this function, you must be familiar with the netCDF C interface. You can access the netCDF documentation at the netCDF website.

    Version History

    Introduced in R2008b

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    See Also

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