ncwriteatt

Write attribute to netCDF file

Syntax

ncwriteatt(filename,location,attname,attvalue)

ncwriteatt(filename,location,attname,attvalue,"Datatype",dtype)

Description

ncwriteatt(filename,location,attname,attvalue) creates or modifies the attribute specified by attname in the group or variable specified by location in the netCDF file specified by filename. 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.

ncwriteatt(filename,location,attname,attvalue,"Datatype",dtype) writes attvalue as the data type specified by dtype. For example, specify dtype as "string" to write the value in attvalue as a string.

Examples

Create Global Attribute

Make a writable local copy of the example.nc file, and examine the value of its global creation_date attribute.

copyfile(which("example.nc"),"myfile.nc")
fileattrib("myfile.nc","+w")
creationDate = ncreadatt("myfile.nc","/","creation_date")

creationDate = 
'29-Mar-2010'

Create a modification_date attribute, set it to the current date, and verify its value.

ncwriteatt("myfile.nc","/","modification_date",datestr(datetime("now")))
modificationDate = ncreadatt("myfile.nc","/","modification_date")

modificationDate = 
'03-Mar-2023 06:28:53'

Modify Attribute of Variable

Make a writable local copy of the example.nc file, and examine the value of the description attribute of the peaks variable.

copyfile(which("example.nc"),"myfile.nc")
fileattrib("myfile.nc","+w")
oldDescription = ncreadatt("myfile.nc","peaks","description")

oldDescription = 
'z = peaks(50);'

Update the attribute and verify its new value.

ncwriteatt("myfile.nc","peaks","description","Output of PEAKS")
newDescription = ncreadatt("myfile.nc","peaks","description")

newDescription = 
'Output of PEAKS'

Write Non-ASCII Data as Attribute

Make a writable local copy of the example.nc file.

copyfile(which("example.nc"),"myfile.nc")
fileattrib("myfile.nc","+w")

Write the string array ["°F" "°C"] as the value of a new attribute Units of the variable /grid1/temp. The array contains non-ASCII string data, which is supported only for files with format netcdf4.

ncwriteatt("myfile.nc","/grid1/temp","Units",["°F" "°C"])

Verify the value of the new attribute.

ncreadatt("myfile.nc","/grid1/temp","Units")

ans = 1x2 string
    "°F"    "°C"

Write Attribute with Specified Data Type

Create a netCDF-4 format file with a variable named Calendar. Then, write the character vector 'July' as type NC_STRING to the attribute named Months by specifying the data type as "string". By default, ncwriteatt writes scalar text data as type NC_CHAR.

nccreate("myfile.nc","Calendar","Format","netcdf4")
ncwriteatt("myfile.nc","Calendar","Months",'July',"Datatype","string")

Verify the value and data type of the new attribute.

ncreadatt("myfile.nc","Calendar","Months")

ans = 
"July"

Input Arguments

filename — Filename

string scalar | character vector

Filename of an existing netCDF file, specified as a string scalar or character vector.

If the netCDF file does not exist, then use the nccreate function to create it first.

Example: "myFile.nc"

location — Location of variable or group

string scalar | character vector

Location of a variable or group in the netCDF file, specified as a string scalar or character vector. To write a global attribute, set location to "/" (forward slash).

Example: "myVar"

Example: "/myGrp/mySubGrp/myNestedVar"

Example: "myGrp"

Example: "/myGrp/mySubGrp"

attname — Attribute name

string scalar | character vector

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

Example: "myAttribute"

attvalue — Attribute value

numeric array | text

Attribute value, specified as a numeric array or text.

Note

If attvalue has more than one dimension, the ncwriteatt function flattens attvalue in column-major order before writing the attribute value. For example, if attvalue is [1 2 3; 4 5 6], then the attribute value written to attname is [1 4 2 5 3 6].

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

dtype — MATLAB data type

"double" | "single" | "int32" | ...

MATLAB data type to use when writing the attribute, specified as one of the values in this table. When ncwriteatt creates or modifies the attribute in the netCDF file, it uses the corresponding netCDF data type.

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

(*) These values of dtype are available only for files with format netcdf4.

Example: "int16"

Data Types: string | char

More About

MATLAB to NetCDF Data Type Conversion

The netCDF-related MATLAB functions automatically choose the netCDF data type that best matches the MATLAB data type according to this table.

MATLAB Data Type	NetCDF Data Type
double	NC_DOUBLE
single	NC_FLOAT
int32	NC_INT
int16	NC_SHORT
int8	NC_BYTE
char	NC_CHAR
string scalar	NC_CHAR
int64 (*)	NC_INT64
uint64 (*)	NC_UINT64
uint32 (*)	NC_UINT
uint16 (*)	NC_USHORT
uint8 (*)	NC_UBYTE
string vector (*)	NC_STRING

(*) These MATLAB data types are available only for files with format netcdf4.

Version History

Introduced in R2011a

R2021b: Write NC_STRING attributes

You can write NC_STRING attributes to netCDF-4 files.