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In general, users set the current variable (which by default is the first variable in the file) to one of those contained in the file and can then apply a number of functions and operators to them :
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| Code Block | ||
|---|---|---|
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setcurrent(netcdf1, 6) |
...
# use the 6th variable in this netCDF file |
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setcurrent(netcdf1, 2) |
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# use the 2nd variable in this netCDF file |
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netcdf3 = netcdf1 op netcdf2 |
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out = function(netcdf1, ...) |
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However, because functions and operators work only on the current variable, when the netCDF contains more than one variable (a multi-variable netcdf), special care must be taken when you need the operator/function to apply to all variables - this is detailed later in this page.
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Example 1 : To operate on a netcdf file which you want to overwrite with the new results
| Code Block | ||
|---|---|---|
| ||
# Derive a cross section of temperature data in a netcdf variable
(...)
# derive the list of netcdf variables
var_list = variables(temp_xs)
# loop over variables and subtract scalar
for i = 1 to count(var_list) do
setcurrent(temp_xs, i)
temp_xs = temp_xs - 273.15 # acts on current variable only
end for |
Example
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2
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:
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To
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operate
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on
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two
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netcdf
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files,
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assigning
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the
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result
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to
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a
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third
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netcdf,
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you
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should
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create
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the
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output
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netcdf
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first
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by
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a
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simple
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copying
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of
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one
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of
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the
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input
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netcdfs
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:
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| Code Block | ||
|---|---|---|
| ||
# Derive cross sections of temperature forecast and analysis
# in two netcdf variables, tfc_xs and tan_xs....
(...)
# derive the list of netcdf variables
var_list = variables(tfc_xs)
# create output netcdf
diff_xs = tfc_xs
# loop over variables and create fc-an difference cross-section
for i = 1 to count(var_list) do
setcurrent(tan_xs, i)
setcurrent(tfc_xs, i)
setcurrent(diff_xs, i)
diff_xs = tfc_xs - tan_xs
end for |
Extracting NetCDF values
If you need to have access to the data values of a netcdf current variable, you can use function values() :
val_list = values(netcdf)
which returns a vector with all the values for the current variable.You can then use the Vector functions to manipulate the data. This technique could even be used to create a new Geopoints variable with the netCDF data, or to insert the values into a GRIB field.
An alternative method for accessing individual values is to the use the function value() :
val = value(netcdf, n)
which returns the nth value from the current netcdf variable.