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.. currentmodule:: cf
.. default-role:: obj
A first example
===============
The cf package allows a data array and its associated metadata to be
contained and manipulated as a single entity called a *field*, which
is stored in a `cf.Field` object.
Much of the basic field manipulation syntax can be seen in this simple
read-modify-write example which:
* Reads a field from a file on disk and find out information about
it.
* Modifies a CF property and the units of its data.
* Modifies the data values.
* Modifies a subspace of the data values.
* Writes it out to another file on disk.
The example may be reproduced by downloading the :download:`sample
netCDF file (file.nc) <../file.nc>` (taking care not to overwrite an
existing file with that name) [#f1]_.
.. .. raw:: html
:file: examples/first_example.html
:download:`1 <examples/first_example.html>`
:download:`2 <examples/regridc_example.html>`
:download:`3 <examples/regrids_example_2.html>`
:download:`4 <examples/regrids_example.html>`
**1.** Import the cf package.
>>> import cf
**2.** Read a field from disk and find a summary of its contents.
>>> f = cf.read('file.nc')
>>> type(f)
cf.field.Field
>>> f
<CF Field: air_temperature(latitude(4), longitude(5)) K>
>>> print f
air_temperature field summary
-----------------------------
Data : air_temperature(latitude(4), longitude(5)) K
Cell methods : time: mean
Axes : time(1) = [2000-01-16 00:00:00] 360_day
: height(1) = [2.0] m
: latitude(4) = [-2.5, ..., 5.0] degrees_north
: longitude(5) = [0.0, ..., 15.0] degrees_east
**3.** Find all of the field's CF properties and its data array as a
numpy array.
>>> f.properties
{'Conventions': 'CF-1.5',
'_FillValue': 1e+20,
'experiment_id': 'stabilization experiment (SRES A1B)',
'long_name': 'Air Temperature',
'standard_name': 'air_temperature',
'title': 'SRES A1B'}
>>> print f.array
[[ 274.15, 276.15, 275.15, 277.15, 278.15],
[ 274.15, 275.15, 276.15, 277.15, 276.15],
[ 277.15, 275.15, 278.15, 274.15, 278.15],
[ 276.15, 274.15, 275.15, 277.15, 274.15]]
**4.**. Modify the field's long name CF property and change the field's
data from units of Kelvin to Celsius.
.. note:: Changing the units automatically changes the data when it is
next accessed.
>>> f.long_name
'Air Temperature'
>>> f.long_name = 'Surface Air Temperature'
>>> f.long_name
'Surface Air Temperature'
>>> print f.Units
K
>>> f.Units -= 273.15
>>> print f.Units
K @ 273.15
>>> print f.array
[[ 1. 3. 2. 4. 5.]
[ 1. 2. 3. 4. 3.]
[ 4. 2. 5. 1. 5.]
[ 3. 1. 2. 4. 1.]]
**5.** Check that the field has 'air_temperature' as its standard name
and that at least one of its latitude coordinate values is
greater than 0.0.
>>> f.match('air_temperature', items={'latitude' : cf.gt(0)})
True
**6.** Modify the data values.
>>> g = f + 100
>>> print g.array
[[ 101. 103. 102. 104. 105.]
[ 101. 102. 103. 104. 103.]
[ 104. 102. 105. 101. 105.]
[ 103. 101. 102. 104. 101.]]
>>> g = f + f
>>> print g.array
[[ 2. 6. 4. 8. 10.]
[ 2. 4. 6. 8. 6.]
[ 8. 4. 10. 2. 10.]
[ 6. 2. 4. 8. 2.]]
>>> g = f / cf.Data(2, 'seconds')
>>> print g.array
[[0.5 1.5 1.0 2.0 2.5]
[0.5 1.0 1.5 2.0 1.5]
[2.0 1.0 2.5 0.5 2.5]
[1.5 0.5 1.0 2.0 0.5]]
>>> print g.Units
s-1.K
**7.** Access and modify a subspace of the data values.
>>> g = f.subspace[0::2, 2:4]
>>> print g.array
[[ 2. 4.]
[ 5. 1.]]
>>> f.subspace[0::2, ...] = -10
>>> print f.array
[[-10. -10. -10. -10. -10.]
[ 1. 2. 3. 4. 3.]
[-10. -10. -10. -10. -10.]
[ 3. 1. 2. 4. 1.]]
**8.** Write the modified field to disk.
>>> cf.write(f, 'newfile.nc')
----
.. rubric:: Footnotes
.. [#f1] This file may be also found in the ``docs/build/_downloads/``
directory of the installation.
|
