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other:python:misc_by_jyp [2022/03/08 16:40]
jypeter [Playing with strings]
other:python:misc_by_jyp [2022/07/08 14:00] (current)
jypeter [numpy related stuff] Added the arbitrary object array
Line 247: Line 247:
  
 ==== numpy related stuff ==== ==== numpy related stuff ====
 +
 +=== Using a numpy array to store arbitrary objects ===
 +
 +The numpy arrays are usually used to store [[https://​numpy.org/​doc/​stable/​reference/​arrays.scalars.html|scalars]] of the same type (see also the [[https://​numpy.org/​doc/​stable/​reference/​arrays.dtypes.html|Data type objects (dtype)]]), very often numerical values.
 +
 +It is also possible to store **arbitrary** Python objects in an array, rather than using nested lists or dictionaries!
 +
 +<​code>>>>​ some_array = np.empty((2,​ 3), dtype=object)
 +>>>​ some_array
 +array([[None,​ None, None],
 +       ​[None,​ None, None]], dtype=object)
 +>>>​ some_array.shape
 +(2, 3)
 +>>>​ print(some_array[-1,​ -1])
 +None
 +>>>​ some_array[-1,​ 0] = filled_contour # e.g. save an existing cartopy filled contour object
 +>>>​ some_array
 +array([[None,​ None, None],
 +       ​[<​cartopy.mpl.contour.GeoContourSet object at 0x2ab679e8bf10>,​
 +        None, None]], dtype=object)</​code>​
 +        ​
 +=== Dealing with a variable number of indices ===
 +
 +[[https://​numpy.org/​doc/​stable/​user/​basics.indexing.html#​dealing-with-variable-indices|Official reference]]
 +
 +<​code>>>>​ i10 = np.identity(10)
 +>>>​ i10
 +array([[1., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
 +       [0., 1., 0., 0., 0., 0., 0., 0., 0., 0.],
 +...
 +       [0., 0., 0., 0., 0., 0., 0., 0., 0., 1.]])
 +>>>​ i10.shape
 +(10, 10)
 +
 +>>>​ i10[3:7, 4:6]
 +array([[0., 0.],
 +       [1., 0.],
 +       [0., 1.],
 +       [0., 0.]])
 +       
 +>>>​ s0 = slice(3, 7)
 +>>>​ s1 = slice(4, 6)
 +>>>​ i10[s0, s1]
 +array([[0., 0.],
 +       [1., 0.],
 +       [0., 1.],
 +       [0., 0.]])
 +       
 +>>>​ my_slices = (s0, s1)
 +>>>​ i10[my_slices]
 +array([[0., 0.],
 +       [1., 0.],
 +       [0., 1.],
 +       [0., 0.]])
 +       
 +>>>​ my_fancy_slices = (s0, Ellipsis)
 +>>>​ i10[my_fancy_slices]
 +array([[0., 0., 0., 1., 0., 0., 0., 0., 0., 0.],
 +       [0., 0., 0., 0., 1., 0., 0., 0., 0., 0.],
 +       [0., 0., 0., 0., 0., 1., 0., 0., 0., 0.],
 +       [0., 0., 0., 0., 0., 0., 1., 0., 0., 0.]])
 +>>>​ i10[my_fancy_slices].shape
 +(4, 10)
 +
 +>>>​ # WARNING! DANGERRRR! NEVER forget that a VIEW is NOT A COPY
 +>>>​ # and that you can change the content of the original array by mistake
 +>>>​ my_view = i10[my_slices]
 +>>>​ my_view[:, :] = -1
 +>>>​ my_view
 +array([[-1.,​ -1.],
 +       [-1., -1.],
 +       [-1., -1.],
 +       [-1., -1.]])
 +>>>​ i10
 +array([[ 1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
 +       [ 0.,  1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
 +       [ 0.,  0.,  1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
 +       [ 0.,  0.,  0.,  1., -1., -1.,  0.,  0.,  0.,  0.],
 +       [ 0.,  0.,  0.,  0., -1., -1.,  0.,  0.,  0.,  0.],
 +       [ 0.,  0.,  0.,  0., -1., -1.,  0.,  0.,  0.,  0.],
 +       [ 0.,  0.,  0.,  0., -1., -1.,  1.,  0.,  0.,  0.],
 +       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  1.,  0.,  0.],
 +       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  1.,  0.],
 +       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  1.]])</​code>​
  
 === Finding and counting unique values === === Finding and counting unique values ===
other/python/misc_by_jyp.1646757652.txt.gz · Last modified: 2022/03/08 16:40 by jypeter