ength of condition array.
axis : {None, int}, optional
    Axis along which the operation must be performed.
out : {None, ndarray}, optional
    Alternative output array in which to place the result. It must have
    the same shape as the expected output but the type will be cast if
    necessary.

Returns
-------
result : MaskedArray
    A :class:`~ma.MaskedArray` object.

Notes
-----
Please note the difference with :meth:`compressed` !
The output of :meth:`compress` has a mask, the output of
:meth:`compressed` does not.

Examples
--------
>>> import numpy as np
>>> x = np.ma.array([[1,2,3],[4,5,6],[7,8,9]], mask=[0] + [1,0]*4)
>>> x
masked_array(
  data=[[1, --, 3],
        [--, 5, --],
        [7, --, 9]],
  mask=[[False,  True, False],
        [ True, False,  True],
        [False,  True, False]],
  fill_value=999999)
>>> x.compress([1, 0, 1])
masked_array(data=[1, 3],
             mask=[False, False],
       fill_value=999999)

>>> x.compress([1, 0, 1], axis=1)
masked_array(
  data=[[1, 3],
        [--, --],
        [7, 9]],
  mask=[[False, False],
        [ True,  True],
        [False, False]],
  fill_value=999999)

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