I was expecting to just say something like
ma.zeros(my_shape, mask=my_mask, hard_mask=True)
(where the mask is the correct shape) but ma.zeros (or ma.ones or ma.empty) rather surprisingly doesn't recognise the mask argument. The simplest I've come up with is
ma.array(np.zeros(my_shape), mask=my_mask, hard_mask=True)
which seems to involve unnecessary copying of lots of zeros. Is there a better way?
Make a masked array:
In [162]: x = np.arange(5); mask=np.array([1,0,0,1,0],bool)
In [163]: M = np.ma.MaskedArray(x,mask)
In [164]: M
Out[164]:
masked_array(data=[--, 1, 2, --, 4],
mask=[ True, False, False, True, False],
fill_value=999999)
Modify x, and see the result in M:
In [165]: x[-1] = 10
In [166]: M
Out[166]:
masked_array(data=[--, 1, 2, --, 10],
mask=[ True, False, False, True, False],
fill_value=999999)
In [167]: M.data
Out[167]: array([ 0, 1, 2, 3, 10])
In [169]: M.data.base
Out[169]: array([ 0, 1, 2, 3, 10])
The M.data is a view of the array used in creating it. No unnecessary copies.
I haven't used functions like np.ma.zeros, but
In [177]: np.ma.zeros
Out[177]: <numpy.ma.core._convert2ma at 0x1d84a052af0>
_convert2ma is a Python class, that takes a funcname and returns new callable. It does not add mask-specific parameters. Study that yourself if necessary.
np.ma.MaskedArray, the function that actually subclasses ndarray takes a copy parameter
copy : bool, optional
Whether to copy the input data (True), or to use a reference instead.
Default is False.
and the first line of its __new__ is
_data = np.array(data, dtype=dtype, copy=copy,
order=order, subok=True, ndmin=ndmin)
I haven't quite sorted out whether M._data is just a reference to the source data, or a view. In either case, it isn't a copy, unless you say so.
I haven't worked a lot with masked arrays, but my impression is that, while they can be convenient, they shouldn't be used where you are concerned about performance. There's a lot of extra work required to maintain both the mask and the data. The extra time involved in copying the data array, if any, will be minor.
