[Numpy-discussion] easy way to change part of only unmasked elements value?

[Richard Hattersley]

Hi Chao,

If you don't mind modifying masked values, then if you write to the
underlying ndarray it won't touch the mask:

>>> a = np.ma.masked_less(np.arange(10),5)
>>> a.base[3:6] = 1
>>> a
masked_array(data = [-- -- -- -- -- 1 6 7 8 9],
             mask = [ True  True  True  True  True False False False False
False],
       fill_value = 999999)

Regards,
Richard Hattersley


On 10 September 2012 17:43, Chao YUE <chaoyuejoy at gmail.com> wrote:

> Dear all numpy users,
>
> what's the easy way if I just want to change part of the unmasked array
> elements into another new value? like an example below:
> in my real case, I would like to change a subgrid of a masked numpy array
> to another value, but this grid include both masked and unmasked data.
> If I do a simple array[index1:index2, index3:index4] = another_value,
> those data with original True mask will change into False. I am using numpy
> 1.6.2.
> Thanks for any ideas.
>
> In [91]: a = np.ma.masked_less(np.arange(10),5)
>
> In [92]: or_mask = a.mask.copy()
> In [93]: a
> Out[93]:
> masked_array(data = [-- -- -- -- -- 5 6 7 8 9],
>              mask = [ True  True  True  True  True False False False False
> False],
>        fill_value = 999999)
>
>
> In [94]: a[3:6]=1
>
> In [95]: a
> Out[95]:
> masked_array(data = [-- -- -- 1 1 1 6 7 8 9],
>              mask = [ True  True  True False False False False False False
> False],
>        fill_value = 999999)
>
>
> In [96]: a = np.ma.masked_array(a,mask=or_mask)
>
> In [97]: a
> Out[97]:
> masked_array(data = [-- -- -- -- -- 1 6 7 8 9],
>              mask = [ True  True  True  True  True False False False False
> False],
>        fill_value = 999999)
>
> Chao
>
> --
>
> ***********************************************************************************
> Chao YUE
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>
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>
>
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