Re: assigning a fill value

hi Dave,

y = ma.masked_where(y == -999.9, y, copy = 0) # avoids data copy

Using the above works, however I do get a warning when doing so:

/Users/jwhitaker/lib/python/numpy/ma/core.py:2304: UserWarning: Warning:
converting a masked element to nan.
   warnings.warn("Warning: converting a masked element to nan.")

Perhaps there is a better way to do what I want. What I'm doing is
reading in data from a netcdf file --- this data already has a fillvalue
associated with it. All I want to do is to transfer a subset of this
data to the array y and then plot y. How can I transfer the fillvalue
of the incoming data to y?

Thanks, Gary

David Brown wrote:
> Hi Gary,
>
> Ordinary NumPy arrays do not support the concept of missing values. To
> get support for missing data you need to use the NumPy MaskedArray
> subclass. The paradigm is a little different from the NetCDF _FillValue
> concept. Associated with the masked array is a 'mask' array, a Boolean
> array with the same dimensions as the data array. It is set to True for
> values that need to be masked and false otherwise. It also includes a
> fill_value attribute, which is only used when converting to a 'filled'
> array.
>
> In your case one way you could get a masked array is as follows (assume
> that you don't know that the array actually contains no values equal to
> the filled value):
>
> from numpy import ma
>
> y= numpy.zeros((plot_len+15),'f')
> y = ma.masked_where(y == -999.9, y, copy = 0) # avoids data copy
>
> # set the fill value
>
> y.set_fill_value(-999.9)
>
> You can also create a masked array directly using ma.array() or its
> equivalent ma.MaskedArray().
>
> The function signature is
> x = MaskedArray(data, mask=nomask, dtype=None, copy=True,
> fill_value=None, keep_mask=True, hard_mask=False, shrink=True)
>
>
> PyNgl (and PyNio) understand masked arrays and will behave appropriately
> when given one.
> PyNIo has various options when reading data from a file concerning how
> or whether to create a masked array.
>
> To get a pure NumPy array that has the masked elements replaced by the
> fill value use:
>
> y_notmasked = y.filled(fill_value)
>
> Hope this helps.
> -dave
>
>
> On Dec 8, 2008, at 5:12 PM, Gary Bates wrote:
>
>> How does one assign a fill value to an array in PyNGL?
>>
>> Here's what I tried and the error I got:
>>
>> y= numpy.zeros((plot_len+15),'f')
>> y._FillValue = -999.9
>>
>> AttributeError: 'numpy.ndarray' object has no attribute '_FillValue'
>>
>> -Gary
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Received on Tue Dec 09 2008 - 10:33:55 MST