Remove masked elements from an array

[John Machin]

holger krekel <pyth at devel.trillke.net> wrote in message news:<mailman.1020102370.5619.python-list at python.org>...
> On Mon, Apr 29, 2002 at 01:26:25PM -0400, PoulsenL at capecon.com wrote:
> > I have a Numeric array with missing elements from which I wish to calculate
> > a histogram.  However, in order to do so I must first remove the 'None'
> > values from the array.  I have a kludgy solution, but I was wondering if
> > someone could point me in the right direction on what is the proper method
> > of collapsing a masked array to just the non-masked values.
> 
> there are too solutions. choose yourself :-)
> 
> nums = [ 1,2,3,None,2,3]
> 
> #version1
> def f(x): return x is not None
> newnums = filter(f, nums)
> 
> #version2
> newnums = [ x for x in nums if x is not None]
> 
> The first version uses filter. 
> 'filter' calls 'f' for every item in 'nums' and puts
> the result into the new list iff it has a 'true' value.
> 
> The second version uses list comprehension.
> It basically does the same if you look closely
> at it. 

*Don't* try this if zero is a valid value for your histogram:

newnums = filter(None, nums)

because it will rip out any "false" entries (including 0 or 0.0) that
you have.

Caveat lector: The OP was talking about a Numeric array. A Numeric
user (i.e. not me) may be able to comment on (a) whether filter()
works on Numeric arrays (b) what the preferred solution is for Numeric
arrays.