[Numpy-discussion] Counting array elements

Todd Miller jmiller at stsci.edu
Thu Oct 21 15:08:52 CDT 2004


On Thu, 2004-10-21 at 18:01, Todd Miller wrote:
> On Thu, 2004-10-21 at 14:30, Todd Miller wrote:
> > On Thu, 2004-10-21 at 13:55, Stephen Walton wrote:
> > > Is there some simple way of counting the number of array elements which
> > > satisfy a certain condition?  It is easy to do
> > > 
> > > A[A<=1].sum()
> > > 
> > > to sum all the values of A which are less than 1, but there doesn't seem
> > > to be a count() method.  I tried
> > > 
> > > (A<=1).sum()
> > >
> > > but this throws an exception at numarray 1.1.  If I try
> > 
> > This works now in CVS and will be part of numarray-1.2.  
> 
> Stephen tried this and it turns out my earlier statement was untrue,
> (A<=1).sum() doesn't do anything reasonable, even in CVS.  The problem
> is that sum() is written (without direct C support) to conserve
> storage.  As a result,  it doesn't do implicit 
> > Another more
> > tedious approach which works for numarray-1.1 is:
> > 
> > (A <= 1).astype('Int32').sum()
> > 
> 
> There's also a prettier approach that works for 1.1 that I forgot about:
> 
> (A <= 1).sum('Int32')
> 
> > > sum(A<=value)
> > > 
> > > I have to nest multiple sums if A has rank greater than 1, plus the sum
> > > overflows if A is large, apparently because boolean gets treated as
> > > Int8.  (Try A=arange(1024,shape=(32,32));sum(sum(A<=1024)).  You get
> > > zero.)  The following works:
> > > 
> > > array(A<=1024,type=Int32).sum()
> > > 
> > > but is awkward.  Am I missing an obvious better alternative?  If not,
> > > I'm going to file an RFE :-) .
> > 
> > I don't think there's any need for an RFE, provided you're satisfied
> > with (A<=1).sum().
> > 
> > Regards,
> > Todd
> > 
> > 
> > 
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