[Numpy-discussion] Counting array elements

Stephen Walton stephen.walton at csun.edu
Thu Oct 21 10:59:05 CDT 2004


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

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 :-) .

-- 
Stephen Walton <stephen.walton at csun.edu>
Dept. of Physics & Astronomy, Cal State Northridge
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