On Fri, Apr 2, 2010 at 10:11 AM, Robert Kern <robert.k...@gmail.com> wrote: > On Thu, Apr 1, 2010 at 22:07, Shailendra <shailendra.vi...@gmail.com> wrote: >> Hi All, >> Below is some array behaviour which i think is odd >>>>> a=arange(10) >>>>> a >> array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) >>>>> b=nonzero(a<0) >>>>> b >> (array([], dtype=int32),) >>>>> if not b[0]: >> ... print 'b[0] is false' >> ... >> b[0] is false >> >> Above case the b[0] is empty so it is fine it is considered false >> >>>>> b=nonzero(a<1) >>>>> b >> (array([0]),) >>>>> if not b[0]: >> ... print 'b[0] is false' >> ... >> b[0] is false >> >> Above case b[0] is a non-empty array. Why should this be consider false. >> >>>>> b=nonzero(a>8) >>>>> b >> (array([9]),) >>>>> if not b[0]: >> ... print 'b[0] is false' >> ... >>>>> >> Above case b[0] is non-empty and should be consider true.Which it does. >> >> I don't understand why non-empty array should not be considered true >> irrespective to what value they have. > > We raise an exception for most arrays. We used to evaluate bool(arr) > as True if any element in arr were nonzero. However, people routinely > got confused and thought that the behavior was that it would return > False if any element were zero. It's ambiguous, so we raise an > exception to prevent errors and misconceptions. Empty arrays are very > rare, so we decided that evaluating bool(arr) as True if there were > any elements had very little use case and would still have the > potential to confuse people. As shown below, there is a better, more > explicit way to test for emptiness. > > However, we do allow single-element arrays to evaluate to True or > False based on its single element because that is unambiguous. This is > possibly a mistake, but it tends not to cause too many problems. I > don't know why we also allow bool(array([])) to evaluate to False, > too; we probably shouldn't. > >> Also, please suggest the best way to differentiate between an empty >> array and non-empty array( irrespective to what is inside array). > > a.size > 0 > > -- > Robert Kern
with examples: >>> bool(0) False >>> bool(1) True >>> bool(np.array([0])) False >>> bool(np.array([1])) True >>> bool([0]) True >>> bool([1]) True >>> bool(np.array([[0]])) False >>> bool(np.array([[1]])) True >>> np.array([[1]]).size 1 Josef > > "I have come to believe that the whole world is an enigma, a harmless > enigma that is made terrible by our own mad attempt to interpret it as > though it had an underlying truth." > -- Umberto Eco > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
