On 03/25/2012 06:55 AM, Pierre Haessig wrote: > Hi, > > I have an off topic but somehow related question : > > Le 19/03/2012 12:04, Matthieu Rigal a écrit : >> array = numpy.logical_and(numpy.logical_and(aBlueChannel< 1.0, aNirChannel> >> (aBlueChannel * 1.0)), aNirChannel< (aBlueChannel * 1.8)) > Is there any significant difference between : > > z = np.logical_and(x,y) and > z= x& y (assuming x and y are already numpy arrays and not just list) > > I've always used the& (and | and ~) operator because it's of course > much shorter ;-) > > I've seen no mention of the "&" operator in np.logical_and docstring so > I wonder...
There is a big difference: &, |, and ~ are bitwise operators, not logical operators, so they work like logical operators only if operating on booleans (or at least arrays containing nothing but integer zeros and ones) and only if you bear in mind that & and | have lower precedence than their logical counterparts. Therefore you often need to use more parentheses than you might have expected. In [1]: a = np.array([1]) In [2]: b = np.array([2]) In [5]: np.logical_and(a,b) Out[5]: array([ True], dtype=bool) In [6]: a & b Out[6]: array([0]) Using the bitwise operators in place of logical operators is a hack to get around limitations of the language; but, if done carefully, it is a useful one. Eric > > Best, > Pierre > > > > > _______________________________________________ > 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
