On Nov 7, 2007 10:35 AM, Sebastian Haase <[EMAIL PROTECTED]> wrote: > On Nov 7, 2007 5:23 PM, Matthieu Brucher <[EMAIL PROTECTED]> > wrote: > > > > > I don't understand. I'm thinking of most math functions in the > > > C-library. In C a boolean is just an integer of 0 or 1 (quasi, by > > > definition). > > > > > > Could you explain what you mean ? > > > > > > > In C++, bool is a new type that has two values, true and false. If you > add > > true and true, it is still true, and not 2. In C, everything that is not > 0 > > is true, not in C++. > > Yes, I know this. But my situation is "the other way around". Lets say > I want to count "foreground pixels" in an image: I would want to "sum" > all the true values, i.e. a true *is* a 1 and a false *is* a 0. > > In other words, I'm really thinking of (older kind of) C, where there > *was* no bool. > I assume this thinking still applies to the internal arithmetic of CPUs > today. > Also the "bit-values" of a boolean array (in memory) are set this way > already anyway ! > > How can I simply call my functions looking at these bit values ? > (essentially interpreting a boolean true as 1 and false as 0)
I'm not sure how well this would work, but could you change the dtype before passing the array to your function? If you wanted a copy, you could just to the equivalent of a.astype(unit8). However, if you didn't want a copy, you could set the dtype to unit8, operate on the array and then reset it to bool: >>> a = np.array([True, True, False, True]) >>> a array([ True, True, False, True], dtype=bool) >>> a.dtype = np.uint8 >>> a array([1, 1, 0, 1], dtype=uint8) >>> # do something with 'a' here >>> a.dtype = bool >>> a array([ True, True, False, True], dtype=bool) This assumes everything is single threaded. If you have multiple threads accessing 'a', this could be a problem... And, you probably want to do this in C, so translate as appropriate. -tim > > > -Sebastian > _______________________________________________ > Numpy-discussion mailing list > [email protected] > http://projects.scipy.org/mailman/listinfo/numpy-discussion > -- . __ . |-\ . . [EMAIL PROTECTED]
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