On Nov 7, 2007 6:46 PM, Timothy Hochberg <tim.hochberg@ieee.org> wrote:
On Nov 7, 2007 10:35 AM, Sebastian Haase <haase@msg.ucsf.edu> wrote:
On Nov 7, 2007 5:23 PM, Matthieu Brucher <matthieu.brucher@gmail.com>
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
Thanks Tim, this sound like a good idea. How about creating an a = a.view() before changing dtype. This should make the proposed solution thread safe again. How "expensive" is the creation of a view (performance wise, e.g. compared to calling a trivial C-function) ? Thanks, Sebastian