On Fri, Jan 23, 2009 at 02:10, V. Armando Sole <s...@esrf.fr> wrote: > At 01:44 23/01/2009 -0600, Robert Kern wrote: >>It is an inevitable consequence of several features interacting >>together. Basically, Python expands "a[b] += 1" into this: >> >> c = a[b] >> d = c.__iadd__(1) >> a[b] = d >> >>Basically, the array c doesn't know that it was created by indexing a, >>so it can't do the accumulation you want. > > Well inevitable would not be the word I would use. I would have expected a > different behaviour between a[b] = a[b] + 1 and a[b] += 1 > > In the second case python (or numpy) does not need to generate an > intermediate array and could be doing in-place operations.
I'm sorry, but that is how Python implements a[b] += 1. We have no control over it. There is simply no hook to implement that statement as an atomic unit. Instead, we have three hooks, __getitem__, __setitem__, and __iadd__. __iadd__ is orthogonal to the former two, so it can't know that it is operating in that context. >> > Is there a way I can achieve the first result >> > without a for loop? In my application the difference is a factor 10 in >> > execution time (1000 secons instead of 100 ...) >> >>In [6]: bincount? >>Type: builtin_function_or_method >>Base Class: <type 'builtin_function_or_method'> >>String Form: <built-in function bincount> >>Namespace: Interactive >>Docstring: >> bincount(x,weights=None) >> >> Return the number of occurrences of each value in x. >> >> x must be a list of non-negative integers. The output, b[i], >> represents the number of times that i is found in x. If weights >> is specified, every occurrence of i at a position p contributes >> weights[p] instead of 1. >> >> See also: histogram, digitize, unique. > > Indeed what I am doing is very close to histogramming. > > Unfortunaly what I have to add is not just one but a value. I have a set of > scattered points (x,y,z) and values corresponding to those points. My goal > is to get a regular grid in which in each voxel I sum the values of the > points falling in them. That's what the weights input is for. > I guess I will have to write a tiny C extension. I > had expected the += sintax would trigger the in-place operation. It does, just a different one than you want. -- Robert Kern "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://projects.scipy.org/mailman/listinfo/numpy-discussion
