Re: [Numpy-discussion] Array accumulation in numpy
Thanks to all for a very quick response. np.bincount does what I need. Tony On 02/19/2013 10:04 AM, Benjamin Root wrote: > > > On Tue, Feb 19, 2013 at 10:00 AM, Tony Ladd <mailto:tl...@che.ufl.edu>> wrote: > > I want to accumulate elements of a vector (x) to an array (f) based on > an index list (ind). > > For example: > > x=[1,2,3,4,5,6] > ind=[1,3,9,3,4,1] > f=np.zeros(10) > > What I want would be produced by the loop > > for i=range(6): > f[ind[i]]=f[ind[i]]+x[i] > > The answer is f=array([ 0., 7., 0., 6., 5., 0., 0., 0., > 0., 3.]) > > When I try to use implicit arguments > > f[ind]=f[ind]+x > > I get f=array([ 0., 6., 0., 4., 5., 0., 0., 0., 0., 3.]) > > > So it takes the last value of x that is pointed to by ind and adds > it to > f, but its the wrong answer when there are repeats of the same > entry in > ind (e.g. 3 or 1) > > I realize my code is incorrect, but is there a way to make numpy > accumulate without using loops? I would have thought so but I cannot > find anything in the documentation. > > Would much appreciate any help - probably a really simple question. > > Thanks > > Tony > > > I believe you are looking for the equivalent of accumarray in Matlab? > > Try this: > > http://www.scipy.org/Cookbook/AccumarrayLike > > It is a bit touchy about lists and 1-D numpy arrays, but it does the job. > Also, I think somebody posted an optimized version for simple sums > recently to this list. > > Cheers! > Ben Root > > > > ___ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion -- Tony Ladd Chemical Engineering Department University of Florida Gainesville, Florida 32611-6005 USA Email: tladd-"(AT)"-che.ufl.edu Webhttp://ladd.che.ufl.edu Tel: (352)-392-6509 FAX: (352)-392-9514 ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] Array accumulation in numpy
Alan Thanks - I felt sure there had to be an easy idea. Best Tony On 02/19/2013 10:17 AM, Alan G Isaac wrote: > x=[1,2,3,4,5,6] > ind=[1,3,9,3,4,1] > f=np.zeros(10) > np.bincount(ind,x) > > hth, > Alan Isaac > > ___ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion -- Tony Ladd Chemical Engineering Department University of Florida Gainesville, Florida 32611-6005 USA Email: tladd-"(AT)"-che.ufl.edu Webhttp://ladd.che.ufl.edu Tel: (352)-392-6509 FAX: (352)-392-9514 ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
[Numpy-discussion] Array accumulation in numpy
I want to accumulate elements of a vector (x) to an array (f) based on an index list (ind). For example: x=[1,2,3,4,5,6] ind=[1,3,9,3,4,1] f=np.zeros(10) What I want would be produced by the loop for i=range(6): f[ind[i]]=f[ind[i]]+x[i] The answer is f=array([ 0., 7., 0., 6., 5., 0., 0., 0., 0., 3.]) When I try to use implicit arguments f[ind]=f[ind]+x I get f=array([ 0., 6., 0., 4., 5., 0., 0., 0., 0., 3.]) So it takes the last value of x that is pointed to by ind and adds it to f, but its the wrong answer when there are repeats of the same entry in ind (e.g. 3 or 1) I realize my code is incorrect, but is there a way to make numpy accumulate without using loops? I would have thought so but I cannot find anything in the documentation. Would much appreciate any help - probably a really simple question. Thanks Tony -- Tony Ladd Chemical Engineering Department University of Florida Gainesville, Florida 32611-6005 USA Email: tladd-"(AT)"-che.ufl.edu Webhttp://ladd.che.ufl.edu Tel: (352)-392-6509 FAX: (352)-392-9514 ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
