Results not repeatable for sum() function on fixed / deterministic vector
in [Matlab]
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From: Bruno Luong on 5 Apr 2010 12:54 "Christopher " <camejia(a)remove.this.alum.mit.edu> wrote in message <hpd3lb$8mc$1(a)fred.mathworks.com>... > Hi, > > I've run into an interesting problem with the following code: > > x1 = (0 : 95999) / 40e6; > x2 = x1 - mean(x1); > sum(x2) > sum(x2) > > As I repeat the sum(x2) command and print out the result, I get slightly different answers, not all of which are zero. I can understand that the result doesn't come out to be exactly zero due to precision issues, but I would like to get the same answer every time. Any ideas what is going on here? > > I thought may it was due to the use of threading, if different numbers of threads would interchange the order of operations and hence the final result. But setting maxNumCompThreads to 1 still gives an answer that varies. > > Here's some output from the 'ver' command: > MATLAB Version 7.8.0.347 (R2009a) > Operating System: Microsoft Windows XP x64 Version 5.2 (Build 3790: Service Pack 2) > I believe you point to an almost death horse. Take a look of this thread: http://www.mathworks.com/matlabcentral/newsreader/view_thread/260828 http://www.mathworks.com/matlabcentral/newsreader/view_thread/261541 There is a serious issue with 2009A. If you want predictable sum result, take a look at Jan's XSum http://www.mathworks.com/matlabcentral/fileexchange/26800-xsum (I have my own FEX for this very reason). Bruno
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