Path: news.mathworks.com!not-for-mail From: <HIDDEN> Newsgroups: comp.soft-sys.matlab Subject: the sign of the sum of the difference different from the sign of the difference of the sums Date: Wed, 23 Dec 2009 15:40:25 +0000 (UTC) Organization: The MathWorks, Inc. Lines: 20 Message-ID: <hgtdl9$212$1@fred.mathworks.com> Reply-To: <HIDDEN> NNTP-Posting-Host: webapp-05-blr.mathworks.com Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Trace: fred.mathworks.com 1261582825 2082 172.30.248.35 (23 Dec 2009 15:40:25 GMT) X-Complaints-To: news@mathworks.com NNTP-Posting-Date: Wed, 23 Dec 2009 15:40:25 +0000 (UTC) X-Newsreader: MATLAB Central Newsreader 2146203 Xref: news.mathworks.com comp.soft-sys.matlab:594531 Dear experts, I'm using matlab 2009b with win xp (coreduo E8500). I should calcolate the difference of the sum of two vectors and I noticed this misbehavior: (you can find the workspace with the two vectors at http://www.box.net/shared/1rtxb8xt28 ): >> format long >> sum(lpt -lvt) ans = -1.455870024968571e-015 >> sum(lpt)-sum(lvt) ans = 1.136868377216160e-013 ok, the numbers are not equal due to the finite precision arithmetic. But, what about the sign? I cannot distinguish which one of the two sums is larger. Is there a way to determine the correct sign of the result, i.e. which one of the sums is larger? Thanks in advance, Lorenzo