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EXTREMELY WEIRD arithmetic bug

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louise
louise on 3 Aug 2011
Hi there,
Basically I'm adding 0.001 to a number over and over again. The series starts with zero so the resulting number should ALWAYS be a multiple of 0.01, but instead it eventually degenerates into numbers like 5.874000000000296 - obviously NOT a multiple of 0.001!
Can you explain why? Can it REALLY BE that Matlab just can't cope with numbers of that precision? It's a mathematics package!!!
Run the following code to reproduce the bug. I'm running 7.10.0(R2010a).
Just run log=testcase() and then inspect the latter half of log in the variable editor- when you double-click on a cell to expand it you will be able to see the lack of precision.
function [ log ] = testcase( )
%RUN LIKE THIS: log=testcase()
%Then inspect the log in the variable editor.
runTime=20;
t=0;
dx=0.001;
i=1;
log(i,1)=t;
it=2;
for i=2:runTime/dx
t=t+dx;
log(i,1)=t;
end
end

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Accepted Answer

Arnaud Miege
Arnaud Miege on 3 Aug 2011

  1 Comment

louise
louise on 3 Aug 2011
I'm being an idiot. It's the floating point problem. I know all about the floating point problem.
SIGH.
I was stupid enough to assume that Matlab was using some futuristic high-precision arithmetic. Nope.
As usual, the problem is BAD ASSUMPTIONS.
Thanks for your fast help.

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