EXTREMELY WEIRD arithmetic bug

1 view (last 30 days)
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

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.

Sign in to comment.

More Answers (0)

Products

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!