Is it safe to compare two numbers in MATLAB compiled DLL file?
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From: Steven Lord on 18 Mar 2010 16:50 "Mathew " <jhwang2k(a)gmail.com> wrote in message news:hnu0sc$hq1$1(a)fred.mathworks.com... > Thanks for everyone. > > It is better to avoid a==b as much as we can. As we can see: > 0.3-0.3-0.1==0 is not true even in MATLAB. 0.3-0.3-0.1 == 0 better not be true OUTSIDE of MATLAB either :) Unless you somehow had something like an "unsigned float" ... But yes, with your typo fixed 0.3-0.2-0.1 is not guaranteed to be exactly equal to 0 in floating-point. > But I want to make sure if we cannot use == at all. In MATLAB, we can see > i==j, i==1, length(x)==100 everywhere, even in function from MATHWORKS. > How safe they are then? As James said, if we KNOW that i and j must be of an integer class or must be floating-point numbers with integer values (flints) then that's pretty safe. In the case with LENGTH, the function by its definition should return a nonnegative flint -- if it returns a negative value, or a noninteger value and x is not an object for which you have SPECIFICALLY overloaded LENGTH (and EQ) to handle that scenario, you probably have other problems to deal with. -- Steve Lord slord(a)mathworks.com comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ
From: Mathew on 18 Mar 2010 17:05 Now I am feeling safe as the flints are OK. Otherwise, it is really scary... "Steven Lord" <slord(a)mathworks.com> wrote in message <hnu3me$9b9$1(a)fred.mathworks.com>... > > "Mathew " <jhwang2k(a)gmail.com> wrote in message > news:hnu0sc$hq1$1(a)fred.mathworks.com... > > Thanks for everyone. > > > > It is better to avoid a==b as much as we can. As we can see: > > 0.3-0.3-0.1==0 is not true even in MATLAB. > > 0.3-0.3-0.1 == 0 better not be true OUTSIDE of MATLAB either :) Unless you > somehow had something like an "unsigned float" ... > > But yes, with your typo fixed 0.3-0.2-0.1 is not guaranteed to be exactly > equal to 0 in floating-point. > > > But I want to make sure if we cannot use == at all. In MATLAB, we can see > > i==j, i==1, length(x)==100 everywhere, even in function from MATHWORKS. > > How safe they are then? > > As James said, if we KNOW that i and j must be of an integer class or must > be floating-point numbers with integer values (flints) then that's pretty > safe. In the case with LENGTH, the function by its definition should return > a nonnegative flint -- if it returns a negative value, or a noninteger value > and x is not an object for which you have SPECIFICALLY overloaded LENGTH > (and EQ) to handle that scenario, you probably have other problems to deal > with. > > -- > Steve Lord > slord(a)mathworks.com > comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ >
From: James Tursa on 18 Mar 2010 17:53 "Mathew " <jhwang2k(a)gmail.com> wrote in message <hnu4ig$oaa$1(a)fred.mathworks.com>... > Now I am feeling safe as the flints are OK. Otherwise, it is really scary... A little fear is a good thing when working with floating point numbers. James Tursa
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