Jump to content

Floating point almost equal


Recommended Posts

The other day I encountered a situation where I wanted to test if two floating point values were equal. Of course, one of the first things that you are told about floating point values is that you should never test for exact equality. So I proceed to wire up:


Is X = Y +/- some acceptable tolerance?


And that worked perfectly for my use case. That is, I knew that the max. value was 1000 and the min value was 0 and the resolution (or increment) was 0.01. A tolerance of 0.001 was sufficient.


This got me thinking. Wouldn't it be nice to have some reusable VIs for testing if floating point values are almost equal? Unfortunately this meant taking a broader view of the problem and lead me to this interesting blog post:




I've attached VIs for the Absolute Epsilon comparison, Relative Epsilon comparison and the ULP comparison described in the blog post for double precision floating point values.




Perhaps this would be a nice addition to the somewhat barren OpenG Comparison Pallet?


Uploaded package to Code Repository: http://lavag.org/files/file/259-floating-point-almost-equal/



Edited by Porter
Link to comment
Thanks for the info.


MGI's solution looks somewhat like the relative epsilon solution proposed in Bruce Dawson's blog post however they use a fixed maximum relative difference of 1%. Perhaps in some cases that would be too high.


Altenbach's version looks identical to the AlmostEqual2sComplement() function described in another article by Bruce Dawson:


Notice at the top of that article he states that you should not be using that method of comparison as it has a couple of major flaws. For example, comparing 2 and -2 will always return true. 


I have added the relative epsilon comparison to the package for completeness. This way, you can try all 3 methods if you are interested. Most of the time I really only need the absolute epsilon comparison because I know beforehand what range of values I'm comparing.

Link to comment
  • 1 month later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.