### Why do floating point numeric equality/inequality tests fail mysteriously?

by itub (Priest)
 on Oct 30, 2004 at 02:49 UTC Need Help??

Rule #1 of floating-point computing in any language, not just Perl: Never compare two floating-point numbers with == or != (or expect >= et al. to work in the edge cases). Use a tolerance range instead.

The reason for this is that floating-point numbers are represented internally as a binary approximation. A consequence of that is that calculations that would be exact in decimal notation are not exact in binary. For example, a simple test like 36.6 + 0.2 == 36.8 may be return false! The same test could be done safely like this:

```\$TOL = 1E-8; # or a sufficiently small number
if (abs((36.6 + 0.2) - 38.2) < \$TOL) {
print "They are (almost) equal!\n";
}

This topic is covered in greater depth in the Q&A QandASection: math section, under the title of : My floating point comparison does not work. Why ?. Please post any further answers there.

• Comment on Why do floating point numeric equality/inequality tests fail mysteriously?

 Re: Why do floating point numeric equality/inequality tests fail mysteriously? by spx2 (Deacon) on Dec 25, 2007 at 23:22 UTC the answer lies here http://dlc.sun.com/pdf/800-7895/800-7895.pdf Originally posted as a Categorized Answer. [reply] Re: Why do floating point numeric equality/inequality tests fail mysteriously? by itub (Priest) on Oct 30, 2004 at 02:54 UTC Silly me. This was already in the Math section. I was browsing the Numbers section and was surprised that this subject had not been addressed. Now I know why... Feel free to move or reap. Originally posted as a Categorized Answer. [reply] Re: Why do floating point numeric equality/inequality tests fail mysteriously? by doxan9 (Initiate) on Sep 16, 2014 at 12:21 UTC thanks you.. if you're not absolutely sure you're posting in the right place. Originally posted as a Categorized Answer. [reply]

Create A New User
Domain Nodelet?
Node Status?
node history
Node Type: perlmeditation [id://403957]
Approved by jdporter
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others exploiting the Monastery: (2)
As of 2022-12-07 02:36 GMT
Sections?
Information?
Find Nodes?
Leftovers?
Voting Booth?

No recent polls found

Notices?