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Answer: My floating point comparison does not work. Why ?

by PhilHibbs (Hermit)
on Sep 23, 2003 at 14:16 UTC ( #293501=categorized answer: print w/replies, xml ) Need Help??

Q&A > math > My floating point comparison does not work. Why ? - Answer contributed by PhilHibbs

The problem with comparing two floats with a tolerance is that the comparison is not transitive. That is, even though a==b and b==c, it may be that a!=c.

This might become important in such situations as passing a sorting or searching function. I am more familiar with this situation in C++ and Java. In Java, there is such a thing as the "equals contract", which states that the equals function must be reflexive (a==a), symmetric (a==b, b==a) and transitive (a==b, b==c, a==c). This is important for building HashSet and HashMap objects, and there are similar rules in C++ for the STL containers and algorithms.

Being something of a perl noob, I don't know if there are similar constraints in Perl libraries, but it's a good rule to bear in mind anyway.

If a comparison function is not transitive, you are just pushing the surprising behaviour into a darker corner.

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