lin0's code above is generally referred as the "Within Epsilon" check, and is an important advancement in understanding how to compare floating point numbers. It may be the best approach you can do in pure perl, at least with any hope of runtime performance.

However, the Within Epsilon family of checks is terribly sensitive to the actual values involved. Properly choosing a threshold epsilon (or the value of $eps in lin0's code) is important and unfortunately, depends on the values you are trying to compare.

An epsilon of 0.00000000000001 will not be useful for larger numbers in the billions, since the floating point number has to hold a larger exponent and can thus not hold as much precision in the mantissa.

Anyone who wants to know more about comparing IEEE floating point numbers in software "the right way" should have a brief read through http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm which includes nice example implementations and discussions of useful (and reasonably fast) C "almost equal" checks that will outperform and adapt better than the classic and naive Within Epsilon technique.

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