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Re: Comparing results of math operations

by Laurent_R (Parson)
on Apr 16, 2014 at 09:26 UTC ( #1082470=note: print w/ replies, xml ) Need Help??

in reply to Comparing results of math operations

This is a usual difficulty with floating point number arithmetics (with most languages). In your first example, try this:

my $x = 0.95*806; my $y = 1.3*589; print $y - $x; # prints something like : 1.13686837721616e-13
So, although the numbers seem to be equal when rounded to 1 or even 5 decimal places, they are not really equal. Even if the numbers were mathematically exactly equal, the simple fact that you derived them from arithmetical operations on other source floating point numbers may lead to different results, because the original numbers are first converted to an internal binary representation which may lead to some internal rounding even before the multiplication is applied.

The lesson is: don't compare for equality floating point numbers derived from arithmetic operations; rather subtract one from the other and check if the absolute value of the difference is smaller than a certain predetermined very small number.

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