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It is quite usual that simple decimal fractions like 0.1 or 0.7 cannot
be converted into their internal binary counterparts without a little loss of precision. This can
lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7
instead of the expected 8 as the result of the internal representation really being
something like 7.9999999999....
This is related to the fact that it is impossible to exactly express some fractions in
decimal notation with a finite number of digits. For instance, 1/3 in decimal form becomes
0.3333333. . ..
So never trust floating number results to the last digit and never compare floating point
numbers for equality. If you really need higher precision, you should use the arbitrary precision math functions or gmp functions
instead.
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