How to fix this problem in PHP?

Question

$onethird = 1.0/3;
$fivethirds = 1.0/3+1.0/3+1.0/3+1.0/3+1.0/3;
$half = 1.0/2;
$threehalf = 1.0/2+1.0/2+1.0/2;
var_dump($onethird + $fivethirds == $half + $threehalf);

which outputs false,but as we all know:5/3+1/3=2=3/2+1/2

How to fix this problem?

Answer 1

This is one of the problems with the IEEE 754 representation of floating point numbers; the representations are not accurate enough to represent all rational numbers.

The way to do it is to compare the difference against a very small number for closeness, rather than equality:

abs(($onethird + $fivethirds) - ($half + $threehalf)) < 1e-8

Answer 2

The problem comes from the small errors introduced by Floating Point arithmetic. This is not specific to PHP.

You can fix this by introducing a small "tolerance" factor, i.e. by checking that the first value in the comparaision is >= the second value minus the tolerance and <= the second value plus the tolerance.