Question

Possible Duplicate:
Strange addition of numeric strings in PHP

$r = 1.0 - 0.8 - 0.2;
var_dump($r);

I get float -5.5511151231258E-17.
The same result i get in C++ and C#.

MySQL i get result 0.0 via query:

SELECT 1.0 - 0.8 - 0.2

Answer 1

Let it be clear: 0.8 has finite representation in decimal because it has value 8/10¹.

But in base 2, it has value: 1/2 + 1/4 + 0/8 + 0/16 + 1/32 + 1/64 + 0/128 + … and it has no end, like one third has no end in decimal representation (0.333…) but would be 0.1 in base 3.

0.8 = 0.110011001100110011001100... in binary
0.2 = 0.001100110011001100110011... in binary

Your computer stores floating point values in binary representation. So, there is a loss of precision when you add, subtract, multiply, etc. numbers.

C++, C and C# are compiled languages, and floats are stored on typically 4 or 8 bytes. You cannot store 0.8 in a float.

MySQL might tell you 0 because it truncates the results of the calculation.

If you want a library that performs calculations without losses of precision in any base, look up GMPlib.