0 isn't 0 in C++ math

[Igor Vartanov]

when add one time its true, but when you do it with cycle it looks something like this

a = -0.1 + 0.01000000001 and so on
and then you compare 0 with a it is looks something like this 0 != 0.0000000000001

Yep, see above: float calculations have finite precision. And the result might fluctuate a little.