0.1 + 0.7 != 0.8 [duplicate]

Question

Why in some programming languages the expression in title evaluates to true? I've tried it in php, ruby and python.

Answer 1

Please read What Every Programmer Should Know About Floating-Point Arithmetic .

Answer 2

double TOLERANCE < 1.0E-10;
if(fabs(0.1+0.7-0.8)< TOLERANCE)
{
    std::cout << "0.1 + 0.7 == 0.8" << std::endl;
}
else
{
    std::cout << "0.1 + 0.7 != 0.8" << std::endl;
}