0.1 + 0.7 != 0.8 [duplicate]

开发者_JAVA技巧This question already has answers here: Is floating point math broken? (31 answers) Closed 11 months ago.

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

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

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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;
}

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Because of internal interpretation of floating-point numbers, they do not exactly equal to what you have wrote.