C++ floating-point console output issue

Question

float x = 384.951257;
std::cout << std::fixed << std::setprecision(6) << x << std::endl;

The output is 384.951263. Why? I'm using gcc.

Answer 1

float is usually only 32-bit. With about 3 bits per decimal digit (2¹⁰ roughly equals 10³) that means it can't possibly represent more than about 11 decimal digits, and accounting for other information it also needs to represent, such as magnitude, let's say 6-7 decimal digits. Hey, that's what you got!

Check e.g. Wikipedia for details.

Use double or long double for better precision. double is the default in C++. E.g., the literal 3.14 is of type double.

Answer 2

Floats have a limited resolution. So it gets rounded when you assing the value to x.