The official references say… it’s a mess. Pre-C++11 and C11: Officially, including <cmath> introduced nothing in ::; all of the functions were in std::. Practically, only export was less respected, and different compilers did very different things. If you included <cmath>, you used std:: everywhere, or what you got varied from compiler to compiler. C … Read more
This is an interesting question, but reading sources of efficient libraries won’t get you very far unless you happen to know the method used. Here are some pointers to help you understand the classical methods. My information is by no means accurate. The following methods are only the classical ones, particular implementations can use other … Read more
In C++, std::abs is overloaded for both signed integer and floating point types. std::fabs only deals with floating point types (pre C++11). Note that the std:: is important; the C function ::abs that is commonly available for legacy reasons will only handle int! The problem with float f2= fabs(-9); is not that there is no … Read more
pow() works with real floating-point numbers and uses under the hood the formula pow(x,y) = e^(y log(x)) to calculate x^y. The int are converted to double before calling pow. (log is the natural logarithm, e-based) x^2 using pow() is therefore slower than x*x. Edit based on relevant comments Using pow even with integer exponents may … Read more
You can use (n & 1) instead of n % 2 and << 1 instead of * 2 if you want to be super-pedantic, er I mean optimized. So the fastest way to compute in an 8086 processor is: 1 – ((n & 1) << 1) I just want to clarify where this answer is … Read more
The header <math.h> is a C std lib header. It defines a lot of stuff in the global namespace. The header <cmath> is the C++ version of that header. It defines essentially the same stuff in namespace std. (There are some differences, like that the C++ version comes with overloads of some functions, but that … Read more
Actually, because of old and annoying legacy, almost none of the math functions can be constexpr, since they all have the side-effect of setting errno on various error conditions, usually domain errors.
The LL makes the integer literal of type long long. So 2LL, is a 2 of type long long. Without the LL, the literal would only be of type int. This matters when you’re doing stuff like this: 1 << 40 1LL << 40 With just the literal 1, (assuming int to be 32-bits, you … Read more
In C++, it’s always sufficient to use std::abs; it’s overloaded for all the numerical types. In C, abs only works on integers, and you need fabs for floating point values. These are available in C++ (along with all of the C library), but there’s no need to use them.