方法一
方法二1.
On some (especially older) platforms (see the comments below) you might need to#define _USE_MATH_DEFINES
and then include the necessary header file:
#include <math.h>
and the value of pi can be accessed via:
M_PI
In mymath.h
(2014) it is defined as:
# define M_PI 3.14159265358979323846 /* pi */
but check yourmath.h
for more. An extract from the "old"math.h
(in 2009):
/* Define _USE_MATH_DEFINES before including math.h to expose these macro
* definitions for common math constants. These are placed under an #ifdef
* since these commonly-defined names are not part of the C/C++ standards.
*/
However:
on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the_USE_MATH_DEFINES
On (recent) Linux platforms there arelong double
values too provided as a GNU Extension:
# define M_PIl 3.141592653589793238462643383279502884L /* pi */
方法三const double PI =3.141592653589793238463;
const float PI_F=3.14159265358979f;
const double pi = boost::math::constants::pi<double>();
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