c++ - 如何在C ++中使用PI常量(How to use the PI constant in C++)

Question

I want to use the PI constant and trigonometric functions in some C++ program.

(我想在一些C ++程序中使用PI常量和三角函数。)

I get the trigonometric functions with include <math.h> .

(我得到了include <math.h>的三角函数。)

However, there doesn't seem to be a definition for PI in this header file.

(但是，此头文件中似乎没有PI的定义。)

How can I get PI without defining it manually?

(如何在不手动定义PI的情况下获取PI？)

  ask by Etan translate from so

Answer 1

On some (especially older) platforms (see the comments below) you might need to

(在某些（特别是较旧的）平台上（请参阅下面的评论），您可能需要这样做)

#define _USE_MATH_DEFINES

and then include the necessary header file:

(然后包含必要的头文件：)

#include <math.h>

and the value of pi can be accessed via:

(pi的值可以通过以下方式访问：)

M_PI

In my math.h (2014) it is defined as:

(在我的math.h （2014）中，它被定义为：)

# define M_PI           3.14159265358979323846  /* pi */

but check your math.h for more.

(但请查看math.h了解更多信息。)

An extract from the "old" math.h (in 2009):

(来自“旧” math.h的摘录（2009年）：)

/* 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:

(然而：)

1. on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the _USE_MATH_DEFINES

   (在较新的平台上（至少在我的64位Ubuntu 14.04上）我不需要定义_USE_MATH_DEFINES)

2. On (recent) Linux platforms there are long double values too provided as a GNU Extension:

   (在（最近的）Linux平台上，作为GNU扩展提供了long double值：)

    # define M_PIl 3.141592653589793238462643383279502884L /* pi */