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How to use the PI constant in C++


Question

I want to use the PI constant and trigonometric functions in some C++ program. I get the trigonometric functions with include <math.h>. However, there doesn't seem to be a definition for PI in this header file.

How can I get PI without defining it manually?

2014/04/04
1
483
4/4/2014 9:08:42 AM

Accepted Answer

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

# define M_PI           3.14159265358979323846  /* pi */

but check your math.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:

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

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

    # define M_PIl          3.141592653589793238462643383279502884L /* pi */
    
2014/04/25
545
4/25/2014 6:54:21 AM

Pi can be calculated as atan(1)*4. You could calculate the value this way and cache it.

2009/11/13

You could also use boost, which defines important math constants with maximum accuracy for the requested type (i.e. float vs double).

const double pi = boost::math::constants::pi<double>();

Check out the boost documentation for more examples.

2009/11/13

Get it from the FPU unit on chip instead:

double get_PI()
{
    double pi;
    __asm
    {
        fldpi
        fstp pi
    }
    return pi;
}

double PI = get_PI();
2015/06/04

I would recommend just typing in pi to the precision you need. This would add no calculation time to your execution, and it would be portable without using any headers or #defines. Calculating acos or atan is always more expensive than using a precalculated value.

const double PI  =3.141592653589793238463;
const float  PI_F=3.14159265358979f;
2014/01/19

Rather than writing

#define _USE_MATH_DEFINES

I would recommend using -D_USE_MATH_DEFINES or /D_USE_MATH_DEFINES depending on your compiler.

This way you are assured that even in the event of someone including the header before you do (and without the #define) you will still have the constants instead of an obscure compiler error that you will take ages to track down.

2009/11/13

Source: https://stackoverflow.com/questions/1727881
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