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====== C - C++ Mathematical Constants ======

The **cmath** library actually provides many mathematical constants that you can make use of.

To include the mathematical constants, you need to use a **#define** macro called **_USE_MATH_DEFINES** and add it before importing the cmath library:

<code cpp>
#define _USE_MATH_DEFINES
 
#include <cmath>
#include <iostream>
 
int main() { 
  std::cout << M_PI << " " << M_E << " " << M_SQRT2 << endl;
  return 0;
}
</code>

----

There are quite a few constants on offer.

^Mathematical Expression^C++ Symbol^Decimal Representation^
|pi|M_PI|3.14159265358979323846|
|pi/2|M_PI_2|1.57079632679489661923|
|pi/4|M_PI_4|0.785398163397448309616|
|1/pi|M_1_PI|0.318309886183790671538|
|2/pi|M_2_PI|0.636619772367581343076|
|2/sqrt(pi)|M_2_SQRTPI|1.12837916709551257390|
|sqrt(2)|M_SQRT2|1.41421356237309504880|
|1/sqrt(2)|M_SQRT1_2|0.707106781186547524401|
|e|M_E|2.71828182845904523536|
|log_2(e)|M_LOG2E|1.44269504088896340736|
|log_10(e)|M_LOG10E|0.434294481903251827651|
|log_e(2)|M_LN2|0.693147180559945309417|
|log_e(10)|M_LN10|2.30258509299404568402|

<WRAP info>
**NOTE:**  It is not best practice within C++ to use **#defines** for mathematical constants!

  * Instead, as an example, you should use **const double pi = 3.14159265358979323846;**.
  * The **#defines** are a legacy feature of C.
</WRAP>