0413 c++ 使用PI

方法一 

1.On some (especially older) platforms (see the comments below) you might need to#define _USE_MATH_DEFINESand then include the necessary header file:
#include <math.h>and the value of pi can be accessed via:
M_PIIn 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:
on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the _USE_MATH_DEFINESOn (recent) Linux platforms there are long double values too provided as a GNU Extension:
# define M_PIl 3.141592653589793238462643383279502884L /* pi */

方法二

const double PI =3.141592653589793238463;const float PI_F=3.14159265358979f;

方法三
      const double pi = boost::math::constants::pi<double>();

0413 vector 常用函數

https://zh.wikipedia.org/wiki/Vector_(STL)

http://manchesteraj.blogspot.tw/2011/12/vector.html

• vec.push_back() - 新增元素至 vector 的尾端，必要時會進行記憶體配置。
• vec.pop_back() - 刪除 vector 最尾端的元素。
• vec.clear() - 清空所有元素
• vec.size() - 取得 vector 目前持有的元素個數。

• 把vector當陣列用以及副函式傳遞
  //main宣告 參考前篇vector用法
  vector<vector<int> > Variance_SD(m, vector<int>(n));//副函式宣告
  void SD(vector<vector<int> >&);
  
  //副函式用&接
  void SD(vector<vector<int> >& variance){
      //可以直接當array用了
      cout << variance[1][1];
  }
  
  

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