计算点与x轴正半轴夹角atan2(double y,double x),返回弧度制(-PI,PI]

精度比acos , asin 什么的高些。

Parameters

y

Value representing the proportion of the y-coordinate.

x

Value representing the proportion of the x-coordinate.

If both arguments passed are zero, a domain error occurs.

所以使用前只需排除x=y=0的情况

例子:

/* atan2 example */
#include <stdio.h>      /* printf */
#include <math.h>       /* atan2 */

#define PI 3.14159265

int main ()
{
  double x, y, result;
  x = -10.0;
  y = 10.0;
  result = atan2 (y,x) *  / PI;
  printf ("The arc tangent for (x=%f, y=%f) is %f degrees\n", x, y, result );
  return ;
}

结果：

The arc tangent for (x=-10.000000, y=10.000000) is 135.000000 degrees.

返回值为(-PI,PI]的例子

printf("%lf",atan2(,-)*/PI);

输出:180.000000