SAS/C Library Reference, Vol. 1

atan2 -- Compute the Trigonometric Arc Tangent of a Quotient

SYNOPSIS

 #include <math.h>

 double atan2(double y, double x);

DESCRIPTION

atan2 computes the angle defined by the positive x axis and a line through the point (x,y) to the point (0, 0). The signs of both values x and y are used to determine the quadrant of the result in a Cartesian system. The result is the inverse trigonometric tangent of y/x if x is not 0.

RETURN VALUE

Provided that it is defined and computable, the return value is the angular position of the point x,y. The return value is a double-precision, floating-point number expressed in radians and lies in the half-open interval (- pi , pi]. For input values (0.0, y), the return value will be either pi/2 or -pi/2 if y does not equal 0.

DIAGNOSTICS

If both x and y are 0, an error message is written to stderr and the function returns 0.0.

If an error occurs in atan2, the _matherr routine is called. You can supply your own version of _matherr to suppress the diagnostic message or modify the value returned.

EXAMPLE

This example converts rectangular coordinates to polar coordinates:

  #include <stdio.h>
  #include <lcmath.h>

  main()
  {
     double x, y;             /* rectangular coordinates (x,y) */
     double r, angle;         /* polar coordinates (r,angle)   */

     puts("Enter the rectangular coordinates please: ");
     scanf("%lf %lf", &x, &y );

     r = sqrt((x*x) + (y*y));

        /* Compute polar coordinates (radians).                */
     angle = atan2(y , x);

        /* Convert radians to degrees.                         */
     angle = (180.0 * angle)/M_PI;

     printf("rect coords(%f,%f) -> polar coords(%f,%f)n",
            x,y,r,angle);
  }

RELATED FUNCTIONS

atan, _matherr