atan2() is a Mathematical Function that returns the arc tangent value of y/x in radians. It uses the signs of both arguments "y" and "x" to find the quadrant.

     float atan2 ( float y, float x );
     double atan2 ( double y, double x );
     long double atan2 ( long double y, long double x );

   #include <stdio.h>
   #include <math.h>
   int main(void)
     {
        double x,y, r;
        x=-4.0;
        y=2.0;
        r=atan2(y, x);
        printf("The arc tangent of y/x is::%lf ", r);
        return 0;
     }

    The arc tangent of y/x is:: 2.677945

In the above example atan2() is used to find the arc tangent value for "2.0/-4.0".