From: Bertrand COCONNIER <bcoconni AT club-internet DOT fr>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: math problem
Date: Tue, 23 Feb 1999 23:59:59 +0100
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Erik Johansson a écrit :

> if i have a line like this
> y
> |     /
> |    /
> |   /
> |  /
> | /
> |/_____________ x
>
> How can I calculate the angle of that line in degrees?
>
> Thanx in advance

You should first compute the slope of the line : slope= (y2-y1)/(x2-x1).
(x1,y1) and (x2,y2) are any plots on your line (they have to be
distinct). On your draw, the line seems to be drawn from the origin to a
point (x,y) so 'slope=y/x'. Then you compute atan(slope). Be careful :
1. The result is in radians
2. If the line is vertical the slope computation will fail but (of
course) the result is 90° or -90° depending on the sign of 'y'.
3. The result of atan() is in between -pi/2 and pi/2. If your angle
should have different values you must care about it. A simple test can
help you to determinate whether the angle is in or out the [-pi/2;pi/2]
interval.
 if (x>0.)
  angle=atan(slope); /* the angle is in between -pi/2 and pi/2*/
else
  angle=atan(slope)+pi; /* the angle is in between pi/2 and 3pi/2*/

    Bertrand.