finding angles 0-360

Question

Need help with a math issue: i need to get the true angle from 0 degrees using x and y cordinates im using this at the moment:

Math.atan((x2-x1)/(y1-y2))/(Math.PI/180)

but /(Math.PI/180) limits results from -90 to 90 i need 0-360

note: im using the angle to indicate direction: 0=up 90=right 135=45 degree right+down 180=down 270=left etc

Answer 1

Math.atan limits you to the two rightmost quadrants on the unit circle. To get the full 0-360 degrees:

if x < 0 add 180 to the angle
else if y < 0 add 360 to the angle.

Your coordinate system is rotated and inverted compared to mine (and compared to convention). Positive x is to the right, positive y is up. 0 degrees is to the right (x>0, y=0, 90 degrees is up (x=0,y>0) 135 degrees is up and to the left (y>0, x=-y), etc. Where are your x- and y-axes pointing?

Answer 2

Also note:

if (y1==y2) {
    if (x1>x2)
        angle = 90;
    else if (x1<x2)
        angle = 270;
    else
        angle = 0;
}

Answer 3

function angle(x1,y1,x2,y2)
{
eangle = Math.atan((x2-x1)/(y1-y2))/(Math.PI/180)
if ( angle > 0 )
{
  if (y1 < y2)
    return angle;
  else
    return 180 + angle;
} else {
  if (x1 < x2)
    return 180 + angle;
  else
    return 360 + angle;
}
}

Answer 4

I would think you are better of using atan2 to get the right quadrant rather than branching yourself, then just scale as you have been s0mething like

Math.atan2(x2-x1,y2-y1)/(Math.PI/180) + 180