MATLAB Central Newsreader - Find angles between two vectors

Re: Find angles between two vectors

Justin Morehouse — Wed, 23 Jan 2008 23:25:04 -0500

"Justin Morehouse" <norman_batez@MSN.com> wrote in message
<fn7egt$447$1@fred.mathworks.com>...

Wow, matlab is pretty neat, thanks for your help guys!

Re: Find angles between two vectors

Roger Stafford — Wed, 23 Jan 2008 21:16:03 -0500

"Justin Morehouse" <norman_batez@MSN.com> wrote in message <fn7egt
$447$1@fred.mathworks.com>...
> Hi there, I have a two vectors (3,5) and (5,6) and I was
> wondering how do I get the angle between them in matlab.
>
> on paper I would multiply both vectors to get (15+30) to =
> 45 and then square both (3,5) and (5,6) to get (9+25) and
> (25+36). Then I would get the square root of both (34) and
> (61) and multiply them together and then divide 45 by that.
>
> Afterwhich I would then use a trusty calc to do cos theta
> of that, but how can math lab do that?
>
> Thanks!
------------
  Another possible solution:

x1 = 3; y1 = 5;
x2 = 5; y2 = 6;
ang = atan2(abs(x1*y2-y1*x2),x1*x2+y1*y2);

where ang is measured in radians. (Multiply by 180/pi to get degrees.)

  This method has a slight advantage over the arccosine method. The acos
function suffers an inherent loss of accuracy near angles 0 and pi, whereas
the atan2 function maintains full accuracy for such cases. (Make a plot of the
acos curve from -1 to +1 to see why.)

  It is important to distinguish between two possible definitions of an angle
between vectors in the x-y plane. Above, the angle is considered as a non-
negative quantity lying between 0 and pi. It can also be defined as the angle
measured counterclockwise from the first vector to the second one, which in
general would be an angle ranging from 0 to 2*pi, or else from -pi to +pi
with clockwise being considered negative. For this latter meaning one would
remove the 'abs' in the above expression.

Roger Stafford

Re: Find angles between two vectors

Anh Huy Phan — Wed, 23 Jan 2008 14:08:02 -0500

"us " <us@neurol.unizh.ch> wrote in message
<fn7gru$7jd$1@fred.mathworks.com>...
> "Justin Morehouse":
> <SNIP many many words...
>
> > Hi there, I have a two vectors (3,5) and (5,6) and I
was
> > wondering how do I get the angle between them in matlab.
> > on paper I would multiply both vectors to get (15+30) to
> =
> > 45 and then square both (3,5) and (5,6) to get (9+25)
and
> > (25+36). Then I would get the square root of both (34)
> and
> > (61) and multiply them together and then divide 45 by
> that.
> > Afterwhich I would then use a trusty calc to do cos
> theta
> > of that, but how can math lab do that...
>
> ML obeys your words... (as one of the solutions...)
>
> v1=[3,5];
> v2=[5,6];
> a=acosd(dot(v1,v2)/(norm(v1)*norm(v2)))
>
> us

The same question was posted and available at the following
address

http://www.mathworks.com/matlabcentral/newsreader/
view_thread/151925

It is better that you search your topic before posting a
question about it.

Anh Huy Phan
RIKEN - BSI

Re: Find angles between two vectors

us — Wed, 23 Jan 2008 13:53:02 -0500

"Justin Morehouse":
<SNIP many many words...

> Hi there, I have a two vectors (3,5) and (5,6) and I was
> wondering how do I get the angle between them in matlab.
> on paper I would multiply both vectors to get (15+30) to
=
> 45 and then square both (3,5) and (5,6) to get (9+25) and
> (25+36). Then I would get the square root of both (34)
and
> (61) and multiply them together and then divide 45 by
that.
> Afterwhich I would then use a trusty calc to do cos
theta
> of that, but how can math lab do that...

ML obeys your words... (as one of the solutions...)

     v1=[3,5];
     v2=[5,6];
     a=acosd(dot(v1,v2)/(norm(v1)*norm(v2)))

us

Re: Find angles between two vectors

tpl@eng.cam.ac.uk (Tim Love) — Wed, 23 Jan 2008 13:34:44 -0500

"Justin Morehouse" <norman_batez@MSN.com> writes:

>Hi there, I have a two vectors (3,5) and (5,6) and I was
>wondering how do I get the angle between them in matlab.

There's a cart2pol function. Or you could use
angle(complex(5,6)-complex(3,5))

Find angles between two vectors

Justin Morehouse — Wed, 23 Jan 2008 13:13:01 -0500

Hi there, I have a two vectors (3,5) and (5,6) and I was
wondering how do I get the angle between them in matlab.

on paper I would multiply both vectors to get (15+30) to =
45 and then square both (3,5) and (5,6) to get (9+25) and
(25+36). Then I would get the square root of both (34) and
(61) and multiply them together and then divide 45 by that.

Afterwhich I would then use a trusty calc to do cos theta
of that, but how can math lab do that?

Thanks!