R help - Apr 2006 - how to draw a circle

how to draw a circle (e.g. radius=10cm) of one point?
And how to choose these points in the circle?
Thanks!

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Hmmm, could you explain this a bit? 
You want to add a circle to a plot? See the examples in help(symbols)

Or you want to calculate the coordinates of the points on a circle?

t <- seq(0,2*pi,length=100)
coords <- t(rbind( 10+sin(t)*2, 20+cos(t)*2))
plot(coords)

Here (10,20) is the center of the circle, and 2 is its radius.
If you set length to a bigger number you'll get smoother resolution.

A points (x,y) is inside the circle if (c1-x)^2+(c2-y)^2 < r^2
(c1, c2) is the center of the circle, r is its radius.

Gabor

On Sat, Apr 22, 2006 at 01:42:46PM -0400, Jian Zhang
wrote:
> how to draw a circle (e.g. radius=10cm) of one point?
> And how to choose these points in the circle?
> Thanks!
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-help
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-- 
Csardi Gabor <csardi at rmki.kfki.hu>    MTA RMKI, ELTE TTK

Dear Jian,

Here's an answer to the first part of what I understood to be your original
question, which entails drawing a circle of a given (absolute) size in cm
(i.e., regardless of the physical dimensions of the device and the units of
the axes):

circle <- function(x, y, radius, units=c("cm", "in"),
segments=100){
    units <- match.arg(units)
    if (units == "cm") radius <- radius/2.54
    plot.size <- par("pin")
    plot.units <- par("usr")
    units.x <- plot.units[2] - plot.units[1]
    units.y <- plot.units[4] - plot.units[3]
    ratio <- (units.x/plot.size[1])/(units.y/plot.size[2])
    size <- radius*units.x/plot.size[1] 
    angles <- (0:segments)*2*pi/segments
    unit.circle <- cbind(cos(angles), sin(angles))
    shape <- matrix(c(1, 0, 0, 1/(ratio^2)), 2, 2)
    ellipse <- t(c(x, y) + size*t(unit.circle %*% chol(shape)))
    lines(ellipse)
    }
    
Regards,
 John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Jian Zhang
> Sent: Saturday, April 22, 2006 12:43 PM
> To: r-help
> Subject: [R] how to draw a circle
> 
> how to draw a circle (e.g. radius=10cm) of one point?
> And how to choose these points in the circle?
> Thanks!
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html

[Jian Zhang]
>how to draw a circle (e.g. radius=10cm) of one point?
>And how to choose these points in the circle?
There also are "ellipse" functions in both packages "car"
and "ellipse".

-- 
Fran?ois Pinard   http://pinard.progiciels-bpi.ca

You may want to have a look at the package pgirmess and the function 
polycirc()

Kind regards,

Patrick