# [R] how to draw a circle

Gabor Csardi csardi at rmki.kfki.hu
Sat Apr 22 20:26:48 CEST 2006

```Fine, than just check every point like suggested below.
(whether (c1-x)^2+(c2-y)^2 < r^2)
and call 'symbols' to add the circle to the plot like this:

For example, if coords is a two column matrix of the coordinates of the
points:

################################
coords <- matrix( runif(100), nc=2 )
c1 <- c2 <- 0.5
r <- 0.2

insiders <- apply(coords, 1,
function(co) (c1-co[1])^2+(c2-co[2])^2 < r^2)
plot(coords)
################################

'insiders' will be a logical vector telling whether the points are inside or
outside the circle.

Gabor

On Sat, Apr 22, 2006 at 02:16:38PM -0400, Jian Zhang wrote:
> Thanks!
> There are many points in our plot. I want to choose the points which
> distance to one point (e.g.(100,200)) is less than 10.
> And I want to draw a circle (radinus=10). The centre of the circle is the
> point (100,200).
>
>
> On 4/22/06, Gabor Csardi <csardi at rmki.kfki.hu> wrote:
> >
> >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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> >
> >--
> >Csardi Gabor <csardi at rmki.kfki.hu>    MTA RMKI, ELTE TTK
> >

--
Csardi Gabor <csardi at rmki.kfki.hu>    MTA RMKI, ELTE TTK

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