# [R] how to draw a circle

Gabor Csardi csardi at rmki.kfki.hu
Sat Apr 22 20:48:23 CEST 2006

```coords[insiders,]

Gabor

On Sat, Apr 22, 2006 at 02:45:24PM -0400, Jian Zhang wrote:
> I want to get the actual value of the points in the circle.
> Can you tell me how to get them?
> Thanks!
>
> >insiders
> [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [25] FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE FALSE FALSE FALSE FALSE FALSE
> [37] FALSE FALSE FALSE FALSE FALSE  TRUE FALSE  TRUE FALSE FALSE  TRUE FALSE
> [49] FALSE  TRUE
>
>
>
> On 4/22/06, Gabor Csardi <csardi at rmki.kfki.hu> wrote:
> >
> >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:
> >symbols(c1, c2, circles=r, inches=FALSE, add=TRUE)
> >
> >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)
> >symbols(c1, c2, circles=r, inches=FALSE, add=TRUE)
> >################################
> >
> >'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]]
> >> >>
> >> >> ______________________________________________
> >> >> R-help at stat.math.ethz.ch mailing list
> >> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> >http://www.R-project.org/posting-guide.html
> >> >
> >> >--
> >> >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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