[R] drawing half-circles

[Barry Rowlingson]

adiamond at fas.harvard.edu wrote:
> Dear wonderful R community,
> 
> I've been creating color-coded concentric circles using the "points" function, 
> but I just realized that what I would really like to do is draw color-coded 
> concentric half-circles.

> What should I do?

Write a 'halfCircle' function.

halfCircle <- function(x,y,r,start=0,end=pi,nsteps=30,...){
   rs <- seq(start,end,len=nsteps)
   xc <- x+r*cos(rs)
   yc <- y+r*sin(rs)
   lines(xc,yc,...)
}

Then try it out:

plot(1:10)

# default draws top halves:
halfCircle(5,5,2)
halfCircle(6,6,1,col="red")

# draw right half:
halfCircle(5,4,1,start=-pi/2,end=pi/2,lwd=2,col="blue")

  Note that you'll have to call halfCircle once for each point, since 
its not 'vectorised' in any way.

  It's also capable of drawing any circle fragments. I should have 
called it 'partCircle'. Oh well.

Barry