get points within a circle

point_in_circle.R

set.seed(123)

x <- rnorm(100)
y <- rnorm(100)
points.df <- data.frame(x=x,y=y)

center <- c(0.2, 0.3)
diameter <- 2

d <- apply(points.df, 1, function(x) x-center)
d <- t(d^2)
idx <- apply(d, 1, sum) < (diameter/2)^2

## points.df[idx,] is the subset of points which locate within the circle.

points.df <- data.frame(points.df, inCircle=factor(idx, levels=c("TRUE", "FALSE")))

require(ggplot2)

p <- ggplot(points.df, aes(x,y))+geom_point(aes(color=inCircle))


circleFun <- function(center = c(0,0),diameter = 1, npoints = 100){
    r = diameter / 2
    tt <- seq(0,2*pi,length.out = npoints)
    xx <- center[1] + r * cos(tt)
    yy <- center[2] + r * sin(tt)
    return(data.frame(x = xx, y = yy))
}

circle.df <- circleFun(center, diameter)


p+geom_path(data=circle.df, aes(x,y))