Plotting 2 out of 3 columns of a data frame

Question

I have a data frame consisting of 400 rows of x, y and z values, and I would like to plot the x column against the y column.

This is what I have got so far:

sample3d = function(n)
{
  df = data.frame()
  while(n>0)
  {
    X = runif(1,-1,1)
    Y = runif(1,-1,1)
    Z = runif(1,-1,1)
    a = X^2 + Y^2 + Z^2 
    if( a < 1 )
    {
      b = (X^2+Y^2+Z^2)^(0.5)
      vector = data.frame(X = X/b, Y = Y/b, Z = Z/b) 
      df = rbind(vector,df)
      n = n- 1
    }
  }
  df
}
sample3d(400)

Answer 1

If you need to draw random points on the surface of a sphere, you need just to extract the polar coordinates. Something like that:

sample3d_v2 <- function(n) {
   phi<-runif(n,0,2*pi)
   cost<-runif(n,-1,1)
   sint<-sqrt(1-cost^2)
   data.frame(X=sint*cos(phi),Y=sint*sin(phi),Z=cost)
}

Just some tests:

system.time(old<-sample3d(4000))
#   user  system elapsed 
#  3.895   0.000   3.879
system.time(new<-sample3d_v2(4000))
#   user  system elapsed 
#  0.000   0.000   0.002

As you can see, a gain of thousands of times. Test the results are correct:

require(rgl)
plot3d(old)
open3d();plot3d(new)

Regarding your question: just name the object resulting for your function and plot the X and Y components.

data<-sample3d_v2(400)
plot(data$X,data$Y)