[R] 2D Random walk

[featherbox]

I've wrote some code to simulate a random walk in 2 dimensions on a lattice.
Basically I want to add something in to make it plot it point by point so
you can see what is going on.
Heres my code for the random walk in 2d


RW2D<-function(N)
  {
    i<-0
    xdir<-0
    ydir<-0
    xpos<-vector()
    xpos[1]<-xdir
    ypos<-vector()
    ypos[1]<-ydir
    for (i in 1:N-1)
      {
	r<-runif(1)
	if(r<=0.25) {xdir<-xdir+1}
	if(r>0.25 && r<=0.5) {xdir<-xdir-1}
	if(r>0.5 && r<=0.75) {ydir<-ydir +1}
	if(r>0.75) {ydir<-ydir-1}
      	xpos[i+1]<-xdir
      	ypos[i+1]<-ydir
      }	
    return(cbind(xpos,ypos))
  }
rw<-RW2D(10000)

xmin<-min(rw[,1])
xmax<-max(rw[,1])
ymin<-min(rw[,2])
ymax<-max(rw[,2])

plot(rw[,1],rw[,2],type="l",xlab="x",ylab="y",main="Random Walk Simulation
In Two Dimensions",col="green4",xlim=range(xmin:xmax),ylim=range(ymin:ymax))

end<-cbind(rw[10000,1],rw[10000,2])
start<-cbind(0,0)

points(start,pch=4,col="red")
points(end,pch=4,col="red")

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