How do I include a step counter in a random walk?

Question

In my random walk loop I need to incorporate a function that uses the step number as a variable.

How do I get R to produce a step number (loop counter may be a better term) during a random walk?

I need it right at the end of this code where I currently have the dummy code step.num

For reference my random walk code is:

    walkW <- function(n.times=125,
               xlim=c(524058,542800),
               ylim=c(2799758,2818500),
               start=c(542800,2815550),
               stepsize=c(4000,4000)) {

    plot(c(0,0),type="n",xlim=xlim,ylim=ylim,
           xlab="Easting",ylab="Northing",col="white",col.lab="white")#arena 
    x <- start[1]
    y <- start[2]
    steps <- 1/c(1,2,4,8,12,16)
    steps.y <- c(steps,-steps,0)
    steps.x <- c(steps[c(1,5,6)],-steps,0)
    points(x,y,pch=16,col="green",cex=1)

for (i in 1:n.times) {
        repeat {
           xi <- stepsize[1]*sample(steps.x,1)
           yi <- stepsize[2]*sample(steps.y,1)
           newx <- x+xi
           newy <- y+yi
           if (newx>xlim[1] && newx<xlim[2] &&
               newy>ylim[1] && newy<ylim[2]) break
        }
        lines(c(x,newx),c(y,newy),col="white")
         x <- newx
           y <- newy
}
  step.num<-(your magical step counter function) #I need the step counter here
 }

set.seed(101)
walkW(s)

You also might like the `all` function. Your break condition can be re-written `if (all(newx > xlim[1], newx < xlim[2], newy > ylim[1], newy < ylim[2]))`, which is a little easier to read. — Gregor Thomas, Oct 29 '14 at 22:32
Fantastic both of you. I actually just popped back on to provide the same answer. It's always great when the magic is as simple as x=y. Thanks a bunch! — Jesse001, Oct 29 '14 at 22:33

IRTFM · Accepted Answer · 2014-10-29T22:59:15.293

After a for loop that was "broken", the index variable persists, so your magical code would just be:

 step.num <- i

Although for-loops are technically functions, sometimes they seem to have different scoping rules, and this is one such time. Your function returns the 'step.num' value, and after exit from walkW, you would not have been able to access 'i', but if the for-loop were done in the .GlobalEnv then the index "i" would still not have been reset or NULL-ed when the for-loop ended via break.

How do I include a step counter in a random walk?

1 Answers1