[R] Beginners problem

[Rolf Wester]

Hi,

I'm new to R and have a problem with a little test program (see below). 
Why doesn't <<- in function rk4
assign the new value to y so that it is seen in rktest. I thought that 
<<- does exactly this. But it seems that I
didn't get it right. I would be very appreciative for an explanation of 
that behaviour of <<-. I know how to
write the whole thing so that it works (return the updated y to rktest) 
but I would like to learn how variable scope
in R works.

Thanks in advance

Rolf

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rk4 <- function(x,y,f,h) {
  n  <- length(y)
  hh <- h*0.5
  k1 <- f(x,y)
  k2 <- f(x,y+k1*hh)
  k3 <- f(x,y+k2*hh)
  k4 <- f(x,y+k3*h)
  y <<- y + h/6.0*(k1 + 2.0*k2 + 2.0*k3 + k4))
}

rkf <- function(x,y) {
  -y
}

rktest <- function(){
  y <- 1.0
  x <- 0.0
  h <- 0.1
  for(i in 1:10) {
    rk4(x,y,rkf,h)
    print(y)
  }
}

rktest()