R help - Apr 2000 - stack operations on vectors

It would be nie to have some basic stack operations for simple vectors and
lists. ie:
> a <- c(1,2,3,4,5)
> b <- pop(a)
> a
[1] 1 2 3 4
>b
[1] 5
> push(b, 6)
> b
[1] 5 6

notice how pop and push modify the original vector and not local copy
inside the functions environment. Is there any way to do this? I tried
this:

push <- function(vec,val) {
  vec <<- c(vec,val)
}

pop <- function(vec) {
  l <- length(vec)
  val <- vec[l]
  vec <<- vec[1:(l-1)]
  val
}

but those only create the object 'vec' in the global environment. is
there
a way to pass an object by reference?

It seems like the only way to do this would be to have it call an
internal function.

PS. please include me in the reply, and I am only on the R-Help digest.
thanks.

-- 
Aidan Dysart | adysart at nonlineardynamics.com | www.nonlineardynamics.com

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On Thu, 20 Apr 2000, Aidan Dysart wrote:
> It would be nie to have some basic stack operations for simple vectors and
> lists. ie:
> 
> > a <- c(1,2,3,4,5)
> > b <- pop(a)
> > a
> [1] 1 2 3 4
> >b
> [1] 5
> > push(b, 6)
> > b
> [1] 5 6
> 
> notice how pop and push modify the original vector and not local copy
> inside the functions environment. Is there any way to do this? I tried
> this:
There are two ways to do this sort of thing.  One is similar to what you
were doing, but is, I think, considered harmful (messing with frames
should be restricted to places where it is essential).

a <- c(1,2,3,4,5)
pop<-function(x)
eval(substitute(x<-x[-1]),parent.frame())
> a
[1] 1 2 3 4 5
> pop(a)
> a
[1] 2 3 4 5



The other is to define a stack class, eg
> makeStack<-function(x){
 pop<-function() if (length(x)>1) x<<-x[-1]
 push<-function(y) x<<-c(y,x)
 value<-function() x
 rval<-list(pop=pop,push=push,value=value)
 class(rval)<-"myStack"
 rval
 }
> print.myStack<-function(x) print(x$value())
> a<-makeStack(1:5)
> a
[1] 1 2 3 4 5
> a$pop()
> a
[1] 2 3 4 5
> a$pop()
> a
[1] 3 4 5
> a$push(3)
> a
[1] 3 3 4 5


	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

 

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