[R] push/pop on a stack

[Marc Schwartz]

>-----Original Message-----
>From: r-help-admin at stat.math.ethz.ch 
>[mailto:r-help-admin at stat.math.ethz.ch] On Behalf Of Barry Rowlingson
>Sent: Tuesday, February 25, 2003 12:10 PM
>To: David Forrest
>Cc: r-help at stat.math.ethz.ch
>Subject: Re: [R] push/pop on a stack
>
>
>
>> It seems like it wouldn't be difficult, and that someone may have 
>> already implemented a set of stack functions, and I wouldn't like
to 
>> duplicate it.
>
>Here's a Tuesday evening lash-up. Usage is:
>
>  > mystack <- stack()   # initialisation
>  > push(mystack,value)  # stores something on the stack
>  > pop(mystack)         # returns 'value'
>
>There's a print method that lists the stack. Its simply a list. Stack

>underflow is reported as 'attempt to select less than one 
>element'. You 
>can push anything on the stack. Almost. I just tried pushing a stack 
>object onto the stack, and got infinite recursion, cant think why. 
>Anyway, this seems to work for most sensible situations!
>
>
>stack <- function(){
>
>   it <- list()
>   res <- list(
>               push=function(x){
>                 it[[length(it)+1]] <<- x
>               },
>               pop=function(){
>                 val <- it[[length(it)]]
>                 it <<- it[-length(it)]
>                 return(val)
>               },
>               value=function(){
>                 return(it)
>               }
>               )
>   class(res) <- "stack"
>   res
>
>}
>
>print.stack <- function(x,...){
>   print(x$value())
>}
>
>push <- function(stack,obj){
>   stack$push(obj)
>}
>
>pop <- function(stack){
>   stack$pop()
>}
>
>
>Baz


Just as an FYI, if you should elect to implement that function, there
is already a stack() function in base R, so you would want to use a
different name.

See ?stack, which is used to transform data.

Regards,

Marc