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\begin{document}

Name: \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Directions: {\bf Work only on this sheet} (on both sides, if needed); do
not turn in any supplementary sheets of paper. There is actually plenty
of room for your answers, as long as you organize yourself BEFORE
starting writing.

{\bf 1.} () Here you will write an R S3 class {\bf "stack"}, do serve as
a stack data structure.  (Review:  Items are {\it pushed} onto the
stack, meaning here that they are added to the right end of a vector,
and they are popped, meaning deleted from the right end.)  

Here is a sample session:

\begin{lstlisting}
> source("stack.R")  # read in code
> ls()
[1] "pop"   "push"  "stack"
> stack("mystack")  # make an object
> ls()  # did we make the object?
[1] "mystack" "pop"     "push"    "stack"  
> push(3,"mystack")  # push 3
> mystack$data  
[1] 3
> push(8,"mystack")
> mystack$data  # check vector is now 3,8
[1] 3 8
> pop("mystack")  # should return the 8
[1] 8
> mystack$data  # check vector is now 3
[1] 3
\end{lstlisting}

The R functions {\bf get()} and {\bf assign()} do what their names
imply, e.g.

\begin{lstlisting}
z <- get("x",envir = .GlobalEnv)
\end{lstlisting}

copies the global variable {\bf x} to {\bf z}, and

\begin{lstlisting}
assign("u",12,envir = .GlobalEnv)
\end{lstlisting}

is the equivalent of

\begin{lstlisting}
u <<- 12
\end{lstlisting}

Fill in the blanks:

\begin{lstlisting}
stack <- function(objname) {
   tmp <-      # blank
   tmp$data <- vector()
   tmp$objname <-   # blank
   # blank; insert 2 lines here
}

push <- function(val,objname) {
   tmp <- get(objname, envir=.GlobalEnv)
   # blank; insert 2 lines here
}

pop <- function(objname) {
   tmp <- get(objname, envir=.GlobalEnv)
   lng <- length(tmp$data)
   val <-    # blank
   # blank; insert 2 lines here
}
\end{lstlisting}

\onecolumn

{\bf Solutions:}

\begin{lstlisting}
# the stack class will (in this implementation) have objects only at the
# global level; an object has 2 member variables, the vector
# representing the stack, and the name of the object; values are pushed
# onto/popped off of the right end of the data vector

# constructor for "stack" class; objname is a string, namely a variable
# to be created at the top level, i.e. global
stack <- function(objname) {
   tmp <- list()
   tmp$data <- vector()
   tmp$objname <- objname
   class(tmp) <- "stack"
   assign(objname, tmp, envir = .GlobalEnv)
}

# pushes val onto the stack named objname
push <- function(val,objname) {
   tmp <- get(objname, envir = .GlobalEnv)
   tmp$data <- c(tmp$data,val)
   assign(objname, tmp, envir = .GlobalEnv)
}

# pops the stack named objname, returns the popped value
pop <- function(objname) {
   tmp <- get(objname, envir = .GlobalEnv)
   lng <- length(tmp$data)
   val <- tmp$data[lng]
   tmp$data <- tmp$data[-lng]
   assign(objname, tmp, envir = .GlobalEnv)
   return(val)
}
\end{lstlisting}

\end{document}