Abstract Data Type Operations in R

2-minute read

This morning, I got a chance to read enough of the R Language Definition to finish my implementations of push and pop. While I was at it, I also wrote implementations of unshift, shift, queue and dequeue. Here they are:

push <- function(vector, item)
{
  vector.lvalue.symbol <- substitute(vector)
  new.expression <- paste(vector.lvalue.symbol,
                          ' <- c(', vector.lvalue.symbol, ', ', item, ')',
                          sep = '')
  eval(parse(text = new.expression),
       sys.frame(sys.parent()))
}

pop <- function(vector)
{
  vector.lvalue.symbol <- substitute(vector)
  temp.env <- new.env()
  last.element <- eval(parse(text = paste(vector.lvalue.symbol,
                                          '[length(', vector.lvalue.symbol, ')]',
                                          sep = '')),
                       sys.frame(sys.parent()))
  assign('tmp', last.element, envir = temp.env)
  eval(parse(text = paste(vector.lvalue.symbol,
                          ' <- ', vector.lvalue.symbol,
                          '[-length(', vector.lvalue.symbol, ')]',
                          sep = '')),
       sys.frame(sys.parent()))
  return(get('tmp', envir = temp.env))
}

unshift <- function(vector, item)
{
  vector.lvalue.symbol <- substitute(vector)
  new.expression <- paste(vector.lvalue.symbol,
                          ' <- c(', item, ', ', vector.lvalue.symbol, ')',
                          sep = '')
  eval(parse(text = new.expression), sys.frame(sys.parent()))
}

shift <- function(vector)
{
  vector.lvalue.symbol <- substitute(vector)
  temp.env <- new.env()
  last.element <- eval(parse(text = paste(vector.lvalue.symbol,
                                          '[1]',
                                          sep = '')),
                       sys.frame(sys.parent()))
  assign('tmp', last.element, envir = temp.env)
  eval(parse(text = paste(vector.lvalue.symbol,
                          ' <- ', vector.lvalue.symbol,
                          '[-1]',
                          sep = '')),
       sys.frame(sys.parent()))
  return(get('tmp', envir = temp.env))
}

queue <- function(vector, item)
{
  vector.lvalue.symbol <- substitute(vector)
  new.expression <- paste(vector.lvalue.symbol,
                          ' <- c(', vector.lvalue.symbol, ', ', item, ')',
                          sep = '')
  eval(parse(text = new.expression), sys.frame(sys.parent()))
}

dequeue <- function(vector)
{
  vector.lvalue.symbol <- substitute(vector)
  temp.env <- new.env()
  last.element <- eval(parse(text = paste(vector.lvalue.symbol,
                                          '[1]',
                                          sep = '')),
                       sys.frame(sys.parent()))
  assign('tmp', last.element, envir = temp.env)
  eval(parse(text = paste(vector.lvalue.symbol,
                          ' <- ', vector.lvalue.symbol,
                          '[-1]',
                          sep = '')),
       sys.frame(sys.parent()))
  return(get('tmp', envir = temp.env))
}

In general, the secret to writing these pseudo-macros is to use substitute. For the three functions that need to return a value as well as edit the passed parameter, you also need to use new.env, assign and get to edit the relevant symbol tables during function execution.

To check that these functions work, try the following examples after defining the functions above:

v <- c(1)
push(v, 2)
v
pop(v) == 2
v
unshift(v, 0)
v
shift(v) == 0
v
queue(v, 2)
v
dequeue(v) == 1
v