to error mess  = 
  fprints 0 mess
  exit  

// programming: make a list of numbers
// print them
// count the lenght of the list

storage = array 1000
free = 2
end = 1000

// a data item has two slots: num, link
// access functions

to setnum a b = storage[a] = b
to setlink a b = storage[a+1] = b
to getnum a = storage[a]
to getlink a = storage[a+1]
to notnull p = p != 0
to isnull p = p == 0

// create a new data item stored a number n
to newitem n | a =
  if free == end
    error "no more memory"
  a = free
  free = free + 2
  setnum a n
  setlink a 0
  a              // return a new data item

to newlist = newitem 0    // create an empty list, return a handle to its header

to insert a k =    // insert an item "a" into the list "k"
  setlink a (getlink k)
  setlink k a

to makelist | k a =
  k = newlist      //  make an empty list
  a = newitem 2    //  make a new node storing 2
  insert a k
  insert (newitem 3) k
  insert (newitem 5) k
  insert (newitem 7) k
  k                //  return the list

to printlist k | p =
  p = getlink k    // get the list
  while notnull p
    print (getnum p) space
    p = getlink p      //  get to the next item
  nl

to printlistrec2 k = {}

to printlistrec k =
  printlistrec2 (getlink k) nl

to printlistrec2 k =
  if notnull k
    print (getnum k) space
    printlistrec2 (getlink k)

to listlen k | c p =
  c = 0
  p = getlink k
  while notnull p
    c = c + 1
    p = getlink p
  c

to listlenrec2 k = {}

to listlenrec k =
  listlenrec2 (getlink k) 

to listlenrec2 k =
  if isnull k
    0
  else
    1 + (listlenrec2 (getlink k))

to listlenreca2 k c = {}

to listlenreca k =
  listlenreca2 (getlink k) 0

to listlenreca2 k c =
  if isnull k
    c
  else
    listlenreca2 (getlink k) (c+1)

to main | k =
  k = makelist    
  printlistrec k
  print listlenreca k nl

// 19 Jan 2010