# LOOK UP A SYMBOL IN THE TABLE USING THE "MOVE TO FRONT" METHOD.  The
# table is a vector held in the global variable sym.table.  The table is 
# modified by moving the symbol found to the front, while symbols before it 
# move back one to make room.  The value returned is the index of the symbol 
# in the table (before it was moved to the front), which is a rough indication 
# of how long it took to find it.
#
# This function will work for symbols that are either character strings or
# integers (and some other things as well).
#
# This function considers it to be an error if the symbol isn't in the table.
# If this happens, the program is terminated with an error message.  (This
# probably isn't what one would want to do in a real application.)
#
# A note on R:  Assignments to global variables inside functions are done 
# using "<<-" rather than "<-".  That's how R can tell that the assignment 
# isn't to a local variable of the same name.

mtf.lookup <- function (s)
{
  # Find where in the table the symbol is found.

  i <- 1
  while (sym.table[i]!=s) 
  { i <- i + 1
    if (i>length(sym.table))
    { stop("The symbol being looked for isn't in the table!")
    }
  }

  # Move symbols before the one found forward one position, and then put
  # the symbol found at the front of the table.

  j <- i
  while (j>1)
  { sym.table[j] <<- sym.table[j-1]
    j <- j - 1
  }
  sym.table[1] <<- s

  # Return the position of the symbol before it was moved to the front. 

  i
}