counter <- function ( )

#*****************************************************************************80
#
## counter() simply reports how many times it has been called.
#
#  Discussion:
#
#    If you can get a function like this to work, you have made a good
#    start at implementing persistent data.
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    12 May 2021
#
#  Author:
#
#    John Burkardt
#
#  Persistent:
#
#    integer CALLS: the number of times this function has been called.
#
{
#
#  Persistent data is stored as attributes associated with this function.
#
  calls <- attr ( counter, "calls" )
#
#  Initialize the persistent data once.
#  Note that we need to use a DOUBLE ARROW assignment here!
#  This makes the assignment "globally".
#
  if ( is.null ( calls ) )
  {
    calls = 0
    attr ( counter, "calls" ) <<- calls
  }
#
#  Update the value.
#
  calls = calls + 1
#
#  Print the updated value.
#
  cat ( "counter() has been called ", calls, " times.\n" )
#
#  Save the updated value.
#
  attr ( counter, "calls" ) <<- calls

  return
}