byname <- function ( action = NULL, name = NULL, value_in = NULL )

#*****************************************************************************80
#
## byname() controls a set of named persistent data.
#
#  Discussion:
#
#    Three values are stored, named ALPHA, BETA and GAMMA.
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    11 May 2021
#
#  Author:
#
#    John Burkardt
#
#  Input:
#
#    string ACTION (case insensitive, only first character important)
#    "Get"     get the value
#    "Print"   print the value
#    "Reset"   reset value to default
#    "Set"     set the value
#
#    string NAME, the name of the parameter (case insensitive)
#    "ALPHA"
#    "BETA"
#    "GAMMA"
#    "*" (all variables)
#    NAME is required for "Get" and "Set".  
#    NAME, if omitted, is assumed "*" for "Print" and "Reset".
#
#    VALUE_IN.
#    Only used for "Set" command.
#
#  Output:
#
#    VALUE_OUT.
#    If NAME was specified on input, and was not "*", then
#    VALUE_OUT is the current value of the corresponding variable.
#    Otherwise, VALUE_OUT is empty.
#
{
#
#  Persistent data is stored as attributes associated with the function.
#
  alpha         <- attr ( byname, "alpha" )
  alpha_default <- attr ( byname, "alpha_default" )
  beta          <- attr ( byname, "beta" )
  beta_default  <- attr ( byname, "beta_default" )
  gamma         <- attr ( byname, "gamma" )
  gamma_default <- attr ( byname, "gamma_default" )
#
#  Initialize the persistent data on first call, and again if x not input.
#  Note that we need to use a DOUBLE ARROW assignment here!
#  This makes the assignment "globally".
#
  if ( is.null ( alpha ) )
  {
    alpha_default = 1.0
    alpha = alpha_default
    beta_default = 2.0
    beta = beta_default
    gamma_default = 3.0
    gamma = gamma_default
    attr ( byname, "alpha" )         <<- alpha
    attr ( byname, "alpha_default" ) <<- alpha_default
    attr ( byname, "beta" )          <<- beta
    attr ( byname, "beta_default" )  <<- beta_default
    attr ( byname, "gamma" )         <<- gamma
    attr ( byname, "gamma_default" ) <<- gamma_default
  }
#
#  Handle the ACTION.
#
  if ( is.null ( action ) )
  {
    action2 = "p"
  }
#
#  Reject bad action.
#
  else
  {
    action2 = substr ( action, 1, 1 )
    action2 = tolower ( action2 )
    match = ( action2 == "g" ) |
            ( action2 == "p" ) |
            ( action2 == "r" ) |
            ( action2 == "s" )

    if ( ! match )
    {
      cat ( '\n' )
      cat ( 'byname: Fatal error!\n' )
      cat ( '  Legal actions are "g", "p", "r", "s"\n' )
      cat ( '  Not "', action, '"\n' )
      stop ( 'byname - Fatal error!' )
    }
  }
#
#  Handle the NAME.
#
  if ( is.null ( name ) )
  {
    name2 = "*"
  }
  else
  {
    name2 = substr ( name, 1, 1 )
    name2 = tolower ( name2 )
    match = 
      ( name2 == "a" ) |
      ( name2 == "b" ) |
      ( name2 == "g" ) |
      ( name2 == "*" ) 

    if ( ! match )
    {
      cat ( '\n' )
      cat ( 'byname: Fatal error!\n' )
      cat ( '  Legal names are "alpha", "beta", "gamma", "*"\n' );
      cat ( '  Not "', name, '"\n' )
      stop ( 'byname - Fatal error!' )
    }
  }
#
#  Handle VALUE_IN.
#
  if ( is.null ( value_in ) )
  {
    value_in = 0.0
  }
#
#  Carry out requested action.
#
  value_out = NULL

  if ( action2 == "g" )
  {
    if ( name2 == "a" )
    {
      value_out = alpha
    }
    else if ( name2 == "b" )
    {
      value_out = beta
    }
    else if ( name2 == "g" )
    {
      value_out = gamma
    }
  }
  else if ( action2 == "p" )
  {
    if ( name2 == "a" | name2 == "*" )
    {
      cat ( '  alpha = ', alpha, '\n' )
    }
    if ( name2 == "a" )
    {
      value_out = alpha
    }
    if ( name2 == "b" | name2 == "*" )
    {
      cat ( '  beta = ', beta, '\n' )
    }
    if ( name2 == "b" )
    {
      value_out = beta
    }
    if ( name2 == "g" | name2 == "*" )
    {
      cat ( '  gamma = ', gamma, '\n' )
    }
    if ( name2 == "g" )
    {
      value_out = gamma
    }
  }
  else if ( action2 == "r" )
  {
    if ( name2 == "a" | name2 == "*" )
    {
      alpha = alpha_default
    }
    if ( name2 == "a" )
    {
      value_out = alpha
    }
    if ( name2 == "b" | name2 == "*" )
    {
      beta = beta_default
    }
    if ( name2 == "b" )
    {
      value_out = beta
    }
    if ( name2 == "g" | name2 == "*" )
    {
      gamma = gamma_default
    }
    if ( name2 == "g" )
    {
      value_out = gamma
    }
  }
  else if ( action2 == "s" )
  {
    if ( name2 == "a" )
    {
      alpha = value_in
      value_out = alpha
    }
    if ( name2 == "b" )
    {
      beta = value_in
      value_out = beta
    }
    if ( name2 == "g" )
    {
      gamma = value_in
      value_out = gamma
    }
  }
#
#  Store persistent values.
#
  attr ( byname, "alpha" )         <<- alpha
  attr ( byname, "alpha_default" ) <<- alpha_default
  attr ( byname, "beta" )          <<- beta
  attr ( byname, "beta_default" )  <<- beta_default
  attr ( byname, "gamma" )         <<- gamma
  attr ( byname, "gamma_default" ) <<- gamma_default

  output = c ( value_out )

  return ( output )
}