function [ m, b, k, t0, y0, tstop ] = spring_parameters ( m_user, b_user, ...
  k_user, t0_user, y0_user, tstop_user )

%*****************************************************************************80
%
%% spring_parameters() returns parameters for the spring ODE.
%
%  Discussion:
%
%    If input values are specified, this resets the default parameters.
%    Otherwise, the output will be the current defaults.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    26 April 2021
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real M_USER: the mass;
%
%    real B_USER: the damping coefficient;
%
%    real K_USER: the spring stiffness.
%
%    real T0_USER: the initial time, in seconds;
%
%    real Y0_USER(2): the initial values.
%
%    real TSTOP_USER: the final time.
%
%  Output:
%
%    real M: the mass;
%
%    real B: the damping coefficient;
%
%    real K: the spring stiffness.
%
%    real T0: the initial time, in seconds;
%
%    real Y0(2): the initial values.
%
%    real TSTOP: the final time.
%
  persistent m_default;
  persistent b_default;
  persistent k_default;
  persistent t0_default;
  persistent y0_default;
  persistent tstop_default;
%
%  Initialize defaults.
%
  if ( isempty ( m_default ) )
    m_default = 5.0;
  end

  if ( isempty ( b_default ) )
    b_default = 2.0;
  end

  if ( isempty ( k_default ) )
    k_default = 3.0;
  end

  if ( isempty ( t0_default ) )
    t0_default = 0.0;
  end

  if ( isempty ( y0_default ) )
    y0_default = [ 0.0, 1.0 ];
  end

  if ( isempty ( tstop_default ) )
    tstop_default = 25.0;
  end
%
%  Update defaults if input was supplied.
%
  if ( 1 <= nargin )
    m_default = m_user;
  end

  if ( 2 <= nargin )
    b_default = b_user;
  end

  if ( 3 <= nargin )
    k_default = k_user;
  end

  if ( 4 <= nargin )
    t0_default = t0_user;
  end

  if ( 5 <= nargin )
    y0_default = y0_user;
  end

  if ( 6 <= nargin )
    tstop_default = tstop_user;
  end
%
%  Return values.
%
  m = m_default;
  b = b_default;
  k = k_default;
  t0 = t0_default;
  y0 = y0_default;
  tstop = tstop_default;
  
  return
end

