function [ m1, m2, k1, k2, t0, y0, tstop ] = spring_double_parameters ( ...
  m1_user, m2_user, k1_user, k2_user, t0_user, y0_user, tstop_user )

%*****************************************************************************80
%
%% spring_double_parameters() returns parameters for spring_double_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:
%
%    29 May 2021
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real M1_USER, M2_USER: the masses;
%
%    real K1_USER, K2_USER: the spring constants.
%
%    real T0_USER: the initial time, in seconds;
%
%    real Y0_USER(4): the initial values.
%
%    real TSTOP_USER: the final time.
%
%  Output:
%
%    real M1, M2: the masses;
%
%    real K1, K2: the spring constants.
%
%    real T0: the initial time, in seconds;
%
%    real Y0(4): the initial values.
%
%    real TSTOP: the final time.
%
  persistent m1_default;
  persistent m2_default;
  persistent k1_default;
  persistent k2_default;
  persistent t0_default;
  persistent y0_default;
  persistent tstop_default;
%
%  Initialize defaults.
%
  if ( isempty ( m1_default ) )
    m1_default = 3.0;
  end

  if ( isempty ( m2_default ) )
    m2_default = 5.0;
  end

  if ( isempty ( k1_default ) )
    k1_default = 1.0;
  end

  if ( isempty ( k2_default ) )
    k2_default = 10.0;
  end

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

  if ( isempty ( y0_default ) )
    u10 = 0.0;
    v10 = 1.0;
    u20 = 0.0;
    v20 = 0.0;
    y0_default = [ u10, v10, u20, v20 ];
  end

  if ( isempty ( tstop_default ) )
    tstop_default = 50.0;
  end
%
%  Update defaults if input was supplied.
%
  if ( 1 <= nargin )
    m1_default = m1_user;
  end

  if ( 2 <= nargin )
    m2_default = m2_user;
  end

  if ( 3 <= nargin )
    k1_default = k1_user;
  end

  if ( 4 <= nargin )
    k2_default = k2_user;
  end

  if ( 5 <= nargin )
    t0_default = t0_user;
  end

  if ( 6 <= nargin )
    y0_default = y0_user;
  end

  if ( 7 <= nargin )
    tstop_default = tstop_user;
  end
%
%  Return values.
%
  m1 = m1_default;
  m2 = m2_default;
  k1 = k1_default;
  k2 = k2_default;
  t0 = t0_default;
  y0 = y0_default;
  tstop = tstop_default;

  return
end

