function dydt = predator_deriv ( t, y )

%*****************************************************************************80
%
%% predator_deriv returns the derivative for the predator-prey ODE.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    22 February 2020
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real T(:), Y(1,2), the arguments of the derivative.
%
%  Output:
%
%    real DYDT(2,1), the value of the derivative.
%
  [ alpha, beta, gamma, delta ] = predator_parameters ( );

  r = y(1);
  f = y(2);

  drdt =   alpha * r - beta  * r * f;
  dfdt = - gamma * f + delta * r * f;

  dydt = [ drdt; dfdt ];

  return
end

