predprey.m

ode routines in matlab

function [out] = PredPrey(x, y, flag)
%
%  Evaluate the right hand side function for the
%  Predator Prey model
%
%    y_1'(x) = alpha*y_1(x) - beta*y_1(x)*y_2(x)
%    y_2'(x) = gamma*y_2(x)  + delta*y_1(x)*y_2(x)
%
%
%

alpha = 0.25;
beta  = 0.01;
gamma = -1.00;
delta = 0.01;

switch( flag )
   case '' % return right hand side

      out  = [ alpha*y(1)-beta*y(1)*y(2);
               gamma*y(2)+delta*y(1)*y(2) ];
   
   case 'jacobian' % return Jacobian of right hand side

      out  = [ alpha-beta*y(2)    -beta*y(1);
               delta*y(2)         gamma+delta*y(1)];
   
   otherwise
      error(['Unknown request.'])
end