MATLAB by rudra pratap

5.5 Ordinary Differential Equations
167
2. isterminal: a. vector of ones and zeros specifying whether occurrence
of an event is terminal (stops the solution) or not. For each element in
the vector value, a 0 or a 1 is required in the vector isterminal. In our
example, we have only one clement in value (x(3) or y), therefore, we
need just one element in ist erminal. Because we would like to stop the
program at y = 0, the zero crossing of y is terminal. Therefore, we set
isterminal=l.
3. direction: a vector of the same length as value, specifying the direction
of zero crossing for each element in value: a. -1 for negative value, a
1 for positive value, and a 0 for I- don'l-care value. In our example,
direction=-1 will imply that the zero crossing of y is a valid event only
when the value of y is decreasing (y crosses zero with iJ < 0). As you
can see, specification of direction prevents the solver from aborting the
solution at the outset when y = 0 but iJ f:. 0.
Of course, your function should provide the event information, in terms of
these three vectors as output, but only when asked. Thus, your function
should check the value of the flag and output the three vectors if the flag
is set to events, otherwise it should output the usual derivative vector xdot.
We can implement this conditional output using switch (you can do it using
if-elseif construction too) , as shown in the following example function.
function [value ,isterminal ,dircn] = proj (t ,z,flag) ;
% PROJ : ODE for proj ectile motion with event detection
g = 9.81;
% specify constant g
if nargin