simulating dynamical systems with arbitrary external inputs using lsode

[Scott Kuntze]

Suppose I have a simple dynamical system

   function xdot = f(x,t)
       xdot = 0 ;
       xdot = -x + du(t)
   endfunction

that I wish to simulate with

   lsode('f',0,t).

Suppose also that the time vector is

   t = [0, 0.1, 0.2, 0.3, 0.4]

and that I have an arbitrary vector, say

   waveform = [2, 3, 5, 6, 2],

that I captured from an experiment; the elements in waveform correspond 
to the
times in t.


How can I simulate 'f' with 'waveform' as the input du(t)?  I don't know 
how to
get t in the function f to pull out the corresponding element in waveform.

I believe this is an indexing problem -- I need to link a particular 
time t1 in
t to its index, so that the index can then be used to pull out the 
element in
waveform at the same index e.g. when t = 0.3, du(0.3) = waveform(4) = 6.

I tried an ugly solution like the following:

   global waveform
   global tt
   tt = t
   
   function result = du(t)
       global waveform
       global tt
       idx = find(tt==t) ;
       result = waveform(idx) ;    
   endfunction,

but lsode stalls -- it doesn't like my "user-defined function," and I 
suspect
that the "find" function is the culprit.

Any suggestions?  I feel this should be an easy thing to do, but I'm stumped
despite searching the archives and the web at large...