Re: data structure for simulation

[Francesco Potorti`]

>What's wrong?

Efficiency :)

This is an issue with cell arrays, in fact.  I realise that my question
can be reformulated more simply like this:

octave> a=cell(1e6,1);
octave> a(1e6)=1;
octave> whos a
Variables in the current scope:

  Attr Name         Size                     Bytes  Class
  ==== ====         ====                     =====  ===== 
       a      1000000x1                          8  cell

Total is 1000000 elements using 8 bytes

So Octave internally knows that only an element in the cell array is
nonempty and does not allocate memory for the whole array.  But if I
want to know the index of the nonempty cell, doing

find (! cellfun (@isempty, a))

needs a non-negligible computing time, because cellfun goes through all
elements, be they empty or not.  I hoped there was an efficient way of
knowing indexes of nonempty elements.

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