Re: data structure for simulation

[Jaroslav Hajek]

On Tue, Mar 17, 2009 at 4:10 PM, Francesco Potorti` <address@hidden> wrote:
>>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.

No, that assumption is wrong. The whos output apparently does not
include memory for array and octave_value internals.
What you have in memory is indeed an array of million octave_value objects.

> 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.
>

Well, you really need to go through all the elements, there's no way
avoiding it. This expression
is slow because you pass in a function handle, even though it's a
built-in function.
Cellfun allows also specifying certain mappers as special string, so you may try

find (cellfun ("prodofsize", a))

which seems to be specialized and thus significantly faster.

cheers

-- 
RNDr. Jaroslav Hajek
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz