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Re: data structure for simulation

From: Francesco Potorti`
Subject: Re: data structure for simulation
Date: Tue, 17 Mar 2009 23:12:40 +0100

>> 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.

I see.  Maybe whos should be more realistic?  I normally use it to know
how much memory my variables need, so I think it is not useful if whos
tells me much less than the real memory usage.

>> 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.

I see.

>Cellfun allows also specifying certain mappers as special string, so you may 
>find (cellfun ("prodofsize", a))
>which seems to be specialized and thus significantly faster.

Yes, it is *much* faster, indeed, thanks!

This is undocumented, right?

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