Re: slow 'eval' function - maybe pointers would do?

[Carlo de Falco]

On 29 Oct 2009, at 18:52, Guido Walter Pettinari wrote:

> Hi everybody!
>
> I need to assign values to a series of variables with numbered names,
> such as
>
> ks_1 = ... ;
> ks_2 = ... ;
> ...
> ks_N = ... ;
>
> where N is big and is determined at runtime. I do not want to use a
> vector named 'ks' such that ks(i) = ks_i and so on. Therefore, I used
> the 'eval' function in this way:
>
> for i=1:N
>    eval ( [ 'k', int2str(i), ' = ... ;' ] );
> endfor
>
> However, using 'eval' the execution time of my loop increases ~6 fold.
> To test this behaviour, please try to run the following script:
>
> tic();
> for i=1:100000   sin(2);   endfor
> toc()
>
> tic();
> for i=1:100000   eval ( 'sin(2);' );   endfor
> toc()
>
> On my machine, the output is:
>
> Elapsed time is 0.8 seconds.
> Elapsed time is 5 seconds.
>
> Could you please suggest a more performance-friendly workaround?
> Actually, my numbered variables are fields of a structure (es.
> struct.k_1, struct.k_2 ... ); I thought that one could solve the
> problem by using pointers to access them, but I could not find support
> for pointers in Octave.
>
> Thank you,
>
> Guido


You can do what you ask by:

for ii = 1:N
names{ii}  = sprintf("ks_%d", ii);
values{ii} = somehow_compute_value (ii);
endfor

S = cell2struct (values, names, 2)

but what is the reason why you don't want to just use

for ii = 1:N
ks{ii}  = somehow_compute_value (ii);
endfor

?

c.