Re: help requested with extremely slow for loop?

[Francesco Potortì]

>add that using sparse row vectors is not the best idea; Octave uses a
>compressed column format for sparse matrices so a sparse row vector is
>not actually compressed at all (i.e. needs O(N) storage, where N is
>the length).

Wow!  So if you want a sparse vector, it should be a column vector.
An in fact I got this:

octave>         tic
        for j = idxs
                x(y(j)) += x(j);
                x(j) = 0;
        endfor
        toc
octave> > > > octave> Elapsed time is 8.4 seconds.

octave> whos
Variables in the current scope:

  Attr Name         Size                     Bytes  Class
  ==== ====         ====                     =====  ===== 
       ans          1x2                         16  double
       idxs         1x37                       296  double
       j            1x1                          8  double
       x            1x15071213            60287148  double
       y            1x15071213            91006944  double

octave> x=sparse(x');
octave> y=sparse(y');
octave> whos
Variables in the current scope:

  Attr Name           Size                     Bytes  Class
  ==== ====           ====                     =====  ===== 
       ans            1x1                          8  double
       idxs           1x37                       296  double
       x       15071213x1                       2732  double
       y       15071213x1                   30722096  double

octave>         tic
        for j = idxs
                x(y(j)) += x(j);
                x(j) = 0;
        endfor
        toc
octave> > > > octave> Elapsed time is 0.31 seconds.

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