Re: [ublas] Filling the elements in a vector without knowing the size

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2007-03-06 02:18:53

Am Dienstag, 6. März 2007 07:40 schrieb Sourabh:
> Hi,
> I have a dense vector whose size is not known at the time of
> creation. However, I want to fill entries into the vector. What is
> the most efficient way of doing this ?

repeat
  x.resize( 2*x.size(), true );
  fill data

gives you amortized constant time for allocation. However, the 
stl::vector type is much better optimized for such operations. uBLAS is 
optimized for linear algebra, where the sizes are usually known at the 
time of creation.

> Also, is there any constant time operation to get the number of
> non-zero elements in a sparse vector ?

A.nnz() returns the number of (structural) non-zeros. If you need the 
exact number of non-zeros (excluding the actually stored zeros) you 
have to loop over all structural nnzs. (There was a discussion about 
that a few month ago on this list.)

mfg
Gunter