C++ STL valarrays vs. vectors

Originally posted by Sean Dettrick

> Michael Aramini <> wrote in message
> news:<SeS4b.16820$>. ..

> >

> > For 1D arrays, the choices are:

> > valarray<type> vs. vector<type>

> >

> > For 2D arrays, the choices are:

> > valarray< valarray<type> > vs. valarray< vector<type> >
> vs.

> > vector< valarray<type> > vs. vector < vector<type> >

> >

>

> If speeds is of the essence, I would suggest you measure the times

> yourself.

> Also consider building classes so that the underlying STL class

> (vector, valarray, deque) can be changed without effecting the rest of

> your code.

>

> For 2D and 3D arrays, I have found it is faster to use a 1D

> vector<double> with a computed index (kept track of manually) than to

> use vector < vector<type> > or

> vector < vector< vector<type> > >. In the 3D case, a 1D vector is

> MUCH faster than a 3D vector. In the 2D case, the speed difference

> was about 10% I think.

>

> Question: Instead of using a vector<vector<type>> for a 2d array,
> we can also use vector<anothertype>, where "anothertype" has two
> members of "type".

>

> My question: Is this way of implementation is more efficient then
> having a long vector of 1d with a computed index in order to manage 2d
> vector?.

>

> Quick replies for this question will be very useful to me. Thanks in
> advance.

>

> -Vijaya.

>

> > In my application, type is will be a scalar type such as bool,
> double,

> > int, or size_t.

>

> Then you need to read about the well known caveat about vector<bool>

> (e.g. in the Meyers book), and consider using deque<bool> for the bool

> case.

>

> > The code is initially being targeted for Linux, but it may also
> be

> > ported to another UNIX such as IRIX, or perhaps even Windows, so
> I'd

> > like to keep things portable by using an STL container rather
> than one

> > from some operating system specific template library.

>

> Sounds like you'll be using a lot of different compilers - beware that

> valarray is not always well supported. I found vector and valarray

> had the same speed, but valarray wasn't fully supported by all

> compilers.

>

> Finally, if you ever plan to interface to legacy C or fortran code, a

> 1D vector<> has the advantage that its contents are guaranteed to be

> contiguous in memory. That simplifies any such interface

> considerably.

>

Sean


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