Re: C++ is Slow?
On 2008-02-06 19:12, Ioannis Vranos wrote:
nw wrote:
I'm talking about contiguous allocation of multidimensional vectors.
There is no such thing. There can be a vector of vectors, but it
is not a "multidimensional vector", unfortunately. The main reason
it isn't that is the ability of all [second-tier] vectors to have
different sizes.
ok, I'm talking about implementing a vectorlike multidimensional
datastructure that uses contiguous memory (see initial message for
details of the problem I'm addressing).
Where computational efficiency is a primary concern, you should use
valarray and its facilities (slice_array etc).
I disagree, unless you have some commercial library which includes
specific optimisations for valarray using vector will be just as good.
The reason being that valarray was designed to allow optimisations but
they do not require it, and since valarray was a bit of a failure no
implementations that I know of perform any kinds of optimisations. The
only advantage of valarray is that it comes with a few mathematical
operators, but at least in the implementation that comes with Visual C++
they are implemented using normal loops.
In fact I would argue that writing your own might quite easily achieve
greater performance for some operations using OpenMP.
As for implementing a two-dimensional array I would do something like this:
template<typename T>
class Matrix
{
std::vector<T> m_vec;
size_t m_rows, m_cols;
public:
Matrix(size_t r, size_t c)
: m_vec(r * c), m_rows(r), m_cols(c)
{ }
T& operator()(size_t r, size_t c)
{
// Perhaps check if r < m_rows && c < m_cols
return m_vec[r * m_cols + c];
}
};
I might have messed up the calculation of where the element is (mixed
rows and columns) but I think that the general idea is clear. To make it
kind of usable with standard algorithms and containers you can offer
begin() and end() which just returns m_vec.begin()/end(), or you can
make more advanced stuff like allowing iterating over a row or column.
--
Erik Wikstr??m