c++ 以简洁的方式对另一个向量进行向量推回切片

Question

I mean to build a vector of M N-sized-vectors, ie, with K=M*N total elements, from a K-sized vector with a linear arrangement of the same set of values. Moreover, I would do that in a class constructor, although I guess that is irrelevant. My code is shown below.

What should I use in the pushback line?

template <int dim>
class vec2d {
  public:
    // Constructor - Version 1: 2D array as initializer
    vec2d(const std::vector<std::vector<double> >& c);
    // Constructor - Version 2: 1D array as initializer
    vec2d(const std::vector<double>& c);
    ...
  protected:
    std::vector<std::vector<double> > _vec2d;
};

// Version 1: 2D array as initializer
...

// Version 2: 1D array as initializer
template <int dim>
vec2d::vec2d(const std::vector<double>& c)
{
    for (size_t i = 0; i < (c.size() / dim); i++)
        _vec2d.push_back(std::vector<double>(c(i * dim), c((i+1) * dim - 1)));  // <- Fix this line
}

Answer 1

std::vector has a constructor that takes an iterator range. Using that you can change you loop to use that like

template <int dim>
vec2d::vec2d(const std::vector<double>& c)
{
    auto begin = c.begin();
    auto end = begin + dim;
    for (size_t i = 0; i < (c.size() / dim); i++) {
        _vec2d.emplace_back(begin, end);
        begin += dim; // move range to next dim sized block
        end += dim;   // this could be moved into the for loops increment section
    }
}

Answer 2

what about:

template <int dim>
vec2d::vec2d(const std::vector<double>& c)
{
    for (auto i = c.begin(); i < c.end(); i+=dim )
        _vec2d.push_back({i, i+dim});
}

or mixing with the hint from @NathanOlivier:

template <int dim>
vec2d::vec2d(const std::vector<double>& c)
{
    for (auto i = c.begin(); i < c.end(); i+=dim )
        _vec2d.emplace_back(i, i+dim);
}

we may debate if "emplace" is longer than "push" + "{}", but it should avoid a call to the move operator on the temporary vector passed as push_back() argument.

As pointed out, it assumes, based on your example that:

• the c size is multiple of dim (or equal to dim*dim)
• the _vec2d has been cleared before

As optimization you should add _vec2d.resize(c.size()/dim) or _vec2d.resize(dim) before performing the series of invocation of the push_back() method.