C++ 将向量向量的元素推入向量中

Question

I'm just wondering whether there is a simple, quick way to insert the elements of a vector of vectors inside a vector.

For example:

std::vector<std::vector<double> > vals
{

    {1, 2, 3, 4, 5, 6},
    {1, 2, 3, 4, 5, 6}
};
std::vector<double> myvector;

so myvector would then contain: 1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6

I've tried using insert and push_back but had no luck.

Edit:

I have used:

myvector.insert(vals.begin(), vals.end()); - No matching member function to call to 'insert'

Answer 1

Try

// c++11
for(auto && v : vals){
  myvector.insert(myvector.end(), v.begin(), v.end());
}

// non c++11
for(std::vector<std::vector<double> >::const_iterator it = vals.begin(), end = vals.end(); it != end; ++it){
  myvector.insert(myvector.end(), it->begin(), it->end());
}

Answer 2

Depending on how your vector of vectors is built in the first place, there may be a more efficient way to approach this.

If your sub-vectors are all the same length (as appears to be the case from the code example), then the best thing may be to view myvector as a 2D array and build directly into this buffer.

If the sub-vector lengths can vary, but the code that builds your vector of vectors only ever actually adds stuff at the end , then you can use something like the 'collapsed vector vector' class I describe here to build this data, instead of a vector of vectors. (And after that, again, your data is already in the kind of contiguous buffer you are looking for.)

Either way, if performance or memory footprint are important here then avoiding the vector of vector construct can eliminate a lot of buffer manipulation overhead, and work out as quite a significant optimisation.

Answer 3

If you want to insert all of the elements of vals into a single vector:

Brute Force Method

for (int i = 0; i < vals.size(); ++i)
{
    for (int j = 0; j < vals[i].size(); ++j)
    {
        myvector.push_back(vals[i][j]);
    }
}

More Elegant Method

struct AddVector
{
    std::vector<double> myvector;
    void operator()(const std::vector<int>& a)
    {
        std::copy(a.begin(), a.end(), std::back_inserter(myvector));
    }
};

// somewhere in your code
AddVector av;
std::for_each(vals.begin(), vals.end(), av);
myvector.swap(av.myvector);

Answer 4

The variable vals is a vector of vectors, but myvector is a vector of doubles. Trying to insert from the first into the second will not work as the contained types are different ( std::vector<double> versus double ).

Instead you have to manually insert from each sub-vector of vals into myvector .

Answer 5

resize myvector to the size of the sum of the sizes of the vectors in vals. For every vector in vals concat it to myvector. To get a idea how to concat:

Concatenating two std::vectors