Best way to concatenate two vectors?

Question

I'm using multitreading and want to merge the results.

ex:

std::vector<int> A;
std::vector<int> B;
std::vector<int> AB;

I want AB to have to contents of A and the contents of B in that order. What's the most efficient way of doing something like this?

Thanks

Answer 1

If your vectors are sorted*, check out set_union from <algorithm>.

set_union(A.begin(), A.end(), B.begin(), B.end(), AB.begin());

There's a more thorough example in the link

*thanks rlbond

Answer 2

AB.reserve( A.size() + B.size() ); // preallocate memory
AB.insert( AB.end(), A.begin(), A.end() );
AB.insert( AB.end(), B.begin(), B.end() );

Answer 3

This is precisely what std::vector::insert is for.

std::vector<int> AB = A;
AB.insert(AB.end(), B.begin(), b.end());

Answer 4

Depends on whether you really need to physically concatenate the two vectors or you want to give the appearance of concatenation of the sake of iteration. The boost::join function

http://www.boost.org/doc/libs/1_43_0/libs/range/doc/html/range/reference/utilities/join.html

will give you this.

std::vector<int> v0;
v0.push_back(1);
v0.push_back(2);
v0.push_back(3);

std::vector<int> v1;
v1.push_back(4);
v1.push_back(5);
v1.push_back(6);
...

BOOST_FOREACH(const int & i, boost::join(v0, v1)){
    cout << i << endl;
}

should give you

1
2
3
4
5
6

Note boost::join does not copy the two vectors into a new container but generates a pair of iterators (range) that cover the span of both containers. There will be some performance overhead but maybe less that copying all the data to a new container first.