C++ push_back many 1D vectors in a 2D vector

Question

Let's say i have a lot of vectors :

vector<int> v1 = ...
vector<int> v2 = ...
...
vector<int> v1000 = ...

Is there a quicker way to populate a 2D vector with those 1D vectors than this :

vector<vector<int>> vGlobal ;
vGlobal.push_back(v1);
vGlobal.push_back(v2);
.......
VGlobal.push_back(v1000);

Answer 1

It will be faster to

• first reserve 1000 slots and
• then move the vectors into vGlobal (iff v1 to v1000 aren't needed anymore; if you still need v1 to be "intact" after the procedure do not use std::move)

if you cannot alter the structure.

vector<vector<int>> vGlobal;
vGlobal.reserve(1000);
vGlobal.push_back(std::move(v1));
vGlobal.push_back(std::move(v2));
...
VGlobal.push_back(std::move(v1000));

If you can alter the structure then I'd advise to use std::vector<std::vector<int>> to hold all 1000 instances in the first place and not go for v1, v2, ..., v1000.

Answer 2

Let's say i have a lot of vectors :

vector<int> v1 = ...
vector<int> v2 = ...
...
vector<int> v1000 = ...

Let's pretend that you don't have to manage and name them individually and instead of v1 to v1000 you could write v[0] to v[999] which makes it easy to process all of them in a loop etc?

There are standard solutions for this:

1. If you have a fixed number of these vectors:

   std::array<std::vector<int>, 1000> v;
   

2. If you may want to add/remove vectors in run-time:

   std::vector<std::vector<int>> v(1000);
   

Now, if you for some reason would like to create vGlobal with a copy of all the vectors in v:

std::vector<std::vector<int>> vGlobal(v.begin(), v.end());