C++: pair<vector<int>,vector<int>> p;

Question

Can we do something like this using c++ STLs. If yes, how am I going to initialize the elements? I was trying to do this but it isn't working.

 pair<vector<int>,vector<int>>p;
 p.first[0]=2;

Answer 1

Can we do something like this using c++ STLs

Yes. Although, you are probably using the standard library instead.

If yes, how am I going to initialize the elements?

You initialize the elements the same way you initialize the elements of a vector that isn't in a pair. List-initialization is a neat option.

I was trying to do this but it isn't working.

You are trying to modify an element of the vector that you never put there. Take a look at the page that desribes what the operator[] does. It doesn't state that it adds elements to the vector. There are however, other functions that do.

Answer 2

Per default the vectors do not have any size, so you should either push_back some elements or resize them first. A way to initialize your p would be:

pair<vector<int>, vector<int>> p = {{1,2,3}, {4,5,6}};

Answer 3

p.first and p.second (with type std::vector<int> ) will be initialized, but they're still empty, there's no elements in them. Then p.first[0] = 2; will lead to UB.

You might want

p.first.push_back(2);

Answer 4

Yes it works, but you have to allocate memory to the std::vector s before you access their elements:

//p now can hold 1 element
p.first.resize(1);

Alternatively, you can use push_back :

//p now has 1 element with value 2
p.first.push_back(2).