vector of vectors push_back

Question

I'm designing a multilevel queue process simulator in C++ but I've got a problem when trying to implement several queues (my queues are vectors).So, "multilevel" is a 4 elements array (not vector). Inside each of those elements there is a vector (type t_PCB).

vector<vector<t_PCB>> multilevel[4];

My question is: How can i insert an element at the end of one these 4 t_PCB vectors? Thank you in advance.

I've tried the code line below but it doesn't work (error: not matching member function for call 'push_back')

multilevel[0].push_back(p); //where "p" is a t_PCB object

The line from above can not be used when talking about "multilevel" because this array only accepts arguments type: vector < t_PCB >

So, as I ask at the beginning: how can I push an object type "t_PCB" inside "multilevel"?

Answer 1

By doing this:

vector<vector<t_PCB> > multilevel[4];

You declare an array of four zero-sized vectors, each of which can contain objects of type vector<t_PCB>. What you probably wanted to do is rather:

vector<vector<t_PCB> > multilevel(4);
//                               ^^^

This will instantiate a vector of four default-initialized objects of type vector<t_PCB>. Then, you can just do:

multilevel[size].push_back(p);

Notice, though, that vector indices (like array indices) are zero-based, so size must be less than the size of the vector.

In the above expression, the sub-expression multilevel[size] returns a reference to the size-th vector inside multilevel, and on that vector you are then invoking the member function push_back(p), which appends element p to it.

Answer 2

Declaring a two dimensional vector is similar to declaring an array. You can also use it in same way...

vector<vector<int> > vec;

for(int i = 0; i < 5; i++)
{
    vector<int> row;
    vec.push_back(row);
}

vec[0].push_back(5);
cout << vec[0][0] << endl;

Answer 3

You are creating a array of vector<vector<t_PCB>> instead of a single object.

I think the right way to do what you want is:

vector<vector<t_PCB>> multilevel(4);
multilevel[0].push_back(p)

Answer 4

You can create a vector instead of array:

std::vector< std::vector<t_PCB>> multilevel(4); // 2 dim array, 1st dim is 4

and then you can push_back at the end of the vector indexed with WHICH this way:

multilevel[WHICH].push_back(p)

Answer 5

And just to put it out there, to access the vector of vectors:

multilevel[outer][inner]

where outer will return the vector at that index and further indexing with inner will return the t_PCB object. You could also replace the array-style indexing with the .at() function for bounds checks.

Answer 6

vector<vector<int>> vec; // declare 2D vector

    for (int i=0; i<=3; i++) {
        vector<int> row;  // create a row vector which adds a row to vec
        for (int j=0; j<=4; j++) {
            row.push_back(j*10); // push elements 0,10,20,30,40 to row 
        }
        vec.push_back(row); // add this row to vec
        // Repeat this procedure 4 times to make a 4*5 2D vector
    }

    cout<<"output is "<<vec[2][4]; // output is 40