Initializing a vector of vectors having a fixed size with boost assign

Question

Having a vector of vector with a fixed size,

vector > v(10);

I would like to initialize it so that it has in all e

Answer 1

#include <vector>
#include <iostream>
using namespace std;


int main(){
    int n; cin >> n;
    vector<vector<int>> v(n);
    //populate
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            int number; cin >> number;
            v[i].push_back(number);
        }
    }
    // display
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            cout << v[i][j] << " ";
        }
        cout << endl;
    }
}

Input:

4
11 12 13 14
21 22 23 24
31 32 33 34
41 42 43 44

Output:

11 12 13 14
21 22 23 24
31 32 33 34
41 42 43 44

Answer 2

You don't need to use boost for the required behaviour. The following creates a vector of 10 vector<int>s, with each vector<int> containing 10 ints with a value of 1:

std::vector<std::vector<int>> v(10, std::vector<int>(10, 1));

Answer 3

I will just try to explain it to those new to C++. A vector of verctors mat has the advantage that you can access its elements directly at almost no cost using the [] operator..

int n(5), m(8);
vector<vector<int> > mat(n, vector<int>(m));

mat[0][0] =4; //direct assignment OR

for (int i=0;i<n;++i)
    for(int j=0;j<m;++j){
        mat[i][j] = rand() % 10;
    }

Of course this is not the only way. And if you do not add or remove elements, one can also use the native containers mat[] which are nothing more than pointers. Here's my fav way, using C++:

int n(5), m(8);
int *matrix[n];
for(int i=0;i<n;++i){
    matrix[i] = new int[m]; //allocating m elements of memory 
    for(int j=0;j<m;++j) matrix[i][j]= rand()%10;
}

This way, you don't have to use #include <vector>. Hopefully, it's clearer!

Answer 4

This doesn't use boost::assign but does what you need:

vector<vector<int>> v(10, vector<int>(10,1));

This creates a vector containing 10 vectors of int, each containing 10 ints.