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

Question

Having a vector of vector with a fixed size,

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

I would like to initialize it so that it has in all elements a one dimensional vector with initialized value (for example 1).

I have used Boost Assign as follows

v = repeat(10,list_of(list_of(1)));

and I've got a compilation error

error: no matching function for call to ‘repeat(boost::assign_detail::generic_list<int>)’

Could you please tell me how to do that. Thanks in advance

Answer 1

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.

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!