Initialization of a vector of vectors?

Question

Is there a way to initialize a vector of vectors in the same ,quick, manner as you initialize a matrix?

typedef int type;

type matrix[2][2]=
{
{1,0},{0,1}
};

vector<vector<type> > vectorMatrix;  //???

sinek · Accepted Answer · 2010-06-17T12:10:24.793

For the single vector you can use following:

typedef int type;
type elements[] = {0,1,2,3,4,5,6,7,8,9};
vector<int> vec(elements, elements + sizeof(elements) / sizeof(type) );

Based on that you could use following:

type matrix[2][2]=
{
   {1,0},{0,1}
};

vector<int> row_0_vec(matrix[0], matrix[0] + sizeof(matrix[0]) / sizeof(type) );

vector<int> row_1_vec(matrix[1], matrix[1] + sizeof(matrix[1]) / sizeof(type) );

vector<vector<type> > vectorMatrix;
vectorMatrix.push_back(row_0_vec);
vectorMatrix.push_back(row_1_vec);

In c++0x, you be able to initialize standard containers in a same way as arrays.

score 4 · Answer 2 · answered Jun 17 '10 at 11:58

std::vector<std::vector<int>> vector_of_vectors;

then if you want to add, you can use this procedure:

vector_of_vectors.resize(#rows); //just changed the number of rows in the vector
vector_of_vectors[row#].push_back(someInt); //this adds a column

Or you can do something like this:

std::vector<int> myRow;
myRow.push_back(someInt);
vector_of_vectors.push_back(myRow);

So, in your case, you should be able to say:

vector_of_vectors.resize(2);
vector_of_vectors[0].resize(2);
vector_of_vectors[1].resize(2);
for(int i=0; i < 2; i++)
 for(int j=0; j < 2; j++)
   vector_of_vectors[i][j] = yourInt;

score 3 · Answer 3 · answered Jun 17 '10 at 11:57

3

In C++0x, I think you can use the same syntax as for your matrix.

In C++03, you have to write some tedious code to populate it. Boost.Assign might be able to simplify it somewhat, using something like the following untested code:

#include <boost/assign/std/vector.hpp>

vector<vector<type> > v;
v += list_of(1)(0), list_of(0)(1);

or even

vector<vector<type> > v = list_of(list_of(1)(0))(list_of(0)(1));

answered Jun 17 '10 at 11:57

Mike Seymour

249,747
28
448
644

I tried that but it does not seem to work. I have a `vector` and I would like to initialize the first element of the variable to one so that I have a vector of just one element vector initialized to one. Thanks in advance. – saloua Oct 29 '12 at 11:33
@tuxworker: In C++11, that's `vector> v {{1}};`. With Boost.Assign in C++03, something like `vector > v = list_of(list_of(1));`. If that doesn't work, ask a new question showing what you're trying and describing what goes wrong. – Mike Seymour Oct 29 '12 at 11:36
Indeed I tried `vector > v(10); v+= list_of(list_of(1));` and `vector > v(10); v = list_of(list_of(1));` and I've got a compilation error. I think it would be also better using in addition repeat but I don't know how. I will ask a new question. Thank you – saloua Oct 29 '12 at 11:53

score 2 · Answer 4 · edited May 23 '17 at 10:27

If the matrix is completely filled -

vector< vector<int> > TwoDVec(ROWS, vector<int>(COLS));
//Returns a matrix of dimensions ROWS*COLS with all elements as 0
//Initialize as -
TwoDVec[0][0] = 0;
TwoDVec[0][1] = 1;
..
.

Update : I found there's a better way here

else if there are variable number of elements in each row(not a matrix) -

vector< vector<int> > TwoDVec(ROWS);
for(int i=0; i<ROWS; i++){
    while(there_are_elements_in_row[i]){           //pseudocode
        TwoDVec[i].push_back(element);
    }
}

Initialization of a vector of vectors?

4 Answers4

Linked