Initializing a two-dimensional std::vector

Question

So, I have the following:

std::vector< std::vector <int> > fog;

and I am initializing it very naively like:

for(int i=0; i<A_NUMBER; i++)
{
    std::vector <int> fogRow;
    for(int j=0; j<OTHER_NUMBER; j++)
    {
         fogRow.push_back(0);
    }
    fog.push_back(fogRow);
}

And it feels very wrong... Is there another way of initializing a vector like this?

Answer 1

Use the std::vector::vector(count, value) constructor that accepts an initial size and a default value:

std::vector<std::vector<int> > fog(
    ROW_COUNT,
    std::vector<int>(COLUMN_COUNT)); // Defaults to zero initial value

If a value other than zero, say 4 for example, was required to be the default then:

std::vector<std::vector<int> > fog(
    ROW_COUNT,
    std::vector<int>(COLUMN_COUNT, 4));

I should also mention uniform initialization was introduced in C++11, which permits the initialization of vector, and other containers, using {}:

std::vector<std::vector<int> > fog { { 1, 1, 1 },
                                    { 2, 2, 2 } };
                           

Answer 2

Let's say you want to initialize a two-dimensional vector, m*n, with the initial value to be 0. We could do this:

#include <iostream>

int main(){
    int m = 2, n = 5;

    vector<vector<int>> vec(m, vector<int> (n, 0));

    return 0;
}

Answer 3

There is no append method in std::vector, but if you want to make a vector containing A_NUMBER vectors of int, each of those containing other_number zeros, then you can do this:

std::vector<std::vector<int>> fog(A_NUMBER, std::vector<int>(OTHER_NUMBER));

Answer 4

The general syntax, as depicted already is:

std::vector<std::vector<int> > v (A_NUMBER, std::vector <int> (OTHER_NUMBER, DEFAULT_VALUE))  

Here, the vector 'v' can be visualised as a two dimensional array, with 'A_NUMBER' of rows, with 'OTHER_NUMBER' of columns with their initial value set to 'DEFAULT_VALUE'.

Also it can be written like this:

std::vector <int> line(OTHER_NUMBER, DEFAULT_VALUE)
std::vector<std::vector<int> > v(A_NUMBER, line)

Inputting values in a 2-D vector is similar to inputting values in a 2-D array:

for(int i = 0; i < A_NUMBER; i++) {
     for(int j = 0; j < OTHER_NUMBER; j++) {
         std::cin >> v[i][j]
     }
}

Examples have already been stated in other answers....!

Answer 5

The recommended approach is to use a fill constructor to initialize a two-dimensional vector with a given default value:

std::vector<std::vector<int>> fog(M, std::vector<int>(N, default_value));

where, M and N are dimensions for your two-dimensional vector.

Answer 6

I think the easiest way to make it done is:

std::vector<std::vector<int>>v(10, std::vector<int>(11, 100));

10 is the size of the outer or global vector, which is the main one, and 11 is the size of inner vector of type int, and initial values are initialized to 100!

Answer 7

Here is what worked for me:

vector<vector<int>> minA{ROW_SIZE, vector<int>(COLUMN_SIZE, VALUE)};

Answer 8

This answer will help to easily initialize 2d vector after declaration.

int n=4,m=3;
int default_value = 0;

std::vector<std::vector<int>> matrix;

matrix.resize(m, std::vector<int>(n, default_value));

Answer 9

Suppose you want to initialize a two-dimensional integer vector with n rows and m column, each having value 'VAL'.

Write it as

std::vector<vector<int>> arr(n, vector<int>(m, VAL));

This VAL can be an integer type variable or constant, such as 100.

Answer 10

This code snippet copies one two-dimensional vector to another. And gives us a clear picture of how to initialize the 2D vector.

void copyVectorToVector(vector<vector<int>> matrix) {

        int rowNumber = matrix.size();
        int columnNumber = matrix[0].size();
        vector<vector<int>> result(rowNumber, vector<int> (columnNumber, 0));

        for(int i=0;i<matrix.size();i++){
            for(int j=0;j<matrix[i].size();j++){
                result[i][j] = matrix[i][j];
                cout<<result[i][j]<<" ";
            }
            cout<<endl;
        }
}

Answer 11

multiplication table with 2D vector the first 10 is for ROW, second 10 for COLUMN

std::vector<std::vector<int>> multiplication (10,std::vector<int>(10));
for(int i{0}; i<multiplication.size(); ++i){
    for(int j{0}; j<multiplication[i].size(); ++j){
        multiplication[i][j] = j * i;
    }
}

for(int i{1}; i < multiplication.size(); ++i){
    for(int j{1}; j < multiplication[i].size(); ++j){
        std::cout<<multiplication[i][j]<<"\t";
    }
    printf("\n");
}

Answer 12

Additionally, if someone wants to initialize only the first col values to some different value(say 2) from the rest of the default values(say 1).

int rows = 3, cols = 4;
std::vector<std::vector<int>> matrix(rows, std::vector<int>(cols, 1));  
for (int r = 0 ; r < rows ; r++) matrix[r][0] = 2;

2 1 1 1
2 1 1 1
2 1 1 1

Answer 13

My C++ STL code to initialise a 5*3 two-dimensional vector with zero:

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

int main()
{
    // If we want to initialise a two-dimensional vector with 0;
    vector<vector<int>> v1(5, vector<int>(3, 0));

    for(int i=0; i<v1.size(); i++)
    {
        for(int j=0; j<v1[i].size(); j++)

            cout << v1[i][j] << " ";

            cout << endl;
        }
    }