CODEWITHSUNDEEP
c++matrixvectorstl
ivanciprian
ivanciprian

Reputation: 83

Creating a matrix using STL vector

I want to create a matrix using "vector":

vector < vector <int> > Mat;

The problem is, when I run this code:

int i ,j;
    for(i = 1  ; i <= 5 ; ++i)
    for(j = 1 ; j <= 5 ; ++j)
        Mat[i][j] = 0;

I would get a pretty nasty error. How can I fix that?

I do NOT want to read the matrix like this:

for(i = 1  ; i <= 5 ; ++i)
    for(j = 1 ; j <= 5 ; ++j)
        M[i].push_back(0);

Upvotes: 2

Views: 21315

Answers (5)

mango
mango

Reputation: 11

It might be better to use std::array。

std::array< std::array<int, 6>, 6> matirx;
for(auto& row: matrix)
    for(auto& col: row)
        col = 0;

Upvotes: 1

Rom098
Rom098

Reputation: 2603

As other answers mention, you need to use the std::vector constructor with initial numbers of elements to avoid push_back'ing the elements into the vector later. However if you want to initialize your matrix with zeros, you don't need to specify the default initializer value, because since C++11 the form vector(size_type count) guarantees zeroing out elements of non-class types such as int:

vector < vector<int> > Mat( 6, vector<int>(6) );

This creates a 6x6 int matrix with zeros.

Upvotes: 0

PYA
PYA

Reputation: 8636

vector < vector <int> > Mat;

This creates an empty Mat;

vector < vector <int> > Mat (5);

will create it with 5 "inside" vectors. The problem still isn't solved, you need to resize the interior vectors as well.

you can do this by: (there are plenty of other better ways)

for(int i =0;i<5;i++)
{
  Mat[i].resize(5); // resizing the interior matrices
}

Upvotes: 1

NathanOliver
NathanOliver

Reputation: 180660

When you create a vector it starts off empty unless you tell it what the size should be. If the vector is empty then you cannot use [] as it doesn't do any range checking and will not grow the vector. That leaves you with two options, use push_back() or supply a size to the vector when you create it. For instance we could use

const int matrix_size = 5;
auto mat = std::vector<std::vector<int>>(matrix_size, std::vector<int>(matrix_size));
//                                       ^# of rows^                   ^# of cols^

Also remember indexes are 0 based in C++. That means for a vector with 5 elements the valid indexes are [0, 4]. Instead of bothering with the indexes of the vector we can use a ranged based for loop to fill the vector like

for(auto& row : mat)
    for(auto& col : row)
        std::cin >> col;

This will fill every element in the vector from cin.

Upvotes: 2

Izuka
Izuka

Reputation: 2612

When you're creating your vectors this way, they have a dimension of 0. You have to initialize them with the good size : 

vector < vector <int> > Mat(6, std::vector<int>(6));

By the way, adding a 0 in the second vector initialization will ensure it will be filled with 0 :

vector < vector <int> > Mat(6, std::vector<int>(6, 0));

Upvotes: 7

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