Is there any good method to initialize 2-D array in C++ 11?

Question

I want to initialize a Identity Matrix (ones on the main diagonal and zeros elsewhere) with a 2-D array. I try to implement this with vector. The code is like this:

vector<vector<int> > matrix;
matrix.resize(n);
for(auto &i: matrix)
    i.resize(n);

// initialize...

This is far more complex than just with ordinary array with just int matrix[n][n]. I don't do this because I want to return this 2-D matrix. So the function is like this :

matrix_type
func(int n)
{
  //intialize the matrix
  ...
}

Because the array in C++11 is like this:

array<Elem,N> arr;

I have no idea how to get the N at runtime. Then I decide to use the vector.

So is there is a better way to handle this?(without using traditional array)

Thanks for the answer. The question can be solved by use

vector<vector<int>> matrix(n, std::vector<int>(n));

And , another question. If I am a stupid guy and I just want to use the interface, that is , I want the return type to be array<int,n>. So is there any method to get the N at runtime and return it?

What is the reason you cannot use the 2D array approach? All of these methods would work for what you need, which one would you prefer to implement? — Easton Bornemeier, Jul 10 '18 at 14:50
The ordinary `int matrix[n][n]` doesn't work with `n` unknown at compile time as well. — V. Kravchenko, Jul 10 '18 at 14:51
Oh this is just for curiosity. Since STL has defined a new interface. So I am wondering if there is a good method to solve this problem by using just STL. — Sut, Jul 10 '18 at 14:52
@V.Kravchenko Yes that't true. Thanks for pointing out. Then things get more complicated~~ — Sut, Jul 10 '18 at 14:53
Note that a vector of vectors is an inefficient representation of a matrix. A flat vector of rows stored consequetively is better. — eerorika, Jul 10 '18 at 14:57
@user2079303 Yes, that's true. But the code to access a matrix is much more natural with a 2-D array. — Sut, Jul 10 '18 at 15:08

R Sahu · Accepted Answer · 2018-07-10T14:56:48.970

2

You can simplify the construction/initialization of the matrix to:

vector<vector<int>> matrix(n, std::vector<int>(n));

The above line will initialize all the elements of the matrix to 0. You can set the values of the diagonal elements to 1 using a for loop.

for ( size_t i = 0; i < n; ++i )
{
   matrix[i][i] = 1;
}

edited Jul 10 '18 at 14:56

answered Jul 10 '18 at 14:51

R Sahu

204,454
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Oh yes! I have forgotten this ! That's a good answer! Thanks~ – Sut Jul 10 '18 at 14:55

WhiZTiM · Answer 2 · 2018-07-10T14:59:50.297

0

You can initialize a 2D array of std::vector<T> as follows:

 template<typename T>
 using Matrix = std::vector<std::vector<T>>;

 template<typename T>
 Matrix<T> make2DArray(int N, int M){
     return Matrix<T>(N, std::vector<T>(M));
 }

Calling it like:

 auto my2Darr = make2DArray<int>(20, 30);

edited Jul 10 '18 at 14:59

answered Jul 10 '18 at 14:51

WhiZTiM

21,207
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There are too much of std::vector> in this solution, consider making a typedef `typedef Matrix std::vector>;` or if template T is really needed `template using Matrix = std::vector>;` – V. Kravchenko Jul 10 '18 at 14:53
@V.Kravchenko, Good catch. updated. Thanks .. – WhiZTiM Jul 10 '18 at 15:00

Is there any good method to initialize 2-D array in C++ 11?

2 Answers2