c++arraysmatrixdynamic-data

How do you dynamically allocate a matrix?

How do you dynamically allocate a 2D matrix in C++? I have tried based on what I already know:

#include <iostream>

int main(){
    int rows;
    int cols;
    int * arr;
    arr = new int[rows][cols];
 }

It works for one parameter, but now for two. What should I do?

Solution

  • A matrix is actually can be represented as an array of arrays.

    int rows = ..., cols = ...;
int** matrix = new int*[rows];
for (int i = 0; i < rows; ++i)
    matrix[i] = new int[cols];

    Of course, to delete the matrix, you should do the following:

    for (int i = 0; i < rows; ++i)
    delete [] matrix[i];
delete [] matrix;

    I have just figured out another possibility:

    int rows = ..., cols = ...;
int** matrix = new int*[rows];
if (rows)
{
    matrix[0] = new int[rows * cols];
    for (int i = 1; i < rows; ++i)
        matrix[i] = matrix[0] + i * cols;
}

    Freeing this array is easier:

    if (rows) delete [] matrix[0];
delete [] matrix;

    This solution has the advantage of allocating a single big block of memory for all the elements, instead of several little chunks. The first solution I posted is a better example of the arrays of arrays concept, though.