C++ 2D dynamic array

(1) Allocate memory in stack for static 2D array (constant dimensions)

const int row=5;
const int col=3;
double matrix[row][col];

(2) Allocate memory in heap for partially dynamic 2D array (if the column dimension is constant)

int row=5;
double (*matrix)[col] = new double[row][col];

(3) Allocate memory for fully dynamic 2D array (continuous memory)

int i,j, row=5, col=3;
double **matrix = new double* [row];
matrix[0] = new double [row*col];
for (i=1; i<row; ++i)
    matrix[i] = matrix[i-1] + col;
for (i=0; i<row; i++) 
    for (j=0; j<col; j++) {
        matrix[i][j] = 0; //do something
    }
delete [] matrix[0];
delete [] matrix;

(4) Allocate memory for fully dynamic 2D array (uncontinuous memory)

int i,j, row=5, col=3;
double **matrix = new double* [row];
for(int i=0;i<row;i++)
    matrix[i]=new double[col];
for (i=0; i<row; i++) 
    for (j=0; j<col; j++) 
        matrix[i][j] = 0;

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

(5) Use 1D array as 2D array

double *array = new double [row*col]; 
for (i=0; i<row; i++) 
    for (j=0; j<col; j++) matrix[i*col+j] = 0;
delete [] array;

(6) Use C++ STL vector container, dimension can be changed during runtime.

vector<vector<double>> matrix(row,vector<double>(col));
matrix[i][j] = 6;

If you want to initialize it then use :

vector<vector<double>> matrix(row,vector<double>(col,0));

Example: 2D Complex matrix using STL

#include <vector>
#include <complex>
using namespace std;
const int N = 2;

typedef std::complex<double>  Complex;
typedef std::vector<vector<Complex> > Complex_Matrix;

int GenInitialCondition(Complex_Matrix &sigma);

int main (int argc, char *argv[]) {
	Complex_Matrix sigma(N,vector<Complex>(N,0.0)); //initialize to be 0
	//to get the values of real or imaginary part:
	double re, im;
	re = sigma[0][0].real() + sigma[1][1].real();
	im = sigma[0][0].imag() + sigma[1][1].imag();
	return 0;
}

int GenInitialCondition(Complex_Matrix &sigma) {
    for (int i = 0 ; i < N ; i++)
        for (int j = 0 ; j < N ; j++)
            sigma[i][j] = 0;
    //to assign real or imaginary part:
    sigma[0][0].real(5.0);
    sigma[0][0].imag(1.0);    
    return 0;
}

For dynamically allocated array in C++ class, see http://www.cs.fsu.edu/~jestes/cop3330/notes/dma.html

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