3D C++ array

JamieAl (236)
This feels like a pretty simple question, but I keep getting my indexing incorrect/confused here. Basically, I have these 2D C++ arrays that I am trying to extend to a 3D C++ array? For example:

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static const int nx = 8; 
static const int ny = 8;

double Lx = 8;
double Ly = 8;
double dx = Lx / nx;
double dy = Ly / ny;

double *xarr;
xarr = (double*) fftw_malloc((nx*ny) *sizeof(double));
memset(XX, 42, (nx*ny) * sizeof(double)); 

double *yarr;
yarr= (double*) fftw_malloc((nx*ny) *sizeof(double));
memset(YY, 42, (nx*ny) * sizeof(double)); 
	
  for(int i = 0; i< nx; i++){
		for(int j = 0; j< ny; j++){ 
			xarr[j + ny*i] = i*dx;
			yarr[j + ny*i] = j * dy; 
 			
		}		
	}

Basically, I am trying to change xarr and yarr into a 3D c++ array (I know it's not the same idea as a tensor or anything) but I am trying to keep my indexing consistent with the above code. So, if I add a third array zarr and create the following nested for loop, is my indexing correct?:
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for(int i = 0; i< nx; i++){
	for(int j = 0; j< ny; j++){
                for(int k=0; k<nz; k++){
                       xarr[k + nz * (j + ny * i)] = i*dx;
                       yarr[k + nz * (j + ny * i)] = j*dy;
                       zarr[k + nz * (j + ny * i)] = k*dz;
                }
           }
      }


I am not really sure if I am conveying what I am trying to do clearly.
Last edited on
coder777 (8444)
What exactly are you trying to do?

A 3-d array is not 3 arrays. Each cell of an 3d array contain a 2d array. Each cell of a 2d array contains a 1d array. Thus when you want a 'flat' 3d array (1d -> 3d) you need to multiply the index of such a cell with the 2d size. If you want the index of a cell within the 2d array you need to multiply the index with the size of the 1d array:

index_1d = z * sizeof_2d + y * sizeof_1d + x

for 3d array size of 8x8x8:

index_1d = z * 8 * 8 + y * 8 + x
JamieAl (236)
What exactly are you trying to do?

Basically, I have the following Eigen tensors that I am trying to rewrite as C++ arrays and get a matching outputs:

Eigen code:
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Eigen::Tensor<double, 3> eXX(nx,ny,nz); 
Eigen::Tensor<double, 3> eYY(nx,ny,nz); 
Eigen::Tensor<double, 3> eZZ(nx,ny,nz); 
for(int i = 0; i< nx; i++){
		for(int j = 0; j< ny; j++){
			for(int k = 0; k< nz; k++){ 
				xarr(k,i,j) = i*dx;
				yarr(j,i,k) = j*dy;
				zarr(j,i,k) = k*dz; 
			}

		}		
	}


C++ Array code:
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double *XX;
	XX = (double*) fftw_malloc((nx*ny*nz) *sizeof(double));
	memset(XX, 42, (nx*ny*nz) * sizeof(double)); 

    double *YY;
	YY = (double*) fftw_malloc((nx*ny*nz) *sizeof(double));
	memset(YY, 42, (nx*ny*nz) * sizeof(double)); //test: XX for some reason isn't the correct dimensions, maybe this is why
	
	double *ZZ;
	ZZ = (double*) fftw_malloc((nx*ny*nz) *sizeof(double));
	memset(ZZ, 42, (nx*ny*nz) * sizeof(double)); //test: XX for some reason isn't the correct dimensions, maybe this is why
	

    for(int i = 0; i< nx; i++){
		for(int j = 0; j< ny; j++){ 
            for(int k =0; k<nz; k++){
                XX[k + nz * (j + ny * i)] = i*dx;
                YY[k + nz * (j + ny * i)] = j*dy;
                ZZ[k + nz * (j + ny * i)] = l*dz;
            }		
 			
	}		
}

The outputs from above codes only match for XX and eXX at least that's what I think is happening, kinda hard to compare the output of a for loop to a structured eigen tensor.

Outputs:
XX and eXX
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0
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0
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  0 160 320 480   0 160 320 480   0 160 320 480   0 160 320 480
  0 160 320 480   0 160 320 480   0 160 320 480   0 160 320 480
  0 160 320 480   0 160 320 480   0 160 320 480   0 160 320 480
  0 160 320 480   0 160 320 480   0 160 320 480   0 160 320 480

However, the other two arrays/tensors aren't matching (YY, eYY and ZZ, eZZ).
dhayden (5798)
Have you considered vector<vector<vector<double> > > ? The code will be easier, although it will have the disadvantage of lots of pointers.

I'd write a little class to represent the 3D array. Test the class and then use it. This isn't up to modern C++ practices, bit it gives you the idea:

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#include <iostream>				 //Required for cin, cout

using std::cin;
using std::cout;

class Arr3D {
public:
    Arr3D(unsigned x, unsigned y, unsigned z);
    ~Arr3D();
    double &operator()(unsigned x, unsigned y, unsigned z);
private:
    Arr3D(const Arr3D&);
    double &operator=(const Arr3D&);
    unsigned nx, ny, nz;
    double *data;
};

Arr3D::Arr3D(unsigned x, unsigned y, unsigned z) :
    nx(x), ny(y), nz(z), data(new double[x*y*z])
{}

Arr3D::~Arr3D()
{
    delete[] data;
}

double &
Arr3D::operator()(unsigned x, unsigned y, unsigned z)
{
    return data[z + y*nz + x*ny*nz];
}

int
main(int argc, char **argv)
{
    unsigned nx, ny, nz;
    cin >> nx >> ny >> nz;
    Arr3D myarr(nx, ny, nz);
    int i=0;
    for (unsigned x=0; x<nx; ++x) {
	for (unsigned y=0; y<ny; ++y) {
	    for (unsigned z=0; z<nz; ++z) {
		myarr(x,y,z) = ++i;
	    }
	}
    }

    for (unsigned x=0; x<nx; ++x) {
	for (unsigned y=0; y<ny; ++y) {
	    for (unsigned z=0; z<nz; ++z) {
		cout << myarr(x,y,z) << ' ';
	    }
	    cout << '\n';
	}
	cout << '\n';
    }
}

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