CODEWITHSUNDEEP
c++multidimensional-arrayarrayaccessinteger-arithmetic
unixman83
unixman83

Reputation: 9963

Indexing a 3 dimensional array using a single contiguous block of memory

vector<bool> working_lattice(box.rect.length * box.rect.height * box.rect.width);

How do I access working_lattice[1][5][3] using the style of declaration above?

Upvotes: 3

Views: 4989

Answers (3)

Xin
Xin

Reputation: 123

i.e considering this one:

A[R][S][T]

assuming that its base address is addr_base_A,

so you hope that you can get the address of one specific element A[i][j][k],

the answer I think is: S*T*i + T*j + k + addr_base_A.

Hope this helps :)

Upvotes: 1

josephthomas
josephthomas

Reputation: 3276

You need to access it as 

(i * length * height) + (j * height) + k

So in your case

working_lattice[(i * box.rect.length * box.rect.height) + (j * box.rect.height) + k);

or

working_lattice[(1 * box.rect.length * box.rect.height) + (5 * box.rect.height) + 3);

EDIT: Since you mentioned x, y, z elsewhere

working_lattice[(x * box.rect.length * box.rect.height) + (y * box.rect.height) + z);

Upvotes: 5

Danica
Danica

Reputation: 28856

This depends on whether you're using row-major or column-major ordering. Row-major is more typical in C/C++, but you can do either if you're doing it manually.

In row-major ordering, to get to the i, j, k'th element, you need to go through box.rect.height * box.rect.width * i elements to get up to the ith row, plus box.rect.width * j elements to get to the jth column of that row, plus k to get back to the kth element depthwise. To be super-explicit:

const size_t n_x = box.rect.length;
const size_t n_y = box.rect.height;
const size_t n_z = box.rect.width;
working_lattice[1 * n_x * n_z + 5 * n_z + 3]

This is obviously pretty annoying, so you might want to define an inline function or something to help out.

Upvotes: 3

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