CODEWITHSUNDEEP
c++arrays
Duane Allman
Duane Allman

Reputation: 571

1D vector - 3D indices

I am using a 1D vector to represent a 3D data structure, with indices 0 to x*y*z, where x, y and z are the number of cells in each direction.

In order to index a specific cell [xi][yi][zi], I use:

index = xi + x*yi + x*y*zi ;

My problem is, I can't work out how to go the other way!

e.g. I want to get the individual coordinates represented in index 43.

I think I've worked out how to get xi:

xi = index % x ;

but I can't for the life of me get the others... :/

EDIT: Hmmm. Is this right?

xi = index % x;
yi = ((index - xi)/x) % y;
zi = ((index - xi - x*yi) / x) / y ;

Upvotes: 2

Views: 2050

Answers (2)

Anthales
Anthales

Reputation: 1158

Try this:

xi =   index % x;
yi =  (index / x) % y;
zi = ((index / x) / y) % z;

This can be easily generalized as one might expect.

Upvotes: 4

SirGuy
SirGuy

Reputation: 10780

Some modular arithmetic should do the trick.

    index % x = (xi + x*yi + x*y*zi) % x = xi % x,
    but 0 <= xi < x therefore xi % x = xi.

Then for yi:

    (index / x) % y = yi.

Then lastly:

    index / (x * y) = zi.

EDIT:

In code (as opposed to math above) it would be:

    xi = index % x;
    yi = (index / x) % y;
    zi = index / (x * y);

Upvotes: 1

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