Re: Doubt - Static multidimension arrays

[Nemo via Digitalmars-d-learn]

On Tuesday, 19 January 2016 at 20:39:37 UTC, tsbockman wrote:
On Tuesday, 19 January 2016 at 19:14:30 UTC, alb wrote:
   [...]

One other thing you may want to keep in mind when working on 
this kind of thing - when you loop over a multi-dimensional 
array, the order matters.

For large arrays, this:

    int[c_max][r_max] arr;

    foreach(r; 0 .. r_max)
    {
        foreach(c; 0 .. c_max)
        {
            // do something with arr[r][c] here
        }
    }

Can be *much* faster than this:

    int[c_max][r_max] arr;

    foreach(c; 0 .. c_max)
    {
        foreach(r; 0 .. r_max)
        {
            // do something with arr[r][c] here
        }
    }

The reason is that the first version access the elements in the 
order that they are actually stored in memory, whereas the 
second forces the CPU to jump between rows for each element.

[...]

You're welcome.

And yes, it can definitely be confusing. I understand why the 
array syntax in D is the way it is, but that still doesn't 
always save me from mixing things up once in a while anyway.

I don't remember where I saw it, but actually, in static 
multi-dimensional arrays, arr[0][1] is next to arr[0][0] in 
memory. You can see it with:

void main () {
  ubyte [7][5] arr;
  import std.stdio : writeln;
  writeln ( & arr[0][0], " ", & arr[0][1], " ", & arr [1][0] );
}