Assigning to slice of array

[Steven Schveighoffer]

On 3/1/18 4:16 PM, Jamie wrote:
> I'm trying to understand arrays and have read a lot of the information 
> about them on this forum. I think I understand that they are set-up like 
> Type[], so that int[][] actually means an array of int[].
> 
> I create an array as per the following:
>      auto arr = new int[3][2][1];
> which produces:
>      [[0, 0, 0], [0, 0, 0]]

No, I think you did int[3][2], if you got that output. Otherwise it 
would have been:

[[[0,0,0],[0,0,0]]]

Looking at the rest of your code, I think it wouldn't work if you had 
done the line above.

> (for each of the following assignments, assume that the array is set 
> back to zeros)
> 
> and I can change the 2nd element of the 1st array using:
>      arr[0][1] = 4;
> which produces:
>      [[0, 4, 0], [0, 0, 0]]
> 
> and I can change the entire 1st array using:
>      arr[0][0 .. 3] = 5;
> which produces:
>      [[5, 5, 5], [0, 0, 0,]]

Yes, all correct.

> 
> however when I try and change elements across arrays rather than within 
> arrays my understanding breaks down..

Well, that's because that type of slicing isn't supported directly. You 
can't slice an array cross-wise like that.

You may be interested in ndslice inside mir: 
http://docs.algorithm.dlang.io/latest/mir_ndslice.html

> when I try
>      arr[0 .. 2][0] = 3;   // which I think is equivalent to arr[0][0] 

Consider the array:

int[] x = new int[2];

Now, what would the slice x[0 .. 2] be? That's right, the same as x.

So when you slice arr[0 .. 2], it's basically the same as arr (as arr 
has 2 elements).

So arr[0 .. 2][0] is equivalent to arr[0].

One thing that is interesting is that you assigned 3 to an array, and it 
wrote it to all the elements. I did not know you could do that with 
static arrays without doing a proper slice assign. But it does compile 
(I learn something new every day).

-Steve