What is the difference between a[x][y] and a[x,y]?

[Adam D. Ruppe]

On Tuesday, 7 January 2020 at 17:38:59 UTC, Robert M. Münch wrote:
> I read all the docs but I'm not toally sure. Is it that [x][y] 
> is a 2D array-index, where as [x,y] is a slice?

So [x][y] indexes an array of arrays. [x,y] indexes a single 
array that has two dimensions.

This can be kinda confusing because we often think of

int[4][4]

as being a 2d array, but the D language actually technically sees 
that as an array of arrays. So it is indexed with [x][y].

There is no built in multi-dimensional array, it is only syntax 
provided for library types to implement with the opIndexAssign 
overloads.

> And what is the difference between opIndexAssign and 
> opIndexOpAssign?

foo[5] = x; // opIndexAssign
foo[4] += x; // opIndexOpAssign

So the operator there like += or -= or *= etc become opAssign, 
whereas plain = is just plain Assign.