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November 19, 2012
declaration/initialization of multidimensional arrays
What is the proper way to declare a multidimensional array?
In Java, I can do:

	double[][] a = new double[M][N];

D does not allow this because of two reasons.

1) M & N are variables and cannot be read at compile time even if they 
are explicitly initialized.

	Error: variable N cannot be read at compile time (sawalks)

2) double[17][17] is a static array and cannot be used to initialize a 
dynamic array.

	Error: cannot implicitly convert expression (new double[17LU][](17LU)) 
of type double[17LU][] to double[][] (sawalks)

Assigning a length and then trying to access the values obviously will 
not work:

	double[][] a;
	a.length = M * N;
	a[i][j] = 17; // core.exception.RangeError@sawalks(19): Range violation

So what is the proper way to do this in D?

Thanks
November 19, 2012
Re: declaration/initialization of multidimensional arrays
Tyro[17]:

> What is the proper way to declare a multidimensional array?
> In Java, I can do:
>
> 	double[][] a = new double[M][N];

In D (hopefully I have not swapped the to sizes, it's a too much 
common mistake in my D code):

auto a = new double[][](M, N);

Bye,
bearophile
November 19, 2012
Re: declaration/initialization of multidimensional arrays
> auto a = new double[][](M, N);

That's a way to declare and initialize a dynamic array of dynamic 
arrays. If you want to allocate something else, you need a 
different syntax.

Example, this creates something rather different:

auto b = new double[][3][](M, N);

In D there are both dynamic arrays and fixed-sized arrays, 
thankfully.

Bye,
bearophile
November 19, 2012
Re: declaration/initialization of multidimensional arrays
> auto b = new double[][3][](M, N);

That doesn't compile, I don't know why. The D array syntax 
allocation syntax is a bit of a mess. So to do that you have to 
write:

auto b = new double[][3][](N);
Or:
auto b = new double[][3][N];

And then fill the arrays with a loop. Why is no one complaining 
about this array allocation syntax?

Bye,
bearophile
November 19, 2012
Re: declaration/initialization of multidimensional arrays
> Why is no one complaining about this array allocation syntax?

A syntax I like is:

new double[][3][](N, M)

Where:
- The numbers inside the [] must be always compile-time 
constants, and they always refer to fixed-sized arrays.
- The numbers inside the () are run-time values and must be used 
for dynamic arrays. (They can be up to the number of empty []. No 
more. But maybe they are allowed to be less).

This means this syntax becomes an error:

int n = 5;
auto a = new int[n];

You must write:

int n = 5;
auto a = new int[](n);

This becomes allowed and allocates a fixed-sized array on the 
heap?

auto a = new int[5];

Bye,
bearophile
November 19, 2012
Re: declaration/initialization of multidimensional arrays
On 11/18/12 7:53 PM, bearophile wrote:
> Tyro[17]:
>
>> What is the proper way to declare a multidimensional array?
>> In Java, I can do:
>>
>>     double[][] a = new double[M][N];
>
> In D (hopefully I have not swapped the to sizes, it's a too much common
> mistake in my D code):
>
> auto a = new double[][](M, N);
>
> Bye,
> bearophile

Thanks... This solved my issue. Much appreciated.
November 19, 2012
Re: declaration/initialization of multidimensional arrays
On 11/18/12 8:14 PM, bearophile wrote:
>> Why is no one complaining about this array allocation syntax?
>
> A syntax I like is:
>
> new double[][3][](N, M)

Why not simply allow new double[N][3][M]; ?
Seems a lot more intuitive/succinct than a separate []() implementation.

> Where:
> - The numbers inside the [] must be always compile-time constants, and
> they always refer to fixed-sized arrays.
> - The numbers inside the () are run-time values and must be used for
> dynamic arrays. (They can be up to the number of empty []. No more. But
> maybe they are allowed to be less).
>
> This means this syntax becomes an error:
>
> int n = 5;
> auto a = new int[n];
>
> You must write:
>
> int n = 5;
> auto a = new int[](n);
>
> This becomes allowed and allocates a fixed-sized array on the heap?
>
> auto a = new int[5];
>
> Bye,
> bearophile
November 19, 2012
Re: declaration/initialization of multidimensional arrays
On Sunday, November 18, 2012 21:06:58 Tyro[17] wrote:
> On 11/18/12 8:14 PM, bearophile wrote:
> >> Why is no one complaining about this array allocation syntax?
> > 
> > A syntax I like is:
> > 
> > new double[][3][](N, M)
> 
> Why not simply allow new double[N][3][M]; ?
> Seems a lot more intuitive/succinct than a separate []() implementation.

Because that declares a dynamic array of static arrays. You'd end up with a 
dynamic array of length N containing static arrays of length 3 containing 
static arrays of length M. The fact that you can declare dynamic arrays of 
static arrays screws with the syntax for declaring multidimensional arrays 
pretty thoroughly.

- Jonathan M Davis
November 19, 2012
Re: declaration/initialization of multidimensional arrays
Tyro[17]:

> Why not simply allow new double[N][3][M]; ?
> Seems a lot more intuitive/succinct than a separate []() 
> implementation.

I think with that there is some ambiguity regarding what 
coordinates are fixed-sized arrays and what dynamic arrays.

Bye,
bearophile
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