What's the syntax for a new empty dynamic array or associative array?

Every time I want to set a AA, I have to say:
(supposing I already have some variable int[int] aa which points to the wrong one)

int[int] throwaway;
aa = throwaway;

Is  there a way to say something like:

aa = new int[int] ?

like I would with a class.

What about for a normal dynamic array?

You can just set it to null. Then, next time you add anything to it, a new one will be automatically created.

On Sunday, 6 April 2014 at 03:23:17 UTC, Adam D. Ruppe wrote:
> You can just set it to null. Then, next time you add anything to it, a new one will be automatically created.

What about if I have a 2D array (ie int[][]) and I want to append an empty int[] on the end?  Will this work?

int[][] nested;

nested ~= null;

~ is overloaded, so how does it know what type I intend null to be?  How can I specify it?

On Sunday, 6 April 2014 at 03:28:50 UTC, dnspies wrote:
> On Sunday, 6 April 2014 at 03:23:17 UTC, Adam D. Ruppe wrote:
>> You can just set it to null. Then, next time you add anything to it, a new one will be automatically created.
>
> What about if I have a 2D array (ie int[][]) and I want to append an empty int[] on the end?  Will this work?
>
> int[][] nested;
>
> nested ~= null;
>
> ~ is overloaded, so how does it know what type I intend null to be?  How can I specify it?


For dynamic arrays you can use the new operator.

nested ~= new int[];

Or in this case, you could just increment the length.

++nested.length;

dnspies:

> What about if I have a 2D array (ie int[][]) and I want to append an empty int[] on the end?  Will this work?
>
> int[][] nested;
>
> nested ~= null;
>
> ~ is overloaded, so how does it know what type I intend null to be?  How can I specify it?

You can increase by one of the length of the outer array, or you can append an empty one:

nested.length++;

or:

nested ~= [];

I don't remember if you can also append a null. Appending a null, if it works, could be more efficient. Take a look at the ASM.

Bye,
bearophile

On Sunday, 6 April 2014 at 03:17:25 UTC, dnspies wrote:
> What's the syntax for a new empty dynamic array or associative array?
>
> Every time I want to set a AA, I have to say:
> (supposing I already have some variable int[int] aa which points to the wrong one)
>
> int[int] throwaway;
> aa = throwaway;
>
> What about for a normal dynamic array?

AA's and Dynamic Arrays are different beasts.

An Dynamic Array is merelly a "fat pointer" that holds both pointer and length. There is no need to create or new a Dynamic Array.

An AA, on the other hand, is completely different. It's a pointer to an implementation, and the implementation does the actual work.

The AA lazily initializes on the first operations. However, until initialized, an AA is pretty much just a null pointer. This can lead to interesting scenarios such as:
//----
    int[int] a; //null
    int[int] b = a; //both null
    assert(a is b); //They are both null
    a[1] = 1; //a gets initialized
    assert(a !is b); //but b remains null.
    int[int] c = a; //c points to the same implementation
    a[2] = 2; //a's implementation is changed
    assert(a is c); //and c "sees" it.
//----

> Is  there a way to say something like:
>
> aa = new int[int] ?
>
> like I would with a class.

Currently, no. However, you can force initialization with a dummy insertion:
int[int] b = [1:1]; b.remove(1);
//b is now an empty, but initialized, AA

It's a bit dirty, but that's how it is.

On Sunday, 6 April 2014 at 09:41:10 UTC, bearophile wrote:
> dnspies:
>
>> What about if I have a 2D array (ie int[][]) and I want to append an empty int[] on the end?  Will this work?
>>
>> int[][] nested;
[...]
>
> You can increase by one of the length of the outer array, or you can append an empty one:
[...]
> nested ~= [];

That doesn't add an element. [] is interpreted to be an empty
int[][]. You need to write [[]] which is an int[][] holding one
empty int[].

anonymous:

>> You can increase by one of the length of the outer array, or you can append an empty one:
> [...]
>> nested ~= [];
>
> That doesn't add an element. [] is interpreted to be an empty
> int[][]. You need to write [[]] which is an int[][] holding one
> empty int[].

Thank you catching my mistake.
But adding a null increases the length by 1:

a ~= null;

This difference is another reason for me to desire the removal of "null" as array literal.

Bye,
bearophile

On 04/06/2014 05:28 AM, dnspies wrote:
>
> int[][] nested;
>
> nested ~= null;
>
> ~ is overloaded, so how does it know what type I intend null to be?  How
> can I specify it?

(int[]).init

On Sunday, 6 April 2014 at 09:52:04 UTC, monarch_dodra wrote:
> An Dynamic Array is merelly a "fat pointer" that holds both pointer and length. There is no need to create or new a Dynamic Array.

new allows for setting the length immediately, though.

     auto arr = new int[](99);
     // arr.length = 99;  // avoided this


Does doing it in two steps allocate twice?