Hi all, I need advice from better developers on this concern.

I'm using an AA to reference another array for quicker access:

[code]
alias contactId = string;
bool[contactId][] matches;
ulong[contactId] idsToMatches;

bool[string] matchesForId(string id) {
  return matches.get(idsToMatches[id], bool[string].init);
}
[/code]

Just wondering, how do I set the keys to avoid copying the id string? So, let's say ids come from another array of structs(e.g. Contact[]). I want to avoid having two copies of the string id value in both of these AAs above (to avoid using extra memory usage).

The reason this is a potential issue is because these arrays can get extremely large. This is to match duplicate contacts and group them in matches. The reason I use bool is because it's the smallest size type and I don't think I can use void.

I guess my question is: does dmd already create pointers to the id from the AA, or is each new key a new allocation?

On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
> Hi all, I need advice from better developers on this concern.
>
> I'm using an AA to reference another array for quicker access:
>
> [...]

Sorry, on second look my explanation isn't very clear. I want to know if:

bool[string] myAA;

myAA[contact.id] = true; // Does this copy contact.id or is this a pointer to contact.id?

On Monday, 19 February 2018 at 14:57:47 UTC, Clinton wrote:
> On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
>> Hi all, I need advice from better developers on this concern.
>>
>> I'm using an AA to reference another array for quicker access:
>>
>> [...]
>
> Sorry, on second look my explanation isn't very clear. I want to know if:
>
> bool[string] myAA;
>
> myAA[contact.id] = true; // Does this copy contact.id or is this a pointer to contact.id?

It's a pointer. In D, string is an alias to immutable(char)[] (Slice of immutable characters). A slice is a combination of pointer and length.

Clinton wrote:

> On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
>> Hi all, I need advice from better developers on this concern.
>>
>> I'm using an AA to reference another array for quicker access:
>>
>> [...]
>
> Sorry, on second look my explanation isn't very clear. I want to know if:
>
> bool[string] myAA;
>
> myAA[contact.id] = true; // Does this copy contact.id or is this a pointer to contact.id?

there is absolutely no reason to copy `string` ever, as it is `immutable`. and compiler knows that. anyway, why don't you just check it by writing the code first?

	import std.stdio;
	void main () {
		int[string] a;
		string s = "test";
		writefln("%08x", s.ptr);
		a[s] = 666;
		s = "test1";
		writefln("%08x", s.ptr);
		a[s] = 42;
		foreach (string k; a.byKey) writefln("%08x", k.ptr);
	}

On Monday, 19 February 2018 at 15:02:29 UTC, ketmar wrote:
> Clinton wrote:
>
>> On Monday, 19 February 2018 at 14:55:01 UTC, Clinton wrote:
>>> [...]
>>
>> Sorry, on second look my explanation isn't very clear. I want to know if:
>>
>> bool[string] myAA;
>>
>> myAA[contact.id] = true; // Does this copy contact.id or is this a pointer to contact.id?
>
> there is absolutely no reason to copy `string` ever, as it is `immutable`. and compiler knows that. anyway, why don't you just check it by writing the code first?
>
> 	import std.stdio;
> 	void main () {
> 		int[string] a;
> 		string s = "test";
> 		writefln("%08x", s.ptr);
> 		a[s] = 666;
> 		s = "test1";
> 		writefln("%08x", s.ptr);
> 		a[s] = 42;
> 		foreach (string k; a.byKey) writefln("%08x", k.ptr);
> 	}

Thanks. I actually did a similar test a little while ago and found it out. Thanks for confirming. I still struggle a bit with these basic things.