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February 19, 2018 How to make AA key a pointer | ||||
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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? |
February 19, 2018 Re: How to make AA key a pointer | ||||
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Posted in reply to Clinton | 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?
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February 19, 2018 Re: How to make AA key a pointer | ||||
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Posted in reply to Clinton | 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.
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February 19, 2018 Re: How to make AA key a pointer | ||||
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Posted in reply to Clinton | 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);
}
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February 20, 2018 Re: How to make AA key a pointer | ||||
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Posted in reply to ketmar | 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.
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