A slice consisting of non-consecutive elements of an array?

[vit]

On Wednesday, 12 January 2022 at 06:58:47 UTC, vit wrote:
> On Wednesday, 12 January 2022 at 06:43:40 UTC, forkit wrote:
>> On Wednesday, 12 January 2022 at 06:16:49 UTC, vit wrote:
>>>
>>> Yes std.algorithm : filter.
>>>
>>> ```d
>>> import std.stdio : writeln;
>>> import std.algorithm : filter;
>>>
>>>     void main()@safe{
>>>         auto a = ["one", "one", "two", "one", "two", "one", 
>>> "one", "two"];
>>>
>>>     	writeln(a);
>>>     	writeln(a.filter!(x => x == "one"));
>>>     }
>>>     ```
>>
>> Interesting. I looked it up.. it says "returns a new range..."
>>
>> Does that mean what I think it means (i.e. a new allocation 
>> takes place) ?
>
> No, it is only view to old slice:
> ```d
> void main()@safe{
>     auto a = ["one", "one", "two", "one", "two", "one", "one", 
> "two"];
>
>     auto b = (()@nogc => a.filter!(x => x == "one"))();
>     writeln(a);
>     writeln(b);
> }
> ```

filter is implemented like this (but more generic):
```d
import std.stdio : writeln;


auto filter(alias fn, T)(T[] slice){

     //InputRange:
     static struct Filter{
     	T[] slice;
         bool empty()const{return slice.length == 0;}
         T front(){return slice[0];}
         void popFront(){
             do{
             	slice = slice[1 .. $];
             }while(slice.length && !fn(slice[0]));
         }
     }

     return Filter(slice);
}

void main()@safe{
     auto a = ["one", "one", "two", "one", "two", "one", "one", 
"two"];

     writeln(a);
     writeln(a.filter!(x => x == "one"));
}
```