Phobos function to remove all occurances from dynamic array?

[Lance Bachmeier]

On Wednesday, 1 May 2024 at 15:18:03 UTC, Steven Schveighoffer 
wrote:
> On Wednesday, 1 May 2024 at 01:09:33 UTC, Liam McGillivray 
> wrote:
>> This is presumably such a common task that I'm surprised it 
>> isn't easy to find the answer by searching;
>>
>> Is there a standard library function that removes all elements 
>> from a dynamic array that matches an input argument?
>>
>> In `std.array` there's the `replace` function which is 
>> supposed to replace all occurrences that match an input with 
>> another. It seems to work as described on strings, but I get 
>> compiler errors when using it on other array types. I've tried 
>> using it to replace occurrences of a certain object in an 
>> array with `[]` in order to remove all occurrences, but it's 
>> not allowed.
>>
>> Is there a Phobos function that does what I want? It would be 
>> crazy if there isn't.
>
> `remove`
>
> https://dlang.org/phobos/std_algorithm_mutation.html#remove
>
> ```d
> arr = arr.remove!(v => shouldBeRemoved(v));
> ```
>
> Why the reassignment? Because `remove` removes elements *in 
> place*, and does not change the range extents. It returns the 
> portion of the range that contains the unremoved elements.
>
> So to give an example:
>
> ```d
> auto arr = [1, 2, 3, 4, 5];
> auto result = arr.remove!(i => i % 2 == 1); // remove odd 
> elements
> assert(result == [2, 4]);
>
> // first 2 are the slice that is stored in result
> // the last three are leftovers.
> assert(arr == [2, 4, 3, 4, 5]);
> ```
>
> -Steve

In case anyone comes upon this in a search, I wanted to point out 
a couple dangers of using remove. The first is that it mutates 
arr, as shown in Steve's example. The second is

```
result[0] = 4;
assert(result == [4, 4]);
assert(arr == [2, 4, 3, 4, 5]); // Fails
arr[0] = 2;
assert(result == [4, 4]); // Fails
```

Any future changes you make to result or arr change the other. 
You can use remove to avoid the allocation of a new array, but 
you better be sure you never read or modify the original array 
again. If you use filter

```
auto result = arr.filter!(i => i % 2 == 0).array;
```

arr is unchanged and you can use arr and result as you want.