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.
