# How to delete element when iterating a vec? **URL:** \ **Category:** uncategorized\ **Created:** [October 13, 2021, 12:03pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862 "2021-10-13T12:03:05Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![harlanc](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/harlanc/32/20575_2.png) [@harlanc](https://users.rust-lang.org/u/harlanc)\ **Post date:** [October 13, 2021, 12:03pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/1 "2021-10-13T12:03:05Z")
For example the following codes will throw exception:index out of bounds: the len is 3 but the index is 3 ``` let mut vecc = vec![1, 2, 3, 4]; for i in 0..vecc.len() { if vecc[i] == 1 { vecc.remove(i); } } ``` ---
**Author:** ![jkugelman](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/jkugelman/32/15839_2.png) [@jkugelman](https://users.rust-lang.org/u/jkugelman)\ **Post date:** [October 13, 2021, 12:12pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/2 "2021-10-13T12:12:54Z")
Mutating while iterating is a tough sell in many languages, Rust among them. It's doable, but I would try to find a way not to. There's often a method you can call to avoid an explicit loop, such as [`retain`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.retain) for your example: ```rust vecc.retain(|&i| i != 1); ``` [Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7f8c1dfc44f3fed0f9c012507371ce3f) ---
**Author:** ![harlanc](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/harlanc/32/20575_2.png) [@harlanc](https://users.rust-lang.org/u/harlanc)\ **Post date:** [October 13, 2021, 12:17pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/3 "2021-10-13T12:17:08Z")
The following can also work: ``` let mut vecc = vec![1, 2, 3, 4]; let mut idx = 0 as usize; while idx < vecc.len() { if vecc[idx] == 1 { vecc.remove(idx); continue; } idx = idx + 1; } ``` ---
**Author:** ![harlanc](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/harlanc/32/20575_2.png) [@harlanc](https://users.rust-lang.org/u/harlanc)\ **Post date:** [October 13, 2021, 12:17pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/4 "2021-10-13T12:17:59Z")
It is great! ---
**Author:** ![steffahn](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/steffahn/32/47569_2.png) [@steffahn](https://users.rust-lang.org/u/steffahn)\ **Post date:** [October 13, 2021, 12:43pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/5 "2021-10-13T12:43:24Z")
Note that calling `remove` in a loop is also more inefficient. `Vec::remove` takes linear1 time (it has to _shift_ all the elements to the right of the removed element one to the left in memory); thus calling `.remove` in a loop can take quadratic1 time; whereas `.retain` only takes linear1, 2 time for the whole process (it can use an implementation that – as an intermediate state – leaves some “slots” of memory empty during the iteration, with the effect of only having to move each item of the vec at most once). 1 linear/quadratic with respect to the length of the vector 2 assuming that the closure given to `.retain` only needs constant time ---
**Author:** ![2e71828](https://avatars.discourse-cdn.com/v4/letter/2/3e96dc/32.png) [@2e71828](https://users.rust-lang.org/u/2e71828)\ **Post date:** [October 13, 2021, 3:36pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/6 "2021-10-13T15:36:22Z")
> [@steffahn](#): > > (it can use an implementation that – as an intermediate state – leaves some “slots” of memory empty during the iteration, with the effect of only having to move each item of the vec at most once). To translate this into code, `Vec::retain()` works similarly¹ to this: ```rust let mut vecc = vec![1, 2, 3, 4]; let mut idx_wr = 0usize; for idx_rd in 0..vecc.len() { if ! (vecc[idx_rd] == 1) { vecc.swap(idx_wr, idx_rd); idx_wr += 1; } } vecc.truncate(idx_wr); ``` ¹ I haven't looked at the actual code for `retain`; it probably uses `unsafe` to perform a move instead of the `swap` call I used. ---
**Author:** ![steffahn](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/steffahn/32/47569_2.png) [@steffahn](https://users.rust-lang.org/u/steffahn)\ **Post date:** [October 13, 2021, 3:38pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/7 "2021-10-13T15:38:35Z")
> [@2e71828](#): > > ¹ I haven't looked at the actual code for `retain` ; it probably uses `unsafe` to perform a move instead of the `swap` call I used. yup, that’s exactly what it does. ---
**Author:** ![system](https://sea1.discourse-cdn.com/flex019/user_avatar/users.rust-lang.org/system/32/51372_2.png) [@system](https://users.rust-lang.org/u/system)\ **Post date:** [January 11, 2022, 3:38pm UTC](https://users.rust-lang.org/t/how-to-delete-element-when-iterating-a-vec/65862/8 "2022-01-11T15:38:38Z")
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