# Efficient popnfirst!?
**URL:** https://discourse.julialang.org/t/efficient-popnfirst/57190
**Category:** Performance
**Created:** [March 15, 2021, 11:59am UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190 "2021-03-15T11:59:16Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)
#### Post date: [March 15, 2021, 11:59am UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/1 "2021-03-15T11:59:16Z")
Hi,
I was wondering if it exists an efficient implementation of `popfirst!` for an array that allows a `n` argument to request multiple pops at once (instead of calling `popfirst!` n times).
Like `popfirst!(a::Array, n::Int = 1)`
If not, is there a reason that this can’t be done efficiently ?
---
### Author:  [@oxinabox](https://discourse.julialang.org/u/oxinabox)
#### Post date: [March 15, 2021, 12:05pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/2 "2021-03-15T12:05:27Z")
What problem do you have with: `[popfirst!(xs) for _ in 1:n]` ?
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### Author:  [@mike](https://discourse.julialang.org/u/mike)
#### Post date: [March 15, 2021, 12:06pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/3 "2021-03-15T12:06:51Z")
`deleteat!(a, 1:n)` _might_ be efficient, I’ve not looked at the implementation, but I’d assume it’s slightly better.
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### Author:  [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)
#### Post date: [March 15, 2021, 12:07pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/4 "2021-03-15T12:07:11Z")
It’s just that I don’t know if it’s efficient to do multiple calls like that instead of removing one by one.
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### Author:  [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)
#### Post date: [March 15, 2021, 12:08pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/5 "2021-03-15T12:08:32Z")
Okay thanks, it will do the trick for what I need. I didn’t know deleteat accepts ranges.
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### Author:  [@oxinabox](https://discourse.julialang.org/u/oxinabox)
#### Post date: [March 15, 2021, 12:10pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/6 "2021-03-15T12:10:16Z")
> It’s just that I don’t know if it’s efficient to do multiple calls like that instead of removing one by one.
It is efficient.
It will be a static dispatch.
and julia’s `Vector` can efficiently grow and shrink at both ends.
Edit:
actually `deleteat!` is much faster. Not sure why
```julia
julia> @btime [popfirst!(x) for _ in 1:1000] setup=(x=rand(1_000_000));
6.954 μs (1 allocation: 7.94 KiB)
julia> @btime deleteat!(x, 1:1000) setup=(x=rand(1_000_000));
136.274 ns (0 allocations: 0 bytes)
```
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### Author:  [@mike](https://discourse.julialang.org/u/mike)
#### Post date: [March 15, 2021, 12:14pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/7 "2021-03-15T12:14:06Z")
Probably the lack of allocation since it’s not creating the comprehension?
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### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [March 15, 2021, 12:14pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/8 "2021-03-15T12:14:52Z")
> [@oxinabox](#):
>
> It is efficient.
Might be better to use `foreach` since otherwise, you create a redundant vector.
But for this specific case, I would think `deleteat!` be significantly faster since it should be O(1) instead of O(n).
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### Author:  [@oxinabox](https://discourse.julialang.org/u/oxinabox)
#### Post date: [March 15, 2021, 12:16pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/9 "2021-03-15T12:16:05Z")
Oh yeah, it doesn’t return the deleted elements.
It returns the remaining elements.
I assume though if you are using `popfirst` you want the deleted elements.
(also why can’t use `foreach`).
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### Author:  [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)
#### Post date: [March 15, 2021, 12:18pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/10 "2021-03-15T12:18:32Z")
Well, not in my case but it’s true that it could be.
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### Author:  [@mike](https://discourse.julialang.org/u/mike)
#### Post date: [March 15, 2021, 12:20pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/11 "2021-03-15T12:20:39Z")
Yeah, true, probably saving `x[1:n]` prior to `deleteat!` would be a better option then, maybe:
```julia
julia> function popnfirst!(xs, n)
out = xs[1:n]
deleteat!(xs, 1:n)
return out
end
```
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### Author:  [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)
#### Post date: [March 15, 2021, 12:24pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/12 "2021-03-15T12:24:07Z")
Do you think it would be worth doing a PR in Base.array.jl ?
It could even be a new method for popfirst!() to avoid a new name.
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### Author:  [@mike](https://discourse.julialang.org/u/mike)
#### Post date: [March 15, 2021, 12:29pm UTC](https://discourse.julialang.org/t/efficient-popnfirst/57190/13 "2021-03-15T12:29:38Z")
`splice!` may be what you want actually…
```julia
julia> splice!([1, 2, 3, 4, 5], 1:3)
3-element Vector{Int64}:
1
2
3
```