# Copy. vs. deepcopy vs. .=
**URL:** https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097
**Category:** General Usage
**Tags:** question, deepcopy
**Created:** [February 23, 2023, 5:40pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097 "2023-02-23T17:40:21Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@prittjam](https://discourse.julialang.org/u/prittjam)
#### Post date: [February 23, 2023, 5:40pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/1 "2023-02-23T17:40:21Z")
Despite many threads on this, I’m still confused. I want to avoid allocation but something is not working in my code. My setup is like the one below. Can someone explain the difference in this context,
```julia
A = (rand(3),rand(3),rand(3))
B = (rand(3),rand(3),rand(3))
```
option 1 works but I want to avoid it because it allocates:
```julia
B = deepcopy(A)
```
option 2 does not allocate but does not work:
```julia
copy!.(B,A)
```
neither does option 3:
```julia
map((b,a)-> b .= a, B, A)
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [February 23, 2023, 5:44pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/2 "2023-02-23T17:44:57Z")
you have a collection of collections, I believe option 3 works:
```julia
julia> @ballocated map((b,a)-> b .= a, B, A) setup = begin A = (rand(3),rand(3),rand(3)); B = (rand(3),rand(3),rand(3))end
0
```
but instead of map, you’re probably looking for `foreach`
```julia
julia> foreach(A,B) do a,b
b .= a
end
```
---
### Author:  [@prittjam](https://discourse.julialang.org/u/prittjam)
#### Post date: [February 23, 2023, 5:48pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/3 "2023-02-23T17:48:59Z")
option 3 doesn’t, if I make a change to A later, then it also changes B. I don’t know why options 2 or 3 don’t work though. I missing something fundamental. I understand that it is a collection of collections, but I thought the map or broadcast would unwrap the outer collection and let me do an in-place copy on the arrays. It doesn’t seem so, and I am confused.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [February 23, 2023, 5:50pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/4 "2023-02-23T17:50:44Z")
> [@prittjam](#):
>
> if I make a change to A later, then it also changes B
it does not:
```julia
julia> A = (rand(3),rand(3),rand(3))
([0.44064579753939914, 0.5513894805471519, 0.14531256392609548], [0.7632771815487763, 0.01759682471452173, 0.5437928857351825], [0.4404378190057092, 0.005535005026375717, 0.09501474232898721])
julia> B = (rand(3),rand(3),rand(3));
julia> B[1][1]
0.67551842978281
julia> map((b,a)-> b .= a, B, A);
julia> B[1][1]
0.44064579753939914
julia> A[1][1] = 0.0
0.0
julia> B[1][1]
0.44064579753939914
```
---
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [February 23, 2023, 6:05pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/5 "2023-02-23T18:05:26Z")
> [@prittjam](#):
>
> `copy!.(B,A)`
I think this does “work”, as you expect:
```julia
julia> A = (zeros(3),zeros(3),zeros(3))
([0.0, 0.0, 0.0], [0.0, 0.0, 0.0], [0.0, 0.0, 0.0])
julia> B = (ones(3),ones(3),ones(3))
([1.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
julia> copy!.(A,B)
([1.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
julia> A
([1.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
julia> B
([1.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
julia> A[1][1] = 5.0
5.0
julia> A
([5.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
julia> B
([1.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
julia> @btime copy!.($A,$B)
13.516 ns (0 allocations: 0 bytes)
([1.0, 1.0, 1.0], [1.0, 1.0, 1.0], [1.0, 1.0, 1.0])
```
---
### Author:  [@prittjam](https://discourse.julialang.org/u/prittjam)
#### Post date: [February 23, 2023, 6:11pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/6 "2023-02-23T18:11:22Z")
I believe there is a reassignment in another part of the code that is causing the problem. Thanks.
---
### Author:  [@prittjam](https://discourse.julialang.org/u/prittjam)
#### Post date: [February 23, 2023, 9:56pm UTC](https://discourse.julialang.org/t/copy-vs-deepcopy-vs/95097/7 "2023-02-23T21:56:59Z")
Yes, I’ve confirmed this was the problem. Thanks for the feedback.