# Bug in copy!(z,view(x,2,:)')?
**URL:** https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753
**Category:** General Usage
**Created:** [October 25, 2024, 3:59pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753 "2024-10-25T15:59:09Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@Paul\_Soderlind](https://discourse.julialang.org/u/Paul_Soderlind)
#### Post date: [October 25, 2024, 3:59pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/1 "2024-10-25T15:59:09Z")
Hi,
I am buffled by what `copy!(z,view(x,1,:)')` does. Seems off. Example:
```julia
x = [11 12;111 112] #to read from
y = [ones(1,2)*1,ones(1,2)*2] #2-vector of matrices, to be pushed to
z = zeros(1,2) #preallocate z
copy!(z,view(x,1,:)') #copy first row of x to z
display(z)
pushfirst!(y,z)
copy!(z,view(x,2,:)') #copy 2nd row
display(z)
pushfirst!(y,z)
display(y)
```
gives
```julia
4-element Vector{Matrix{Float64}}:
[111.0 112.0]
[111.0 112.0]
[1.0 1.0]
[2.0 2.0]
```
Replacing by `z = x[1,:]'` etc does the right thing. Am I messing things up?
---
### Author:  [@Benny](https://discourse.julialang.org/u/Benny)
#### Post date: [October 25, 2024, 4:09pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/2 "2024-10-25T16:09:46Z")
Looks like mutation semantics to me. You don’t reassign `z` in the given example, just mutate its preallocated instance. `y` gets the same instance assigned to `z` pushed to its beginning twice, in short `y[1] === y[2]`. If you want to see different values, you do need to allocate more.
---
### Author:  [@Paul\_Soderlind](https://discourse.julialang.org/u/Paul_Soderlind)
#### Post date: [October 25, 2024, 4:56pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/3 "2024-10-25T16:56:49Z")
Yes, the whole idea was to (keep) mutating an existing array, so as to avoid allocations in a long loop. Still, the `display(z)` (twice in the example) suggests that `z` is changed. It’s just at the stage of `pushfirst!()` that it somehow isn’t.
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [October 25, 2024, 5:13pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/4 "2024-10-25T17:13:06Z")
When you `pushfirst!(y, z)`, you’re putting _the matrix named `z`_ inside y. If you do it twice, `y` will contain `[#= the matrix named z =#, #= the matrix named z =#, ...]`. If you mutate the matrix named `z`, then that matrix changes. And since `y` has that matrix placed in _two locations_, you’ll see that change in both those locations. Because it’s just one matrix.
If you decide to do `z = x[1,:]'` in between, you’re deciding to repurpose the name `z` for a brand spanking new matrix — and that matrix will be completely independent from anything you might have called `z` in the past.
---
### Author:  [@eldee](https://discourse.julialang.org/u/eldee)
#### Post date: [October 25, 2024, 5:28pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/5 "2024-10-25T17:28:29Z")
To possibly further clarify what @Benny and in the mean time also @mbauman are saying, the variable `z` just points to some location in memory. When you mutate `z` using `copy!` (or some `.=`-like operation), `z` still points to the same location, but the values in memory have now changed.
```julia-repl
julia> z = ones(1, 2)
1×2 Matrix{Float64}:
1.0 1.0
julia> pointer(z) # Memory location where z points to
Ptr{Float64} @0x000002245d8f0900
julia> z .*= 2
1×2 Matrix{Float64}:
2.0 2.0
julia> pointer(z) # Same location (but different values in memory)
Ptr{Float64} @0x000002245d8f0900
julia> z = [2. 2.] # New variable coincidentally also called z, with the same values as before
1×2 Matrix{Float64}:
2.0 2.0
julia> pointer(z) # but new location
Ptr{Float64} @0x000002245da1c4a0
```
When you `pushfirst!` `z` into `y` the new first entry of `y` should be found at the memory location corresponding to `z`. Mutating `z` and using `pushfirst!` again, the first two entries both point to the same location in memory, so have the same values.
---
### Author:  [@Paul\_Soderlind](https://discourse.julialang.org/u/Paul_Soderlind)
#### Post date: [October 25, 2024, 6:23pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/6 "2024-10-25T18:23:22Z")
Thanks. I get it now. May I ask if there is a straightforward way around this? (The point is to avoid allocations.)
---
### Author:  [@Benny](https://discourse.julialang.org/u/Benny)
#### Post date: [October 25, 2024, 6:30pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/7 "2024-10-25T18:30:57Z")
If you intend to have 2 different mutable matrices, you’ll need to allocate 2 mutable matrices because it’s the memory-safe way for references to share data, as you inadvertently ran into. If you only need the two, you can preallocate those. If you have a loop with indeterminate iterations, then you have a situation where you need to allocate frequently. Is it possible for the matrices to be immutable? That is far more feasible to store directly in `y` instead of a separate allocation, though you’ll probably want to preallocate elements for `y` because resizing needs allocation.
---
### Author:  [@Paul\_Soderlind](https://discourse.julialang.org/u/Paul_Soderlind)
#### Post date: [October 25, 2024, 6:51pm UTC](https://discourse.julialang.org/t/bug-in-copy-z-view-x-2/121753/8 "2024-10-25T18:51:32Z")
Thanks, I’ll follow your idea and probably set up multidimensional array `y` and change elements in that. (I was probably trying to be too clever with my vector of matrices…)