# Copying of arrays and array elements

**URL:** <https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897>\
**Category:** New to Julia\
**Created:** [September 18, 2019, 2:47pm UTC](https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897 "2019-09-18T14:47:30Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Ahmed\_Salih](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ahmed_salih/32/206579_2.png) [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)\
**Post date:** [September 18, 2019, 2:47pm UTC](https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897/1 "2019-09-18T14:47:30Z")

</div>

Hey guys!

I discovered this kind of behaviour in Julia. Say that I define a vector as as:

```julia
a = [1 2 3]

```

Then I define:

```julia
b = a[2]

```

And:

```julia
c = a

```

If I now change a by:

```julia
a[2] = 4
a
1×3 Array{Int64,2}:
 1 4 3

```

Then c becomes:

```julia
c
1×3 Array{Int64,2}:
 1 4 3

```

But b is still:

```julia
b = 2

```

So my question is, how do I turn of this behaviour / or control it? Could I copy a without later a change in a would become a change in c? And am I able to do it in a way so that b changes corresponding to the element change at index 2 of a, as defined initially (b = a[2])?

I hope my question makes sense. I am using Julia 1.2.0

Kind regards

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [September 18, 2019, 3:14pm UTC](https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897/2 "2019-09-18T15:14:24Z")

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> [@Ahmed\_Salih](#):
>
> Could I copy a without later a change in a would become a change in c?

Yes. As you said yourself, you can `copy(a)` and then changing `a` won’t change in the copied `a` that you assign to `c`.

> [@Ahmed\_Salih](#):
>
> And am I able to do it in a way so that b changes corresponding to the element change at index 2 of a, as defined initially (b = a[2])?

You cannot do `b = 2` and observe it in `a` or assign to `a[2] = 3` while making `b == 2`.  
You can create a view of the array as `b = @view a[2]`. Note that `b` will **NOT** be a number anymore.

---

<div class="post-metadata">

**Author:** ![Ahmed\_Salih](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ahmed_salih/32/206579_2.png) [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)\
**Post date:** [September 18, 2019, 4:02pm UTC](https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897/3 "2019-09-18T16:02:42Z")

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Thanks for clarifying! I have one follow up question. When I use `@view` you say that it is not a number anymore. Could you elaborate on that a bit? I am still able to say `b+b` and it will output a number.

Kind regards

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [September 18, 2019, 4:21pm UTC](https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897/4 "2019-09-18T16:21:05Z")

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Well,

```julia
julia> a = [1, 2, 3]
3-element Array{Int64,1}:
 1
 2
 3

julia> b = @view a[1]
0-dimensional view(::Array{Int64,1}, 1) with eltype Int64:
1

julia> b + 1
ERROR: MethodError: no method matching +(::SubArray{Int64,0,Array{Int64,1},Tuple{Int64},true}, ::Int64)
Closest candidates are:
  +(::Any, ::Any, ::Any, ::Any...) at operators.jl:529
  +(::Complex{Bool}, ::Real) at complex.jl:297
  +(::Missing, ::Number) at missing.jl:115
  ...
Stacktrace:
 [1] top-level scope at REPL[3]:1

julia> b + b
0-dimensional Array{Int64,0}:
2

julia> isa(b, Number)
false

julia> isa(b, AbstractArray)
true

```

It’s not a `Number`, it’s an (abstract) array.

Behavioral wise, just because you can add it to itself doesn’t means it’s a number and indeed the result of that isn’t a number (this might have produced a number on 1.0/1.1 and was changed/fixed on master) In either case, adding a number to it doesn’t work.

---

<div class="post-metadata">

**Author:** ![Ahmed\_Salih](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ahmed_salih/32/206579_2.png) [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)\
**Post date:** [September 18, 2019, 4:34pm UTC](https://discourse.julialang.org/t/copying-of-arrays-and-array-elements/28897/5 "2019-09-18T16:34:54Z")

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Great, thanks once again.

Kind regards