# Weird behavior assigning values to initialized vector of matrices?

**URL:** <https://discourse.julialang.org/t/weird-behavior-assigning-values-to-initialized-vector-of-matrices/79128>\
**Category:** New to Julia\
**Tags:** array, arrays\
**Created:** [April 6, 2022, 10:07pm UTC](https://discourse.julialang.org/t/weird-behavior-assigning-values-to-initialized-vector-of-matrices/79128 "2022-04-06T22:07:57Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![poopsilon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/poopsilon/32/35351_2.png) [@poopsilon](https://discourse.julialang.org/u/poopsilon)\
**Post date:** [April 6, 2022, 10:07pm UTC](https://discourse.julialang.org/t/weird-behavior-assigning-values-to-initialized-vector-of-matrices/79128/1 "2022-04-06T22:07:57Z")

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I came across a behavior of arrays of arrays that I find unintuitive. I suspect I might be missing something basic here.

I think the following illustrates the behavior. Suppose I make a vector of matrices

```julia
julia> V = Vector{Matrix{Float64}}(undef, 2)
2-element Vector{Matrix{Float64}}:
 #undef
 #undef

```

and I would like to initialize each component of this vector to have size 2\times 2, say

```julia
julia> fill!(V, Matrix{Float64}(undef, 2, 2));

```

Then, I’d like to set the upper left element of the matrix in `V[1]` to be `42.0`:

```julia
julia> V[1][1,1] = 42.0;

```

I find that it works, but it also sets the upper left element of the matrix in `V[2]` to `42.0`

```julia
julia> V[1]
2×2 Matrix{Float64}:
 42.0 1.36587e-314
  1.36587e-314 5.84085e-315

julia> V[2]
2×2 Matrix{Float64}:
 42.0 1.36587e-314
  1.36587e-314 5.84085e-315

```

Is there a way to assign values to the elements of these matrices independently? Is there an explanation for this behavior?

---

<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [April 6, 2022, 10:19pm UTC](https://discourse.julialang.org/t/weird-behavior-assigning-values-to-initialized-vector-of-matrices/79128/2 "2022-04-06T22:19:06Z")

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This is basically the same as the recent thread: [Current behavior of fill and the like..?](https://discourse.julialang.org/t/current-behavior-of-fill-and-the-like/78888)

The explanation of this behavior is already included in the documentation of `fill!`:

> help?\> fill!  
> search: fill! fill finally findall
> 
> fill!(A, x)
> 
> Fill array A with the value x. If x is an object reference, all elements will refer to the same object. fill!(A, Foo()) will return A filled with the result of evaluating Foo() once.

If you want to initialize something creating multiple distinct objects instead of using always the same, either use an array comprehension or `map(copy, ...)` as it is suggested in that thread. In your specific case, as the container already exist, maybe the best solution is `V .= copy.(value)`.