# Vector of vectors access to undefined reference

**URL:** <https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057>\
**Category:** General Usage\
**Created:** [August 9, 2018, 3:57am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057 "2018-08-09T03:57:04Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![microlifecc](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@microlifecc](https://discourse.julialang.org/u/microlifecc)\
**Post date:** [August 9, 2018, 3:57am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/1 "2018-08-09T03:57:04Z")

</div>

Hi Guys!!!  
I am trying to create a vector of vectors since I need the data structure to have a fixed number of rows(6) but variable number of columns. And in each of the units of the data structure I want to store an instance of a structure. My code is:

```julia
struct abc
    a::Float64
    b::Float64
    c
end

k = Vector{Vector{abc}}(6);

for i = 1:6
   for j = 1:6
        aa, bb, cc = i+1, j+1, rand(3);
        instance = abc(aa, bb, cc);
        k[i][j] = instance;
    end
end

```

I’m getting this error:

> UndefRefError: access to undefined reference  
> Stacktrace:  
> [1] getindex(::Array{Array{abc,1},1}, ::Int64) at ./array.jl:549  
> [2] macro expansion at ./In[7]:5 [inlined]  
> [3] anonymous at ./:?

Can you guys tell me what I am doing wrong? Thanks!

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<div class="post-metadata">

**Author:** ![laborg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laborg/32/5474_2.png) [@laborg](https://discourse.julialang.org/u/laborg)\
**Post date:** [August 9, 2018, 4:56am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/3 "2018-08-09T04:56:07Z")

</div>

Your k is created with 6 undefined values, so when you index into k[i][j] you won’t access a properly initialized array.

Just create the vector with `fill`, and then use the `push!` to append to the inner vector.

btw. semicolons aren’t needed in Julia

```julia
struct abc
    a::Float64
    b::Float64
    c
end

k = fill(Vector{abc}(),6)

for i = 1:6
   for j = 1:6
        aa, bb, cc = i+1, j+1, rand(3);
        instance = abc(aa, bb, cc);
        push!(k[i], instance);
    end
end

```

---

<div class="post-metadata">

**Author:** ![greg\_plowman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/greg_plowman/32/8100_2.png) [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)\
**Post date:** [August 9, 2018, 10:36pm UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/4 "2018-08-09T22:36:40Z")

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> [@laborg](#):
>
> k = fill(Vector{abc}(),6)

Be wary of `fill`.  
It will create a `Vector` with all 6 elements referring to the _same_ empty `Vector`.  
If you want independent vectors, you could create with a comprehension:

```julia
k = [Vector{abc}() for i = 1:6]

```

---

<div class="post-metadata">

**Author:** ![microlifecc](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@microlifecc](https://discourse.julialang.org/u/microlifecc)\
**Post date:** [August 10, 2018, 4:26am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/6 "2018-08-10T04:26:26Z")

</div>

Hi @greg_plowman, thanks a lot for the help!

That works!

I have just one doubt. Actually I need to have variable number of columns for each of the rows. I will be storing an instance of the structure abc in each cell of the 2D structure but in the actual problem I need to calculate the number of columns in each iteration of the outer loop(based on the value of i). I tried:

```julia
kk = [Vector{abc}() for i = 1:3]
for i = 1:3
    for j = 1:i+1
        aa, bb, cc = i+1, j+1, rand(3);
        instance = abc(aa, bb, cc);
        push!(kk[i], instance);
    end
end

```

I’m getting kk as:

> 3-element Array{Array{abc,1},1}:  
> abc[abc(2.0, 2.0, [0.750578, 0.158137, 0.664006]), abc(2.0, 3.0, [0.293539, 0.479623, 0.932276])]  
> abc[abc(3.0, 2.0, [0.701739, 0.411975, 0.719304]), abc(3.0, 3.0, [0.351443, 0.149604, 0.131821]), abc(3.0, 4.0, [0.780022, 0.281377, 0.924904])]  
> abc[abc(4.0, 2.0, [0.212862, 0.150822, 0.784564]), abc(4.0, 3.0, [0.880618, 0.752912, 0.634355]), abc(4.0, 4.0, [0.0175809, 0.388531, 0.882258]), abc(4.0, 5.0, [0.407847, 0.919263, 0.207803])]  
> which means each row is also of type abc. But I just need the cells of the datastructure to be of type abc not the whole row.  
> Can you please suggest what I should change? Thanks!

---

<div class="post-metadata">

**Author:** ![microlifecc](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@microlifecc](https://discourse.julialang.org/u/microlifecc)\
**Post date:** [August 10, 2018, 4:42am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/7 "2018-08-10T04:42:59Z")

</div>

I got it.

```julia
kk = [Vector{Any}() for i = 1:3]

```

Thanks anyways!

---

<div class="post-metadata">

**Author:** ![greg\_plowman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/greg_plowman/32/8100_2.png) [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)\
**Post date:** [August 10, 2018, 4:52am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/8 "2018-08-10T04:52:56Z")

</div>

> [@microlifecc](#):
>
> which means each row is also of type abc. But I just need the cells of the datastructure to be of type abc not the whole row.

No, that’s just the way Julia displays it.  
kk is a `3-element Array{Array{abc,1},1}`, which means each of the 3 elements are of type `Array{abc,1}`.  
You can check this by looking at an individual element, say `kk[1]`.

> [@microlifecc](#):
>
> kk = [Vector{Any}() for i = 1:3]

For performance, it’s generally better to avoid abstract types (e.g. `Any`) for containers if possible.

---

<div class="post-metadata">

**Author:** ![microlifecc](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@microlifecc](https://discourse.julialang.org/u/microlifecc)\
**Post date:** [August 10, 2018, 5:07am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/9 "2018-08-10T05:07:04Z")

</div>

@greg_plowman, Thanks!  
So

```julia
kk = [Vector{abc}() for i = 1:3]

```

would give me a vector(kk) with 3 elements each of type `Vector{abc}`? On running I’m getting:

```julia
abc[]
abc[]
abc[]

```

---

<div class="post-metadata">

**Author:** ![greg\_plowman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/greg_plowman/32/8100_2.png) [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)\
**Post date:** [August 10, 2018, 5:20am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/10 "2018-08-10T05:20:09Z")

</div>

That’s how Julia usually displays vectors:  
`Type[x, y, z]`  
An empty vector displays as `Type[]`

Try typing `abc[]` at the REPL.

To further convince yourself, try at the REPL:

```julia
typeof(kk)
eltype(kk)

typeof(kk[1])
eltype(kk[1])

```

---

<div class="post-metadata">

**Author:** ![microlifecc](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@microlifecc](https://discourse.julialang.org/u/microlifecc)\
**Post date:** [August 10, 2018, 5:27am UTC](https://discourse.julialang.org/t/vector-of-vectors-access-to-undefined-reference/13057/11 "2018-08-10T05:27:24Z")

</div>

Thanks!