# Indexed variable names, or vector of vectors?

**URL:** <https://discourse.julialang.org/t/indexed-variable-names-or-vector-of-vectors/82240>\
**Category:** New to Julia\
**Tags:** performance, metaprogramming, array, vector\
**Created:** [June 4, 2022, 4:14pm UTC](https://discourse.julialang.org/t/indexed-variable-names-or-vector-of-vectors/82240 "2022-06-04T16:14:41Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![byrnoga](https://avatars.discourse-cdn.com/v4/letter/b/858c86/32.png) [@byrnoga](https://discourse.julialang.org/u/byrnoga)\
**Post date:** [June 4, 2022, 4:14pm UTC](https://discourse.julialang.org/t/indexed-variable-names-or-vector-of-vectors/82240/1 "2022-06-04T16:14:41Z")

</div>

I have to update values of a number of vectors, say s1 … s20. I need a way to access arbitrarily s\_n, the nth index of variable s.

```julia
s1=[1 2 3]
s2=[11 12 13 14 15]

# calculations lead to "n" and "val"
n=2
val=16

# I append "val" to the "n"th variable "s".
if n == 1
  append!(s1, val)
elseif n == 2
  append!(s2, val)
# elseif ... same for n=3, n=4 ...
end

```

I would like to reduce code duplication. I found on this forum : [How to change the name of a variable in a for loop](https://discourse.julialang.org/t/how-to-change-the-name-of-a-variable-in-a-for-loop/28510) So I rewrote as:

```julia
@eval append!($(Symbol(:s,n)),value)

```

It works, but it is 10x slower (I have a loop over millions of data points).

I was previously working with scilab. The same program as above can be done in scilab by defining variable s as a list, in which the elements of any type can be indexed.

```julia
// Scilab 6.1.1 console
s=list()
s(1)=[1 2 3]
s(2)=[11 12 13 14 15]
// ... we can append to s by calling s($+1), for example in a loop

n=2
val=16

// we call the "n"th index of list "s" and append "val" in the end
s(n)($+1)=val

```

Is it possible to do something similar in julia? I mean defining a heterogeneous list or a vector of vectors where each of the elements can grow dynamically? (without compromising too much of execution performance)

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<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [June 4, 2022, 4:18pm UTC](https://discourse.julialang.org/t/indexed-variable-names-or-vector-of-vectors/82240/2 "2022-06-04T16:18:26Z")

</div>

> [@byrnoga](#):
>
> I mean defining a heterogeneous list or a vector of vectors where each of the elements can grow dynamically? (without compromising too much of execution performance)

yes

```julia
julia> s = [Int[] for _ = 1:20]; #vector of vector

julia> append!(s[3], 20);

julia> s[2]
Int64[]

julia> s[3]
1-element Vector{Int64}:
 20

```

or you can use a `Dict()` with `Int` as keys and `Vector{Int}` as values