How to make a list of data frames in Julia? - dataframe

Question

Given that I have some data frames with a single dimension, how can I create a list of all the data frames? Is it really as simple as just making a list and adding them in?

Answer 1

You could also use vcat to combine these data frames into a single one with an extra column indicating the source like this:
julia> c = vcat(a, b, source=:source => ["a", "b"])
8×2 DataFrame
Row │ A source
│ Int64 String
─────┼───────────────
1 │ 1 a
2 │ 2 a
3 │ 3 a
4 │ 4 a
5 │ 1 b
6 │ 2 b
7 │ 3 b
8 │ 4 b
This form is often easier to work with later. In particular if you then groupby the c data frame by :source like this:
julia> groupby(c, :source)
GroupedDataFrame with 2 groups based on key: source
First Group (4 rows): source = "a"
Row │ A source
│ Int64 String
─────┼───────────────
1 │ 1 a
2 │ 2 a
3 │ 3 a
4 │ 4 a
⋮
Last Group (4 rows): source = "b"
Row │ A source
│ Int64 String
─────┼───────────────
1 │ 1 b
2 │ 2 b
3 │ 3 b
4 │ 4 b
As a result you also get a collection of data frames (like the list that was created in the other answer), but this time you can apply functions supporting the split-apply-combine to it, see https://dataframes.juliadata.org/stable/man/split_apply_combine/.

Answer 2

One possible option that appears to work is the straightforward, "just add them to the list" method mentioned above. This would look like:
julia> a = DataFrame(A = 1:4)
4×1 DataFrame
Row │ A
│ Int64
─────┼───────
1 │ 1
2 │ 2
3 │ 3
4 │ 4
julia> b = DataFrame(A = 1:4)
4×1 DataFrame
Row │ A
│ Int64
─────┼───────
1 │ 1
2 │ 2
3 │ 3
4 │ 4
julia> c = [a, b]
2-element Vector{DataFrame}:
4×1 DataFrame
Row │ A
│ Int64
─────┼───────
1 │ 1
2 │ 2
3 │ 3
4 │ 4
4×1 DataFrame
Row │ A
│ Int64
─────┼───────
1 │ 1
2 │ 2
3 │ 3
4 │ 4
julia> typeof(c)
Vector{DataFrame} (alias for Array{DataFrame, 1})