julia create an empty dataframe and append rows to it

Question

I am trying out the Julia DataFrames module. I am interested in it so I can use it to plot simple simulations in Gadfly. I want to be able to iteratively add rows to the dataframe and I want to initialize it as empty.

The tutorials/documentation on how to do this is sparse (most documentation describes how to analyse imported data).

To append to a nonempty dataframe is straightforward:

df = DataFrame(A = [1, 2], B = [4, 5])
push!(df, [3 6])

This returns.

3x2 DataFrame
| Row | A | B |
|-----|---|---|
| 1   | 1 | 4 |
| 2   | 2 | 5 |
| 3   | 3 | 6 |

But for an empty init I get errors.

df = DataFrame(A = [], B = [])
push!(df, [3, 6])

Error message:

ArgumentError("Error adding 3 to column :A. Possible type mis-match.")
while loading In[220], in expression starting on line 2

What is the best way to initialize an empty Julia DataFrame such that you can iteratively add items to it later in a for loop?

question from:https://stackoverflow.com/questions/26201005/julia-create-an-empty-dataframe-and-append-rows-to-it

Answer 1

A zero length array defined using only [] will lack sufficient type information.

julia> typeof([])
Array{None,1}

So to avoid that problem is to simply indicate the type.

julia> typeof(Int64[])
Array{Int64,1}

And you can apply that to your DataFrame problem

julia> df = DataFrame(A = Int64[], B = Int64[])
0x2 DataFrame

julia> push!(df, [3  6])

julia> df
1x2 DataFrame
| Row | A | B |
|-----|---|---|
| 1   | 3 | 6 |