[SOLVED] Extract value from DataFrame as Float64 value not as Vector

Extract value from DataFrame as Float64 value not as Vector

The below is the sample dataframe and I want to extract the value bw of Rat as Float64 not as Vector{Float64}.

df = DataFrame(id=["Mouse","Rat"],
               time=[1,1],
               bw=[25.0,100.45])

I get a Vector{Float64} when I use the below code.

df[in.(df.id, Ref(["Rat"])), :bw]

Can yo please tell me how can I get only the value of bw=100.45 from the dataframe. I don't want to use the below code, instead I want to reference id=="Rat" and take it that way to be sure I am getting the correct value from a larger dataset.

df[2, :bw]

Thanks again...!

Solution

You can do:

julia> only(df[in.(df.id, Ref(["Rat"])), :bw])
100.45

The reason why you are getting a vector back is that you are indexing with a vector. This is consistent with base Julia:

julia> x = ["a", "b"]
2-element Vector{String}:
 "a"
 "b"

julia> x[[true, false]]
1-element Vector{String}:
 "a"

So another option is to index with a scalar:

julia> df[findfirst(in.(df.id, Ref(["Rat"]))), :bw]
100.45

here the findfirst returns the index of the first true element of your indexing vector, which is a scalar.