DataFrames reorder columns in place

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1

If a dataframe is defined as:
df_dummy = DataFrame(A=[1,2,3],B=["A","B","C"],ID =[121,321,421])
How can I change the order of one or more columns and modify the dataframe in place? If I want to make ID the first column and B the last column.

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2 
julia> df_dummy = DataFrame(A=[1,2,3],B=["A","B","C"],ID =[121,321,421])
3×3 DataFrame
 Row │ A      B       ID
     │ Int64  String  Int64
─────┼──────────────────────
   1 │     1  A         121
   2 │     2  B         321
   3 │     3  C         421

julia> select!(df_dummy, :ID, Not([:ID, :B]), :B)
3×3 DataFrame
 Row │ ID     A      B
     │ Int64  Int64  String
─────┼──────────────────────
   1 │   121      1  A
   2 │   321      2  B
   3 │   421      3  C

Note that select!(df_dummy, :ID, Not(:B), :B) is a bit shorter and would also work because the selection is evaluated left to right. But I think explicitly excluding :ID is more clear in conveying the intention.

Similarly if you knew you have exactly 3 columns you can just write select!(df_dummy, :ID, :A, :B). The expression I have given above is general and would allow any number of columns in the middle.

12 Likes
4

Thank you. I use select a lot but never realised it can be used this way

5

We try to minimize the number of functions that users need to learn as they are already quite complex :smiley:.

3 Likes
6

I don’t know if it is exactly the columns permutation you are looking for, but it could be an equivalent way of achieving the same result

select!(df_dummy,circshift(names(df_dummy),1))
#or 
df_dummy=df_dummy[:,circshift(names(df_dummy),1)]