Non-systematic "New columns must have the same length as old columns" error

[nalimilan]

Creating a column with only one value is only an error if it's the first column:

julia> df = DataFrame(A=1:3)
3x1 DataFrames.DataFrame
│ Row │ A │
┝━━━━━┿━━━┥
│ 1   │ 1 │
│ 2   │ 2 │
│ 3   │ 3 │

julia> @select(df, B=1, :A)
ERROR: New columns must have the same length as old columns
 in insert_single_column!(::DataFrames.DataFrame, ::DataArrays.DataArray{Int64,1}, ::Symbol) at /home/milan/.julia/DataFrames/src/dataframe/dataframe.jl:309
 in setindex!(::DataFrames.DataFrame, ::DataArrays.DataArray{Int64,1}, ::Symbol) at /home/milan/.julia/DataFrames/src/dataframe/dataframe.jl:368
 in #select#17(::Array{Any,1}, ::Any, ::DataFrames.DataFrame) at /home/milan/.julia/DataFramesMeta/src/DataFramesMeta.jl:450
 [inlined code] from ./boot.jl:307
 in (::###8232#20{DataFrames.DataFrame})(::DataArrays.DataArray{Int64,1}) at /home/milan/.julia/DataFramesMeta/src/DataFramesMeta.jl:55
 in eval(::Module, ::Any) at ./boot.jl:243

julia> @select(df, :A, B=1)
3x2 DataFrames.DataFrame
│ Row │ A │ B │
┝━━━━━┿━━━┿━━━┥
│ 1   │ 1 │ 1 │
│ 2   │ 2 │ 1 │
│ 3   │ 3 │ 1 │

Would it make sense to always recycle the columns so that the new data frame has the same number of columns as the original? Other constructs could be recommended for aggregation operations.

[tshort]

I agree that this should recycle. The following PR in DataFrames was meant to address this issue:

JuliaData/DataFrames.jl#882

[tshort]

That PR was on hold because I decided we probably shouldn't recycle except for the case where the value to be put in the new column is a single value.