Allow end to select last column.

[nathanrboyer]

I was trying to hcat the last column of a data frame onto another data frame and was surprised that the end keyword does not work for selecting data frame columns. df[:, end] selected the correct data, but it drops the column name. It would be convenient if end worked as an alias for the last integer index like it does for array indexing. (The ncol(df) workaround is easy enough if this is difficult to implement though.)

julia> df = DataFrame(rand(3,3), :auto)
3×3 DataFrame
 Row │ x1        x2        x3       
     │ Float64   Float64   Float64
─────┼──────────────────────────────
   1 │ 0.993444  0.609605  0.32879
   2 │ 0.195981  0.417102  0.843341
   3 │ 0.441776  0.205019  0.975706

julia> select(df, 1)
3×1 DataFrame
 Row │ x1       
     │ Float64
─────┼──────────
   1 │ 0.993444
   2 │ 0.195981
   3 │ 0.441776

julia> select(df, end)
ERROR: syntax: unexpected "end"
Stacktrace:
 [1] top-level scope
   @ none:1

julia> select(df, Cols([end]))
ERROR: syntax: unexpected "end"
Stacktrace:
 [1] top-level scope
   @ none:1

julia> select(df, ncol(df))
3×1 DataFrame
 Row │ x3       
     │ Float64
─────┼──────────
   1 │ 0.32879
   2 │ 0.843341
   3 │ 0.975706

[bkamins]

What you propose is syntactically not possible (this is disallowed by Julia). What you can do is df[:, [end]] instead.