Dataframes.jl: Improvement of rename!

[bkamins]

rename!(f::Function, df) is not really convenient as f normally has to convert column names to string, operate on a string and convert it back to Symbol.

Two ideas:

1. (more radical) maybe we should change rename! to assume that f takes a string a returns a string and Symbol unwrapping/wrapping happens automatically
2. (less radical) maybe we can add a kwarg to rename! that would allow the user to choose if f is from string to string or from Symbol to Symbol

[nalimilan]

At least we could allow returning a string in addition to a symbol. Not sure about converting the argument to string before passing it, as it would be inconsistent with the fact that we use symbols everywhere.

[bkamins]

Actually in this case the change could be quite small and non breaking - we can simply cast whatever is returned by f to Symbol (or restrict casting only to AbstractString).

[kescobo]

Would also be great to be able to do pairs of Int=>Symbol so that one can rename columns by index rather than name. I'm often in a situation where I need to rename the first column of a bunch of tables where I don't necessarily know the name. So something like rename(df, 1=>:a) and rename(df, [1=>:a, 2=>:b])

[bkamins]

Now we allow string or Symbol as "oldcol", do we still want to add integers?

[nalimilan]

Why not, if somebody wants to implement that...

[bkamins]

I have opened https://github.com/JuliaData/DataFrames.jl/pull/2030 for this (as the docstrings for rename and rename! needed decoupling and a clean-up anyway).

[bkamins]

all is done here. closing.