add ready examples to docstring for rselect, rtransform (#321)

[mahiki]

I added REPL jldoctest examples for the @rselect and @rtransform macros to demonstrate the easy syntax for operations that need a ByRow treatment.

I choose these examples to show an easy way to post a value to a new column based on the row value in another column or combination of columns.

[mahiki]

There was a doctest failure in my macro examples for @rtransform, and also @rselect. I'm not sure how this works so not able to determine what is missing.

The dataframe operations are correct in project REPL, I must be missing some syntax in the docsstring?

[bkamins]

Probably julia> using DataFramesMeta; is needed (normally it should be set up to be globally loaded for all doctest strings, but I do not know if @pdeffebach did this)

[mahiki]

Now I see. The other jldoctest sections are like this:

```jldoctest
julia> using DataFramesMeta      # <--- I didnt do this

julia> df = DataFrame(A = 1:3, B = [2, 1, 2]);

julia> @transform df begin
           :a = 2 * :A
           :x = :A .+ :B
# ..etc

@pdeffebach I will push another commit and try to update the PR or file a new one. Alternately, I could make @bkamins suggested change to make.jl:

I think @bkamins is saying that the following added to the make.jl file will alleviate the need to place `using DataFramesMeta for each doctest:

DocMeta.setdocmeta!(DataFramesMeta, :DocTestSetup, :(using DataFramesMeta); recursive=true)

Following the DataFrames.jl/doc/make.jl file.

[bkamins]

Yes, this is how we do it in https://github.com/JuliaData/DataFrames.jl/blob/5233159d470c8e5a9a2e2f93cd3ea67a7fff1769/docs/make.jl#L5.

[pdeffebach]

Yes we should definitely fix that. But just do the using DataFramesMeta line for now and we will simplify the docstrings in a later PR.

[pdeffebach]

Thanks a ton for this!

Do you think you could add these for @rorderby and @rsubset as well?

[mahiki]

Do you think you could add these for @rorderby and @rsubset as well?

Very happy to. I'll commit those in the same branch, for this PR.

[mahiki]

Added jldoctest examples for @rorderby, @rsubset. I was very glad to learn what the purpose of these macros is, its very convenient to write like this.

[mahiki]

added minor change to df output.

[mahiki]

Let me know if there are needed changes before running the workflow tests.

[pdeffebach]

Sorry about the delay on this! Will merge tomorrow

[pdeffebach]

Thanks! Sorry about the delay.

[mahiki]

Awesome, ping me if you need some other help!