I can’t find any docs for the by() function in DataFrames. There is an example of usage here, but no docs:
At the REPL,
?by works for me.
I guess the REPL is the only documentation. The REPL showed this which was helpful:
by(d, cols, f) is equivalent to combine(map(f, groupby(d, cols)))
Found this while searching for the DataFrames.nrow function: