based_on not working in @linq macro

[ElOceanografo]

I've been playing around with this package, and it's already made some of my real-world work a lot easier. I came across this error just now--based_on does not seem to work when chained together with other operations using @linq.

With R and dplyr, I can do the following:

> iris %>%  group_by(Species) %>%  summarise(x=mean(Sepal.Width))

but when I try the (I think?) equivalent in Julia, I get an error:

julia> @linq iris |> groupby(:Species) |> based_on(x = mean(SepalWidth))
ERROR: UndefVarError: based_on not defined

Is this something that should work? (Or am I doing something incorrectly?)

[nalimilan]

Looks like only the @based_on form (with the @) works:

iris=dataset("datasets", "iris")
@linq iris |> groupby(:Species) |> @based_on(x = mean(:SepalWidth))

But this syntax is probably better, right?

@linq iris |> by(:Species, x=mean(:SepalWidth))

Or maybe I'm missing the difference between by and based_on (@tshort again, do we need both?).

[ElOceanografo]

If you want to do transformations or calculations in between grouping and summarizing, a sand-alone based_on is nice. For an example, say you've got a data frame with the positions (x) of a bunch of different objects at different times (t), and you want to calculate each object's average speed:

speeds = @linq trajectory |>
    group_by(:object) |>
    transform(timespan = max(:t) - min(:t), displacement = last(:x) - first(:x)) |>
    based_on(avg_speed = :displacement ./ :timespan)

I this could of course all be done on one line, or by defining a helper function, but if you're working in the data-frame-piping style I think a separate based_on or summarise makes things a little clearer...

[ElOceanografo]

👍

[nalimilan]

OK, I see. Though I don't understand the name: what is "based on" what in this operation? Isn't summarise a better term?

[ElOceanografo]

I believe the ideas is "based on this data frame, calculate these things." I'd tend to agree that summarize is a better term. In dplyr, all the data frame operations are verbs, which is nice for readability.

(Prepare for the coming flame war over whether it should be summarise or summarize ;) )