Question

Hoping that someone could post an example of a custom function which acts similar to “.” in plyr.

What I have is a data frame. Where I am continuously running queries such as:

sqldf("select * from event.df where Date in (select Date from condition.df where C_1 = 1 and (C_2 = 1 OR C_3 = 3)")

What I would like is to have a function which basically acts as follows:

.(C_1, C_2 + C_3)

Specifically, a vector of formulas which define the attributes I use to select my data. I can treat “+” as OR “*” as AND etc…

I tried looking at the return type for “.” from plyr but did not understand it.

Answer 1

A function similar to plyr:::. is plyr:::.:

plyr:::.
function (..., .env = parent.frame()) 
{
    structure(as.list(match.call()[-1]), env = .env, class = "quoted")
}
<environment: namespace:plyr>

This returns a list and assigns it a class “quoted”. All it does, is to match the arguments of .() to the column names in the enclosing environment. Try it in a different context:

with(iris, .(Sepal.Length, Species))
List of 2
 $ Sepal.Length: symbol Sepal.Length
 $ Species     : symbol Species
 - attr(*, "env")=<environment: 0x2b33598> 
 - attr(*, "class")= chr "quoted"

What you do with this object next, depends on your purpose. Several methods exist for working with this class:

methods(class="quoted")
[1] as.quoted.quoted* c.quoted*         names.quoted*     print.quoted*     [.quoted*        

   Non-visible functions are asterisked

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So, if you’re looking for a function like .(), perhaps you can simply use .()