[R] Why does debugging print() change output of function?

[David Winsemius]

On Sep 9, 2014, at 4:07 PM, peter dalgaard wrote:

>
> On 07 Sep 2014, at 00:31 , David Winsemius <dwinsemius at comcast.net>  
> wrote:
>
>> The goal:
>> to create a function modeled after `subset` (notorious for its non- 
>> standard evaluation) that will take a series of logical tests as  
>> unquoted expressions to be evaluated in the framework of a  
>> dataframe environment and return a dataframe of logicals:
> ...
>
> A belated peep from the author of subset(): Don't!
>
> I think we learned the hard way by now that it is much easier to  
> pass unevaluated expressions in the shape of formula objects or  
> maybe expression objects. Lots of pain can be avoided by slipping in  
> a simple "~".

It's taken me several years to understand why you are probably correct  
in this regard. I needed to learn that `~` is actually a function that  
creates a language object.

 > is.function(`~`)
[1] TRUE
 > is.language( ~ x > 5 & x < 10)
[1] TRUE

... and that it's rather easy to extract the object somewhat like but  
not really an expression embedded in such an object:

 > is.expression( ~ x > 5 & x < 10)
[1] FALSE

 > is.call( (~ x > 5 & x < 10)[2] )
[1] TRUE

The task of learning the various types of language objects is not an  
easy one.

-- 

David Winsemius, MD
Alameda, CA, USA