Um... Let's get the concepts straight:
The "function" function doesn't evaluate anything. It just takes the list of
formal arguments (including default expressions), the function body, and the
current evaluation environment, and stiches them together into a function
object, known as a "closure".
The action happens when a function is _called_. Then the actual arguments are
combined with the formals, and the body expression is evaluated. You will find
the source code for this in src/main/eval.c. It is a pretty complex beast, but
the essential point for the present discussion is that actual arguments in
function calls are passed in the form of so-called promises. These contain the
expression passed, so that substitute() can extract it. It also enables lazy
evaluation: putting off argument evaluation until the value is actually needed
(possibly never).
-pd
> On 6 Sep 2019, at 23:10 , David Winsemius <dwinsem...@comcast.net> wrote:
>
>
> On 9/6/19 1:07 PM, Golden, Shelby wrote:
>> Thank you all for your reply. I should clarify, that I am looking to
>> understand why the keyword function can take a logical argument (eg: x<4)
>> and use that later inside the function's definition for logical evaluations.
>>
>> Consider this example, which is a simplification of
>> getAnywhere(subset.data.frame):
>> x = data.frame("Col1" = c(1, 2, 3, 4, 5), "Col2" = c(6, 7, 8, 9, 10))
>> test <- function(x, logic){
>> e <- substitute(logic)
>> r <- eval(e, x, parent.frame())
>> r[r]
>> }
>
> x<4 is not really a logical argument in that context. It is rather an
> expression and will remain an expression until it needs to be evaluated. See
> this even simpler example:
>
>
> test <- function(x, logic, ... ){
> e <- deparse( substitute(logic))
> #r <- eval(e, x, parent.frame())
> e
> }
> test(4, x<4)
> #[1] "x < 4"
>
>
> test(4, is.logical(x < 4) )
> [1] "is.logical(x < 4)"
>
>
> Some of this you have already been told, but appears necessary to repeat.
> Expressions given to `function` are not necessarily evaluated. They will be
> evaluated if assigned names.
>
>
> test(4, zed = is.logical(x < 4) )
> #[1] ""
>
> The function()-function will parse the contents of the parentheses for number
> of arguments and for parse()-ability. It will evaluate named arguments
> created with "=". In the context of parsing the formals of a function the "="
> operator is different than the "<-" function.
>
> The substitute function will not evaluate (since in the language of R
> operations it is "special"), but rather checks that the expression can be
> parsed by R's rules, i.e. is a valid parse tree. `deparse` returns the
> original character representation.
>
> --
>
> David
>
>
>>
>> Shelby
>>
>> On 9/6/19, 1:02 PM, "R-help on behalf of Richard M. Heiberger"
>> <r-help-boun...@r-project.org on behalf of r...@temple.edu> wrote:
>>
>> You might also want to look at the codetools package, for example the
>> showTree function " Prints a Lisp-style representation of R
>> expression."
>> > library(codetools)
>> > showTree(quote(x %*% x))
>> (%*% x x)
>> > showTree(quote(a+b))
>> (+ a b)
>> > showTree(quote(y ~ a+b))
>> (~ y (+ a b))
>> On Fri, Sep 6, 2019 at 2:30 PM Bert Gunter <bgunter.4...@gmail.com>
>> wrote:
>> >
>> > The following may be of use (it gives the parse tree of the text):
>> >
>> > > z <- as.list(parse(text = "function(x)x %*% x"))
>> > > z[[1]]
>> > function(x) x %*% x
>> > > z[[c(1,1)]]
>> > `function`
>> > > z[[c(1,2)]]
>> > $x
>> > > z[[c(1,3)]]
>> > x %*% x
>> > > z[[c(1,3,1)]]
>> > `%*%`
>> > > z[[c(1,3,2)]]
>> > x
>> > > z[[c(1,3,3)]]
>> > x
>> >
>> >
>> > Bert Gunter
>> >
>> >
>> >
>> > On Fri, Sep 6, 2019 at 10:14 AM Wang Jiefei <szwj...@gmail.com> wrote:
>> >
>> > > If you are looking for an R code parser, I think the `parse` and
>> `eval`
>> > > function might be a good start point. See the example below.
>> > >
>> > > > parse(text="function(x)message(x)")
>> > > expression(function(x)message(x))
>> > > > eval(parse(text="function(x)message(x)"))
>> > > function(x)message(x)
>> > >
>> > > Best,
>> > > Jiefei
>> > >
>> > > On Fri, Sep 6, 2019 at 12:55 PM Golden, Shelby <gold...@njhealth.org>
>> > > wrote:
>> > >
>> > >> Hello Bert,
>> > >>
>> > >> Thank you for the reply and your clarifications. Yes, it might be
>> helpful
>> > >> to look into R’s formal grammar to see how “function” parses input
>> to
>> > >> delegate correct syntax. Is that accessible online?
>> > >>
>> > >> Thank you,
>> > >> Shelby
>> > >>
>> > >>
>> > >> From: Bert Gunter <bgunter.4...@gmail.com>
>> > >> Date: Friday, September 6, 2019 at 10:44 AM
>> > >> To: "Golden, Shelby" <gold...@njhealth.org>
>> > >> Cc: "r-help@R-project.org" <r-help@r-project.org>, "Gillenwater,
>> Lucas" <
>> > >> gillenwat...@njhealth.org>
>> > >> Subject: Re: [R] [R-devel] Source Code for function
>> > >>
>> > >> 1. This is a plain text list; all html is stripped. So there is no
>> red
>> > >> highlighting.
>> > >>
>> > >> 2. There is no "source code" for "function" -- it is a reserved
>> keyword.
>> > >> Or are you looking for R's formal grammar -- e.g. how it parses
>> input to
>> > >> determine correct syntax?
>> > >>
>> > >>
>> > >>
>> > >> Bert Gunter
>> > >>
>> > >> "The trouble with having an open mind is that people keep coming
>> along
>> > >> and sticking things into it."
>> > >> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>> > >>
>> > >>
>> > >> On Fri, Sep 6, 2019 at 8:51 AM Golden, Shelby <gold...@njhealth.org
>> > >> <mailto:gold...@njhealth.org>> wrote:
>> > >> Hi all,
>> > >>
>> > >> I have been attempting to access the source code for the keyword
>> > >> “function” to better understand how it assigns and stores logical
>> inputs,
>> > >> like in the subset() [base] function. Does anyone know how I can
>> access the
>> > >> source code for this?
>> > >>
>> > >> For example, if I have
>> > >> norm <- function(x){
>> > >> sqrt(x%*%x))
>> > >> }
>> > >> I am looking for the source code for the “function” portion,
>> highlighted
>> > >> in red.
>> > >>
>> > >> Thank you for your time and assistance,
>> > >> Shelby Golden
>> > >> Lab Researcher Technician
>> > >> Dr. Russell Bowler’s Lab
>> > >> Department of Medicine
>> > >> National Jewish Health in Denver, CO
>> > >> Phone: (303) 270-2598
>> > >>
>> > >> NOTICE: This email message is for the sole use of the intended
>> > >> recipient(s) and may contain confidential and privileged
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>> > >> unauthorized review, use, disclosure or distribution is prohibited.
>> If you
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>> email
>> > >> and destroy all copies of the original message.
>> > >> [[alternative HTML version deleted]]
>> > >>
>> > >> ______________________________________________
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>> > >>
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>> > >> >
>> > >> PLEASE do read the posting guide
>> > >> http://www.R-project.org/posting-guide.html<
>> > >>
>> http://mx2.njhealth.org:32224/?dmVyPTEuMDAxJiZlMTkwYmMwMzFlNjk4ZTAzNz01RDcyOEQwN18yMjk2OF8zOTk2XzEmJmFkYTkxMWRkMWRhZTFkNz0xMjMzJiZ1cmw9aHR0cCUzQSUyRiUyRnd3dyUyRVItcHJvamVjdCUyRW9yZyUyRnBvc3RpbmctZ3VpZGUlMkVodG1s
>> > >> >
>> > >> and provide commented, minimal, self-contained, reproducible code.
>> > >>
>> > >> [[alternative HTML version deleted]]
>> > >>
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>> > >>
>> > >
>> >
>> > [[alternative HTML version deleted]]
>> >
>> > ______________________________________________
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>> > https://stat.ethz.ch/mailman/listinfo/r-help
>> > PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> > and provide commented, minimal, self-contained, reproducible code.
>> ______________________________________________
>> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>> NOTICE: This email message is for the sole use of the intended recipient(s)
>> and may contain confidential and privileged information. Any unauthorized
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>> intended recipient, please contact the sender by reply email and destroy all
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>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd....@cbs.dk Priv: pda...@gmail.com
______________________________________________
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