[R] Return value from function with For loop

[peter dalgaard]

> On 17 Apr 2017, at 10:04 , Ramnik Bansal <ramnik.bansal at gmail.com> wrote:
> 
> This is my output for is.function
> 
>> is.function("for")
> [1] FALSE
>> is.function(for)
> Error: unexpected ')' in "is.function(for)"
>> is.function("next")
> [1] FALSE
>> is.function(next)
> Error: no loop for break/next, jumping to top level
> 
> *I did not get the TRUE value. R version 3.3.3 on Mac. What am I doing
> different ?*

You need backticks:

> is.function(`for`)
[1] TRUE
> is.function(`next`)
[1] TRUE
> `for`
.Primitive("for")
> `next`
.Primitive("next")

David either should have typed backticks, or he did type them, only to have a friendly e-mail client autocorrect them into quotes...

-pd

> 
> Packages detail
>> search()
> [1] ".GlobalEnv"        "package:pryr"
> [3] "tools:RGUI"        "package:stats"
> [5] "package:graphics"  "package:grDevices"
> [7] "package:utils"     "package:datasets"
> [9] "package:methods"   "Autoloads"
> [11] "package:base"
> 
> thanks
> Ramnik
> 
> On Mon, Apr 17, 2017 at 12:06 PM, David Winsemius <dwinsemius at comcast.net>
> wrote:
> 
>> Both 'for' and 'next' return TRUE from is.function
>> 
>> is.function('for')
>> is.function('next')
>> 
>> Not at an R console at the moment but I did check this earlier today.
>> Thinking of it as different is definitely the way to think about it. (ISTR
>> Bert and I have had this exchange in the past.)
>> 
>> --
>> Best
>> David
>> 
>> Sent from my iPhone
>> 
>>> On Apr 16, 2017, at 9:50 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:
>>> 
>>> David et. al.:
>>> 
>>> "this levels is the level where you realize that the `for` function is
>>> different from most other R functions.  It is really a
>>> side-effect-fucntion. "
>>> 
>>> for(), while(), if(), next, etc. are *not* functions.
>>> 
>>> ?for says: "These are the basic control-flow constructs of the R
>> language."
>>> 
>>> They do not "return" values. They control program flow, whence what
>>> you call "side effects" are actually expressions that are parsed and
>>> evaluated
>>> 
>>> viz.
>>> 
>>>> if(TRUE)10
>>> [1] 10
>>> 
>>> ## but
>>> 
>>>> if(FALSE) 5
>>> ## nothing is returned, not even NULL
>>>> for(i in 1:3) i
>>> ## Ditto
>>> 
>>>> z <- NULL
>>>> z <- for(i in 1:3)i
>>>> z
>>> NULL ## still
>>> 
>>> Cheers,
>>> Bert
>>> 
>>> 
>>> 
>>> 
>>> Cheers,
>>> Bert
>>> 
>>> 
>>> 
>>> 
>>> 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 Sun, Apr 16, 2017 at 8:12 PM, David Winsemius <
>> dwinsemius at comcast.net> wrote:
>>>> 
>>>>> On Apr 16, 2017, at 7:26 PM, Ramnik Bansal <ramnik.bansal at gmail.com>
>> wrote:
>>>>> 
>>>>> In the code below
>>>>> 
>>>>> 
>>>>> *ff <- function(n){ for(i in 1:n) (i+1)}*
>>>>> 
>>>>> *n<-3;ff(n)->op;print(op)*
>>>>> 
>>>>> Why doesnt *print(op) * print 4 and instead prints NULL.
>>>>> Isnt the last line of code executed is *i+1 * and therefore that
>> should be
>>>>> returned instead of NULL
>>>>> 
>>>>> instead if I say
>>>>> *ff <- function(n){ (n+1) }*
>>>>> 
>>>>> Then
>>>>> *n<-3;ff(n)->op;rm(n);print(op)*
>>>>> gives 4 as output.
>>>>> 
>>>>> My question is *Which *is considered as the last line in a functoin
>> for the
>>>>> purpsoe of default return ? And under what conditions ?
>>>> 
>>>> It's probably a good thing that you are confused. It suggests that you
>> are actually "getting" the R-paradigm. Unfortunately for the new user of R,
>> there are several levels of understanding to pass through. First, you
>> realize that function-results need to be assigned to names in order to
>> persist. Then there is the next level where you discover that there are
>> exceptions to that rule: this levels is the level where you realize that
>> the `for` function is different from most other R functions.  It is really
>> a side-effect-fucntion. The assignments made within its body actually
>> persist in the global environment. AND it returns NULL. It shares this
>> anomalous behavior with `while` and `repeat`.n Almost all functions are
>> invoked with a possibly empty argument list.  The next and break functions
>> have implicit paired (empty) parentheses.
>>>> 
>>>> (My personal opinion is that this is not adequately advertised. Perhaps
>> it is an attempt to get people to migrate away from "Fortran-coding"
>> behavior?)
>>>> 
>>>> --
>>>> David.
>>>> 
>>>> 
>>>>> 
>>>>> -Thanks,
>>>>> Ramnik
>>>>> 
>>>>>     [[alternative HTML version deleted]]
>>>>> 
>>>>> ______________________________________________
>>>>> R-help at 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.
>>>> 
>>>> David Winsemius
>>>> Alameda, CA, USA
>>>> 
>>>> ______________________________________________
>>>> R-help at 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.
>> 
>> 
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at 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.mes at cbs.dk  Priv: PDalgd at gmail.com