R help - Apr 2017 - Different output for "if else" and "ifelse" that I don't understand

Dear list-members,

During the test phase of a function, I run it interactively (in Rstudio) 
and the ... produces an error. Then I use this to read it:

if (class(try(list(...), silent=TRUE))=="try-error") p3p <- list()
else
p3p <- list(...)

It works fine; interactively I will get

 > if (class(try(list(...), silent=TRUE))=="try-error") p3p <-
list()
else p3p <- list(...)
 > p3p
list()

and within a function I will get a list with the ... value. Perfect.

I wanted to simplify the line by doing:

p3p <- ifelse(class(try(list(...), silent=TRUE))=="try-error",
list(),
list(...))

In interactive mode, it works but within a function, I don't get the 
names of the parameters. I don't understand the logic behind this 
difference. Have you an idea ?

Thanks

Marc


 > Try1 <- function(...) {
+   if (class(try(list(...), silent=TRUE))=="try-error") p3p <-
list()
else p3p <- list(...)
+   return(p3p)
+ }
 > Try1(k=100)
$k
[1] 100

 > Try1()
list()
 > Try2 <- function(...) {
+   p3p <- ifelse(class(try(list(...), silent=TRUE))=="try-error", 
list(), list(...))
+   return(p3p)
+ }
 > Try2(k=100)
[[1]]
[1] 100

 > Try2()
[[1]]
NULL

ifelse's vectorization messes this up.

You could replace your original
  if (class(try(list(...), silent=TRUE))=="try-error") p3p <-
list() else
p3p <- list(...)
with
  p3p <- if (class(try(list(...), silent=TRUE))=="try-error")
list() else
list(...)
instead of using ifelse, since the return value of 'if ... else ...' is
the
value of whichever branch was taken.

Even better, use
  p3p <- try(list(...), error=function(e) list())


Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Fri, Apr 28, 2017 at 2:21 AM, Marc Girondot via R-help <
r-help at r-project.org> wrote:
> Dear list-members,
>
> During the test phase of a function, I run it interactively (in Rstudio)
> and the ... produces an error. Then I use this to read it:
>
> if (class(try(list(...), silent=TRUE))=="try-error") p3p <-
list() else
> p3p <- list(...)
>
> It works fine; interactively I will get
>
> > if (class(try(list(...), silent=TRUE))=="try-error") p3p
<- list() else
> p3p <- list(...)
> > p3p
> list()
>
> and within a function I will get a list with the ... value. Perfect.
>
> I wanted to simplify the line by doing:
>
> p3p <- ifelse(class(try(list(...), silent=TRUE))=="try-error",
list(),
> list(...))
>
> In interactive mode, it works but within a function, I don't get the
names
> of the parameters. I don't understand the logic behind this difference.
> Have you an idea ?
>
> Thanks
>
> Marc
>
>
> > Try1 <- function(...) {
> +   if (class(try(list(...), silent=TRUE))=="try-error") p3p
<- list()
> else p3p <- list(...)
> +   return(p3p)
> + }
> > Try1(k=100)
> $k
> [1] 100
>
> > Try1()
> list()
> > Try2 <- function(...) {
> +   p3p <- ifelse(class(try(list(...),
silent=TRUE))=="try-error", list(),
> list(...))
> +   return(p3p)
> + }
> > Try2(k=100)
> [[1]]
> [1] 100
>
> > Try2()
> [[1]]
> NULL
>
> ______________________________________________
> 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/posti
> ng-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

Typo: last line should be

Even better, use
  p3p <- tryCatch(list(...), error=function(e) list())


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 Fri, Apr 28, 2017 at 8:34 AM, William Dunlap via R-help
<r-help at r-project.org> wrote:
> ifelse's vectorization messes this up.
>
> You could replace your original
>   if (class(try(list(...), silent=TRUE))=="try-error") p3p <-
list() else
> p3p <- list(...)
> with
>   p3p <- if (class(try(list(...), silent=TRUE))=="try-error")
list() else
> list(...)
> instead of using ifelse, since the return value of 'if ... else
...' is the
> value of whichever branch was taken.
>
> Even better, use
>   p3p <- try(list(...), error=function(e) list())
>
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
> On Fri, Apr 28, 2017 at 2:21 AM, Marc Girondot via R-help <
> r-help at r-project.org> wrote:
>
>> Dear list-members,
>>
>> During the test phase of a function, I run it interactively (in
Rstudio)
>> and the ... produces an error. Then I use this to read it:
>>
>> if (class(try(list(...), silent=TRUE))=="try-error") p3p
<- list() else
>> p3p <- list(...)
>>
>> It works fine; interactively I will get
>>
>> > if (class(try(list(...), silent=TRUE))=="try-error") p3p
<- list() else
>> p3p <- list(...)
>> > p3p
>> list()
>>
>> and within a function I will get a list with the ... value. Perfect.
>>
>> I wanted to simplify the line by doing:
>>
>> p3p <- ifelse(class(try(list(...),
silent=TRUE))=="try-error", list(),
>> list(...))
>>
>> In interactive mode, it works but within a function, I don't get
the names
>> of the parameters. I don't understand the logic behind this
difference.
>> Have you an idea ?
>>
>> Thanks
>>
>> Marc
>>
>>
>> > Try1 <- function(...) {
>> +   if (class(try(list(...), silent=TRUE))=="try-error") p3p
<- list()
>> else p3p <- list(...)
>> +   return(p3p)
>> + }
>> > Try1(k=100)
>> $k
>> [1] 100
>>
>> > Try1()
>> list()
>> > Try2 <- function(...) {
>> +   p3p <- ifelse(class(try(list(...),
silent=TRUE))=="try-error", list(),
>> list(...))
>> +   return(p3p)
>> + }
>> > Try2(k=100)
>> [[1]]
>> [1] 100
>>
>> > Try2()
>> [[1]]
>> NULL
>>
>> ______________________________________________
>> 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/posti
>> ng-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.