Adding an optional argument to get (and mget) like
val <- get(name, where, ..., value.if.not.found=NULL ) (*)
would be useful for many. HOWEVER, it is possible that there could be
some confusion here: (*) can give a NULL because either x exists and
has value NULL, or because x doesn't exist. If that matters, the user
would need to be careful about specifying a value.if.not.found that cannot
be confused with a valid value of x.
To avoid this difficulty, perhaps we want both: have Martin's getifexists(
)
return a list with two values:
- a boolean variable 'found' # = value returned by exists( )
- a variable 'value'
Then implement get( ) as:
get <- function(x,...,value.if.not.found ) {
if( missing(value.if.not.found) ) {
a <- getifexists(x,... )
if (!a$found) error("x not found")
} else {
a <- getifexists(x,...,value.if.not.found )
}
return(a$value)
}
Note that value.if.not.found has no default value in above.
It behaves exactly like current get does if value.if.not.found
is not specified, and if it is specified, it would be faster
in the common situation mentioned below:
if(exists(x,...)) { get(x,...) }
John
P.S. if you like dromedaries call it valueIfNotFound ...
..............................................................
John P. Nolan
Math/Stat Department
227 Gray Hall, American University
4400 Massachusetts Avenue, NW
Washington, DC 20016-8050
jpno...@american.edu voice: 202.885.3140
web: academic2.american.edu/~jpnolan
..............................................................
-----"R-devel" <r-devel-boun...@r-project.org> wrote: -----
To: Martin Maechler <maech...@stat.math.ethz.ch>, R-devel@r-project.org
From: Duncan Murdoch
Sent by: "R-devel"
Date: 01/08/2015 06:39AM
Subject: Re: [Rd] RFC: getifexists() {was [Bug 16065] "exists" ...}
On 08/01/2015 4:16 AM, Martin Maechler wrote:
> In November, we had a "bug repository conversation"
> with Peter Hagerty and myself:
>
> https://bugs.r-project.org/bugzilla/show_bug.cgi?id=16065
>
> where the bug report title started with
>
> --->> "exists" is a bottleneck for dispatch and package loading, ...
>
> Peter proposed an extra simplified and henc faster version of exists(),
> and I commented
>
> > --- Comment #2 from Martin Maechler <maech...@stat.math.ethz.ch>
---
> > I'm very grateful that you've started exploring the bottlenecks of
loading
> > packages with many S4 classes (and methods)...
> > and I hope we can make real progress there rather sooner than
later.
>
> > OTOH, your `summaryRprof()` in your vignette indicates that
exists() may use
> > upto 10% of the time spent in library(reportingTools), and your
speedup
> > proposals of exist() may go up to ca 30% which is good and well
worth
> > considering, but still we can only expect 2-3% speedup for
package loading
> > which unfortunately is not much.
>
> > Still I agree it is worth looking at exists() as you did ... and
> > consider providing a fast simplified version of it in addition to
current
> > exists() [I think].
>
> > BTW, as we talk about enhancements here, maybe consider a further
possibility:
> > My subjective guess is that probably more than half of exists()
uses are of the
> > form
>
> > if(exists(name, where, .......)) {
> > get(name, whare, ....)
> > ..
> > } else {
> > NULL / error() / .. or similar
> > }
>
> > i.e. many exists() calls when returning TRUE are immediately
followed by the
> > corresponding get() call which repeats quite a bit of the lookup
that exists()
> > has done.
>
> > Instead, I'd imagine a function, say getifexists(name, ...) that
does both at
> > once in the "exists is TRUE" case but in a way we can easily keep
the if(.) ..
> > else clause above. One already existing approach would use
>
> > if(!inherits(tryCatch(xx <- get(name, where, ...),
error=function(e)e), "error")) {
>
> > ... (( work with xx )) ...
>
> > } else {
> > NULL / error() / .. or similar
> > }
>
> > but of course our C implementation would be more efficient and use
more concise
> > syntax {which should not look like error handling}. Follow ups
to this idea
> > should really go to R-devel (the mailing list).
>
> and now I do follow up here myself :
>
> I found that 'getifexists()' is actually very simple to implement,
> I have already tested it a bit, but not yet committed to R-devel
> (the "R trunk" aka "master branch") because I'd like to get
> public comments {RFC := Request For Comments}.
>
I don't like the name -- I'd prefer getIfExists. As Baath (2012, R
Journal) pointed out, R names are very inconsistent in naming
conventions, but lowerCamelCase is the most common choice. Second most
common is period.separated, so an argument could be made for
get.if.exists, but there's still the possibility of confusion with S3
methods, and users of other languages where "." is an operator find it a
little strange.
If you don't like lowerCamelCase (and a lot of people don't), then I
think underscore_separated is the next best choice, so would use
get_if_exists.
Another possibility is to make no new name at all, and just add an
optional parameter to get() (which if present acts as your value.if.not
parameter, if not present keeps the current "object not found" error).
Duncan Murdoch
> My version of the help file {for both exists() and getifexists()}
> rendered in text is
>
> ---------------------- help(getifexists) -------------------------------
> Is an Object Defined?
>
> Description:
>
> Look for an R object of the given name and possibly return it
>
> Usage:
>
> exists(x, where = -1, envir = , frame, mode = "any",
> inherits = TRUE)
>
> getifexists(x, where = -1, envir = as.environment(where),
> mode = "any", inherits = TRUE, value.if.not = NULL)
>
> Arguments:
>
> x: a variable name (given as a character string).
>
> where: where to look for the object (see the details section); if
> omitted, the function will search as if the name of the
> object appeared unquoted in an expression.
>
> envir: an alternative way to specify an environment to look in, but
> it is usually simpler to just use the ‘where’ argument.
>
> frame: a frame in the calling list. Equivalent to giving ‘where’ as
> ‘sys.frame(frame)’.
>
> mode: the mode or type of object sought: see the ‘Details’ section.
>
> inherits: should the enclosing frames of the environment be searched?
>
> value.if.not: the return value of ‘getifexists(x, *)’ when ‘x’ does not
> exist.
>
> Details:
>
> The ‘where’ argument can specify the environment in which to look
> for the object in any of several ways: as an integer (the position
> in the ‘search’ list); as the character string name of an element
> in the search list; or as an ‘environment’ (including using
> ‘sys.frame’ to access the currently active function calls). The
> ‘envir’ argument is an alternative way to specify an environment,
> but is primarily there for back compatibility.
>
> This function looks to see if the name ‘x’ has a value bound to it
> in the specified environment. If ‘inherits’ is ‘TRUE’ and a value
> is not found for ‘x’ in the specified environment, the enclosing
> frames of the environment are searched until the name ‘x’ is
> encountered. See ‘environment’ and the ‘R Language Definition’
> manual for details about the structure of environments and their
> enclosures.
>
> *Warning:* ‘inherits = TRUE’ is the default behaviour for R but
> not for S.
>
> If ‘mode’ is specified then only objects of that type are sought.
> The ‘mode’ may specify one of the collections ‘"numeric"’ and
> ‘"function"’ (see ‘mode’): any member of the collection will
> suffice. (This is true even if a member of a collection is
> specified, so for example ‘mode = "special"’ will seek any type of
> function.)
>
> Value:
>
> ‘exists():’ Logical, true if and only if an object of the correct
> name and mode is found.
>
> ‘getifexists():’ The object-as from ‘get(x, *)’- if ‘exists(x, *)’
> is true, otherwise ‘value.if.not’.
>
> Note:
>
> With ‘getifexists()’, instead of the easy to read but somewhat
> inefficient
>
> if (exists(myVarName, envir = myEnvir)) {
> r <- get(myVarName, envir = myEnvir)
> ## ... deal with r ...
> }
>
> you now can use the more efficient (and slightly harder to read)
>
> if (!is.null(r <- getifexists(myVarName, envir = myEnvir))) {
> ## ... deal with r ...
> }
>
> References:
>
> Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
> Language_. Wadsworth & Brooks/Cole.
>
> See Also:
>
> ‘get’. For quite a different kind of “existence” checking, namely
> if function arguments were specified, ‘missing’; and for yet a
> different kind, namely if a file exists, ‘file.exists’.
>
> Examples:
>
> ## Define a substitute function if necessary:
> if(!exists("some.fun", mode = "function"))
> some.fun <- function(x) { cat("some.fun(x)\n"); x }
> search()
> exists("ls", 2) # true even though ls is in pos = 3
> exists("ls", 2, inherits = FALSE) # false
>
> ## These are true (in most circumstances):
> identical(ls, getifexists("ls"))
> identical(NULL, getifexists(".foo.bar.")) # default value.if.not =
NULL(!)
>
> ----------------- end[ help(getifexists) ] -----------------------------
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
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