Martin Maechler
2015-Jan-08 09:16 UTC
[Rd] RFC: getifexists() {was [Bug 16065] "exists" ...}
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 <maechler at
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}.
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) ] -----------------------------
Duncan Murdoch
2015-Jan-08 11:38 UTC
[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 <maechler at 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 at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel >
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
jpnolan at american.edu voice: 202.885.3140
web: academic2.american.edu/~jpnolan
..............................................................
-----"R-devel" <r-devel-bounces at r-project.org> wrote: -----
To: Martin Maechler <maechler at stat.math.ethz.ch>, R-devel at
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 <maechler at
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 at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
______________________________________________
R-devel at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
Duncan Murdoch
2015-Jan-08 14:36 UTC
[Rd] RFC: getifexists() {was [Bug 16065] "exists" ...}
On 08/01/2015 9:03 AM, John Nolan wrote:> Adding an optional argument to get (and mget) like > > val <- get(name, where, ..., value.if.not.found=NULL ) (*)That would be a bad idea, as it would change behaviour of existing uses of get(). What I suggested would not give a default. If the arg was missing, we'd get the old behaviour, if the arg was present, we'd use it. I'm not sure this is preferable to the separate function implementation. This makes the documentation and implementation of get() more complicated, and it would probably be slower for everyone. Duncan Murdoch> > 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 > > jpnolan at american.edu voice: 202.885.3140 > web: academic2.american.edu/~jpnolan > .............................................................. > > > -----"R-devel" <r-devel-bounces at r-project.org> wrote: ----- > To: Martin Maechler <maechler at stat.math.ethz.ch>, R-devel at 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 <maechler at 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 at r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-devel > > > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel
Michael Lawrence
2015-Jan-08 14:58 UTC
[Rd] RFC: getifexists() {was [Bug 16065] "exists" ...}
If we do add an argument to get(), then it should be named consistently
with the ifnotfound argument of mget(). As mentioned, the possibility of a
NULL value is problematic. One solution is a sentinel value that indicates
an unbound value (like R_UnboundValue).
But another idea (and one pretty similar to John's) is to follow the SYMSXP
design at the C level, where there is a structure that points to the name
and a value. We already have SYMSXPs at the R level of course (name
objects) but they do not provide access to the value, which is typically
R_UnboundValue. But this does not even need to be implemented with SYMSXP.
The design would allow something like:
binding <- getBinding("x", env)
if (hasValue(binding)) {
x <- value(binding) # throws an error if none
message(name(binding), "has value", x)
}
That I think it is a bit verbose but readable and could be made fast. And I
think binding objects would be useful in other ways, as they are
essentially a "named object". For example, when iterating over an
environment.
Michael
On Thu, Jan 8, 2015 at 6:03 AM, John Nolan <jpnolan at american.edu>
wrote:
> 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
>
> jpnolan at american.edu voice: 202.885.3140
> web: academic2.american.edu/~jpnolan
> ..............................................................
>
>
> -----"R-devel" <r-devel-bounces at r-project.org> wrote:
-----
> To: Martin Maechler <maechler at stat.math.ethz.ch>, R-devel at
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 <maechler at
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 at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
[[alternative HTML version deleted]]
Martin Maechler
2015-Jan-08 15:02 UTC
[Rd] RFC: getifexists() {was [Bug 16065] "exists" ...}
> 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.Exactly -- well, of course: That problem { NULL can be the legit value of what you want to get() } was the only reason to have a 'value.if.not' argument at all. Note that this is not about a universal replacement of the if(exists(..)) { .. get(..) } idiom, but rather a replacement of these in the cases where speed matters very much, which is e.g. in the low level support code for S4 method dispatch. 'value.if.not.found': Note that CRAN checks requires all arguments to be written in full length. Even though we have auto completion in ESS, Rstudio or other good R IDE's, I very much like to keep function calls somewhat compact. And yes, as you mention the dromedars aka 2-hump camels: getIfExist is already horrible to my taste (and "_" is not S-like; yes that's all very much a matter of taste and yes I'm from the 20th century).> 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) > }Interesting... Note that the above get() implementation would just be "conceptually", as all of this is also quite a bit about speed, and we do the different cases in C anyway [via 'op' code].> 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,...) }Good... Let's talk about your getifexists() as I argue we'd keep get() exactly as it is now anyway, if we use a new 3rd function (I keep calling 'getifexists()' for now): I think in that case, getifexists() would not even *need* an argument 'value.if.not' (or 'value.if.not.found'); it rather would return a list(found = *, value = *) in any case. Alternatively, it could return structure(<found>, value = *) In the first case, our main use case would be if((r <- getifexists(x, *))$found) { ## work with r$value } in the 2nd case {structure} : if((r <- getifexists(x, *))) { ## work with attr(r,"value") } I think that (both cases) would still be a bit slower (for the above most important use case) but probably not much and it would like slightly more readable than my if (!is.null(r <- getifexists(x, *))) { ## work with r } After all of this, I think I'd still somewhat prefer my original proposal, but not strongly -- I had originally also thought of returning the two parts explicitly, but then tended to prefer the version that behaved exactly like get() in the case the object is found. ... Nice interesting ideas! ... let the proposals and consideration flow ... Martin> John> P.S. if you like dromedaries call it valueIfNotFound ...:-) ;-) I don't .. as I said above, I already strongly dislike more than one hump. [ Each capital is one key stroke ("Shift") more , and each "_" is two key strokes more on most key boards..., and I do like identifiers that I can also quickly pronounce on the phone or in teaching .. ]> .............................................................. > John P. Nolan > Math/Stat Department > 227 Gray Hall, American University > 4400 Massachusetts Avenue, NW > Washington, DC 20016-8050 > ..............................................................> -----"R-devel" <r-devel-bounces at r-project.org> wrote: ----- > To: Martin Maechler <maechler at stat.math.ethz.ch>, R-devel at 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 <maechler at 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) ] -----------------------------