R help - Oct 2010 - Testing for existence of object within a function

I'm trying to test for the existence of an object within a function,
but despite searching the help files and R-list, I can't figure out
how to do it.

Here is some test code:

#-------------------------
a=1 #now I have a in the global environment

tst <- function(a,b=1) {
# but a is not in the local function environment
print(exists("a", inherits=FALSE)) #This is how I think I should be
able to test if it is in the local function environment
print(a)
}
tst()
#----------------------

#Here's the output
[1] TRUE
Error in print(a) : argument "a" is missing, with no default

It says TRUE, even though I specified inherits=FALSE and "a" is not in
the function environment.  I've tried to pass in where and envir
arguments to exists() but I haven't been able to solve the problem.

Thanks in advance.

Eli -
    I think you're looking for missing():
> tst = function(a,b=1)
+          if(missing(a))print("You must provide a") else
print(a)
> tst()
[1] "You must provide a"
> tst(7)
[1] 7

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu



On Thu, 7 Oct 2010, Eli Holmes wrote:
> I'm trying to test for the existence of an object within a function,
> but despite searching the help files and R-list, I can't figure out
> how to do it.
>
> Here is some test code:
>
> #-------------------------
> a=1 #now I have a in the global environment
>
> tst <- function(a,b=1) {
> # but a is not in the local function environment
> print(exists("a", inherits=FALSE)) #This is how I think I should
be
> able to test if it is in the local function environment
> print(a)
> }
> tst()
> #----------------------
>
> #Here's the output
> [1] TRUE
> Error in print(a) : argument "a" is missing, with no default
>
> It says TRUE, even though I specified inherits=FALSE and "a" is
not in
> the function environment.  I've tried to pass in where and envir
> arguments to exists() but I haven't been able to solve the problem.
>
> Thanks in advance.
>
> ______________________________________________
> R-help at r-project.org mailing list
> 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.
>

On Oct 7, 2010, at 7:27 PM, Eli Holmes wrote:
> I'm trying to test for the existence of an object within a function,
> but despite searching the help files and R-list, I can't figure out
> how to do it.
>
> Here is some test code:
>
> #-------------------------
> a=1 #now I have a in the global environment
>
> tst <- function(a,b=1) {
> # but a is not in the local function environment
> print(exists("a", inherits=FALSE)) #This is how I think I should
be
> able to test if it is in the local function environment
> print(a)
> }
> tst()
> #----------------------
>
> #Here's the output
> [1] TRUE
> Error in print(a) : argument "a" is missing, with no default
>
> It says TRUE, even though I specified inherits=FALSE and "a" is
not in
> the function environment.
Yes it is, or at least its name is known, even if it doesn't have a  
value:

a=1 #now I have a in the global environment

tst2 <- function(a,b=1) {
# but a is not in the local function environment
print(ls())
}
tst2()
[1] "a" "b"


tst3 <- function(a,b=1) {
# but a is not in the local function environment
print(formals("tst3"))
} ; tst3()

$a


$b
[1] 1
>  I've tried to pass in where and envir
> arguments to exists() but I haven't been able to solve the problem.
Exactly what IS the problem? If you tested for "d" there would be a  
FALSE return. And does this solve it....

 > tst4 <- function(a,b=1,d=missing(a)) {
+ print(d)
+ } ; tst4()
[1] TRUE

 > tst4 <- function(a,b=1,d=missing(a)) {
+ print(d)
+ } ; tst4(2)
[1] FALSE

-- 

David Winsemius, MD
West Hartford, CT