R help - Oct 2010 - Assigning value to a vector from within a function

Hi all,

I am having trouble assigning a value within a vector using a variable
defined within a function.  I suspect this has something to do with
namespaces but I cannot figure it out.  Please reply directly to me if you
can help.

###begin code
#simple vector
test<-c(4,5,6)

#simple function to get a value from a vector
#works just like I would expect, has no problem indexing using the local
function variable name "index"

getvalue<-function(index){
print(test[index])
}
getvalue(2)


#another simple function to update the value in a vector
update<-function(index){
test[index]<- 20
}
update(2)
test
#The update() function silently fails to accomplish the update
###

I don't understand why the function has no problem accessing the value, but
cannot accomplish the assignment.  I need to be able to update vectors using
index values which I pass from within a function.

Thanks for your help.

Andrew Barr

	[[alternative HTML version deleted]]

>
> #another simple function to update the value in a vector
> update<-function(index){
> test[index]<- 20
> }
> update(2)
> test
> #The update() function silently fails to accomplish the update
Replace the '<-' by '<<-' and you'll be good to go
if you call the
function from a global environment. More generally, look up
environments since you may want to update the variable in the
environment of the calling code which may not be the global
environment.

Remember that when you call a function, all arguments are copied, so
you do your operation on a copy that's discarded after the function
ends. The most transparent way to get what you want is to re-code the
function as

update<-function(test, index){
test[index]<- 20
test
}

and then write

test = update(test, index)

Peter

See the replace function:

index <- 3
test <- replace(test, index, 20)


On Wed, Oct 6, 2010 at 2:31 PM, Andrew Barr <wabarr@gmail.com> wrote:
> Hi all,
>
> I am having trouble assigning a value within a vector using a variable
> defined within a function.  I suspect this has something to do with
> namespaces but I cannot figure it out.  Please reply directly to me if you
> can help.
>
> ###begin code
> #simple vector
> test<-c(4,5,6)
>
> #simple function to get a value from a vector
> #works just like I would expect, has no problem indexing using the local
> function variable name "index"
>
> getvalue<-function(index){
> print(test[index])
> }
> getvalue(2)
>
>
> #another simple function to update the value in a vector
> update<-function(index){
> test[index]<- 20
> }
> update(2)
> test
> #The update() function silently fails to accomplish the update
> ###
>
> I don't understand why the function has no problem accessing the value,
but
> cannot accomplish the assignment.  I need to be able to update vectors
> using
> index values which I pass from within a function.
>
> Thanks for your help.
>
> Andrew Barr
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@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.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

	[[alternative HTML version deleted]]

On Oct 6, 2010, at 1:31 PM, Andrew Barr wrote:
> Hi all,
>
> I am having trouble assigning a value within a vector using a variable
> defined within a function.  I suspect this has something to do with
> namespaces but I cannot figure it out.  Please reply directly to me  
> if you
> can help.
>
> ###begin code
> #simple vector
> test<-c(4,5,6)
>
> #simple function to get a value from a vector
> #works just like I would expect, has no problem indexing using the  
> local
> function variable name "index"
>
> getvalue<-function(index){
> print(test[index])
> }
> getvalue(2)
But, but , but that just the "[" function.

 > test<-c(4,5,6)
 > `[`(test, 2)
[1] 5
> #another simple function to update the value in a vector
> update<-function(index){
> test[index]<- 20
> }
> update(2)
> test
> #The update() function silently fails to accomplish the update
More "buts", ... that's just the "[<-" function with
a constant 20 or
perhaps it's an unsuccessful effort to make one.

 > test[2] <- 20   # which gets parsed to something like `[<-`(test,  
2, 20)
 > `[<-`(test, 2, 20)
[1]  4 20  6
 > test
[1]  4 20  6
> ###
>
> I don't understand why the function has no problem accessing the  
> value, but
> cannot accomplish the assignment.
Scoping. You made an assignment inside a function and then did not  
return a full object that was assignable as "test". Look what happens
if you do an assignment of the returned value of your function

 > newtest <-  update(2)
 >
 > newtest
[1] 20
> I need to be able to update vectors using
> index values which I pass from within a function.
Pass the object to the function and return what you want. If you want  
the original to be altered, then do it like:

test <- modfn(test)  # which is slightly more general than `[<-`(test,  
2, 20)

But do it with:
.
.
return(test) }

at the end of the operations, i.e., return the full object, not just  
its second element.
>
> Thanks for your help.
>
> Andrew Barr
-- 
David Winsemius, MD
West Hartford, CT