[R] Assigning value to a vector from within a function

[David Winsemius]

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