[Rd] Array changing address unexpectedly

[luke-tierney at uiowa.edu]

A simpler version with annotation:

library(pryr)

data <- 1:5          # new allocation, referenced from one variable

add <- address(data)

data[1] <- 6L        # only one reference; don't need to duplicate

add == address(data) # TRUE

print(data)          # inside the print call the local variable also references
                      # the object, so there are two references

data[1] <- 7L        # there have been two references to the object, so need
                      # to duplicate

add == address(data) # FALSE

R objects are supposed to be immutable, so conceptually
assignments create a new object. If R is sure it is safe to do
so, it will avoid making a copy and re-use the original
object. In the first assignment there is only one reference to
the object, the variable data, and it is safe to re-use the
object, so the address of the new value of `data` is the same as
the address of the old value. It would not be safe to do this if
the assignment happened inside print(), since there are then two
references -- the original variable and the local variable for
the argument. R currently doesn't have enough information to know
that there is only one reference after the call to print()
returns, so it has to be safe and make a copy, and the new value
has a different address than the old one. R may be able to be
less conservative in the future, but for now it cannot.

[As an exercise you can try to work out why the calls to
address() don't play a role in this.]

As a rule you shouldn't need to worry about addresses, and R might
make changes in the future that would: cause the address in this
version of the example to change on the first assignments; cause the
address on the second assignment to not change; cause the information
provided by pryr to be inaccurate or misleading; result in the use of
a tool like pryr::address to change the internal structure of the
object.

Best,

luke

On Sun, 12 Nov 2017, lille stor wrote:

> Hi David,
> 
> Thanks for the correction concerning the "else" issue.
> 
> Taking your code and removing some lines (to increase readability):
> 
>
> library(pryr)
> 
> data <- array(dim = c(5))
> for(x in 1:5)
> {
>    data[x] <- as.integer(x * 2)
> }
> 
> #print(data)
> 
> add = address(data)
> for(x in 1:5)
> {
>    data[x] <- as.integer(0)
> }
> 
> if (add == address(data))
> {
> print("Address did not change")
> } else {
> print("Address changed")
> }
>
> 
> If one runs this code everything works as expected, i.e. message "Address did not change" is printed. However, if one uncomments line "#print(data)", message "Address is changed" is printed instead. Any idea why this happens as it is a bit counter-intuitive? Is it something to do with some kind of lazy-evaluation mechanism R has that makes the array to be filled-up only when needed (in this case, when printing it) thus changing the array's address?
> 
> Thank you once more!
> 
>
>
> 
>
> Sent: Sunday, November 12, 2017 at 6:02 PM
> From: "David Winsemius" <dwinsemius at comcast.net>
> To: "lille stor" <lille.stor at gmx.com>
> Cc: r-devel at r-project.org
> Subject: Re: [Rd] Array changing address unexpectedly
>> On Nov 12, 2017, at 8:47 AM, lille stor <lille.stor at gmx.com> wrote:
>>
>> Hi,
>>
>> Given the following R code:
>>
>> library(pryr)
>>
>> data <- array(dim = c(5))
>>
>> for(x in 1:5)
>> {
>> data[x] <- as.integer(x * 2)
>> }
>>
>> add = address(data) # save address of "data"
>>
>> for(x in 1:5)
>> {
>> data[x] <- as.integer(0)
>> }
>>
>> if (add == address(data))
>> {
>> print("Address did not change")
>> }
>> else
>> {
>> print("Address changed")
>> }
>>
>> If one runs this code, message "Address changed" is printed. However, if one comments line "data[x] <- as.integer(0)" the address of "data" does not change and message "Address did not change" is printed instead. Why? The datatype of the array should not change with this line and hence no need for R to convert the array to a different type (and have the array's address changing in the process).
>
> I'm guessing you didn't take note of the error message:
>
>> else
> Error: unexpected 'else' in " else"
>
> It's always good practice to investigate errors. The else function needs to come immediately after the "{".
>
> Here's a more complete test of what I take to be your question:
>
> library(pryr)
>
> data <- array(dim = c(5))
> add = address(data)
> for(x in 1:5)
> {
> data[x] <- as.integer(x * 2)
> }
> if (add == address(data))
> {
> print("Address did not change")
> } else {
> print("Address changed")
> }
>
>
> data <- array(dim = c(5)) # reset
> add = address(data)
> for(x in 1:5)
> {
> data[x] <- as.integer(0)
> }
>
> if (add == address(data))
> {
> print("Address did not change")
> } else {
> print("Address changed")
> }
>
> # changes in both situations.
>
>
>>
>> Thank you!
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
> David Winsemius
> Alameda, CA, USA
>
> 'Any technology distinguishable from magic is insufficiently advanced.' -Gehm's Corollary to Clarke's Third Law
>
>
>
>
> 
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu