R devel - Jun 2009 - reference counting bug: overwriting for loop 'seq' variable

It looks like the 'seq' variable to 'for' can be altered from
within the loop, leading to incorrect answers.  E.g., in
the following I'd expect 'sum' to be 1+2=3, but R 2.10.0
(svn 48686) gives 44.5.

   > x = c(1,2);  sum = 0; for (i in x) { x[i+1] = i + 42.5; sum = sum +
i }; sum
   [1] 44.5
or, with a debugging cat()s,
   > x = c(1,2);  sum = 0; for (i in x) { cat("before, i=", i,
"\n");
x[i+1] = i + 42.5; cat("after, i=", i,"\n"); sum = sum + i
}; sum
   before, i= 1
   after, i= 1
   before, i= 43.5
   after, i= 43.5
   [1] 44.5
 
If I force the for's 'seq' to be a copy of x by adding 0 to it, then
I
do get the expected answer.

   > x = c(1,2);  sum = 0; for (i in x+0) { x[i+1] = i + 42.5; sum = sum
+ i }; sum
   bbbbb[1] 3

It looks like an error in reference counting. 

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com

William Dunlap wrote:
> It looks like the 'seq' variable to 'for' can be altered
from
> within the loop, leading to incorrect answers.  E.g., in
> the following I'd expect 'sum' to be 1+2=3, but R 2.10.0
> (svn 48686) gives 44.5.
>
>    > x = c(1,2);  sum = 0; for (i in x) { x[i+1] = i + 42.5; sum = sum +
> i }; sum
>    [1] 44.5
> or, with a debugging cat()s,
>    > x = c(1,2);  sum = 0; for (i in x) { cat("before, i=", i,
"\n");
> x[i+1] = i + 42.5; cat("after, i=", i,"\n"); sum = sum
+ i }; sum
>    before, i= 1
>    after, i= 1
>    before, i= 43.5
>    after, i= 43.5
>    [1] 44.5
>  
> If I force the for's 'seq' to be a copy of x by adding 0 to it,
then I
> do get the expected answer.
>
>    > x = c(1,2);  sum = 0; for (i in x+0) { x[i+1] = i + 42.5; sum = sum
> + i }; sum
>    bbbbb[1] 3
>
> It looks like an error in reference counting. 
>   
indeed;  seems like you've hit the issue of when r triggers data
duplication and when it doesn't, discussed some time ago in the context
of names() etc.  consider:

    x = 1:2
    for (i in x)
       x[i+1] = i-1
    x
    # 1 0 1

    y = c(1, 2)
    for (i in y)
       y[i+1] = i-1
    y
    # -1 0


vQ

Thanks for the report.  Should be fixed in teh devel and 2.9 branches.

luke

On Mon, 1 Jun 2009, William Dunlap wrote:
> It looks like the 'seq' variable to 'for' can be altered
from
> within the loop, leading to incorrect answers.  E.g., in
> the following I'd expect 'sum' to be 1+2=3, but R 2.10.0
> (svn 48686) gives 44.5.
>
>   > x = c(1,2);  sum = 0; for (i in x) { x[i+1] = i + 42.5; sum = sum +
> i }; sum
>   [1] 44.5
> or, with a debugging cat()s,
>   > x = c(1,2);  sum = 0; for (i in x) { cat("before, i=", i,
"\n");
> x[i+1] = i + 42.5; cat("after, i=", i,"\n"); sum = sum
+ i }; sum
>   before, i= 1
>   after, i= 1
>   before, i= 43.5
>   after, i= 43.5
>   [1] 44.5
>
> If I force the for's 'seq' to be a copy of x by adding 0 to it,
then I
> do get the expected answer.
>
>   > x = c(1,2);  sum = 0; for (i in x+0) { x[i+1] = i + 42.5; sum = sum
> + i }; sum
>   bbbbb[1] 3
>
> It looks like an error in reference counting.
>
> Bill Dunlap
> TIBCO Software Inc - Spotfire Division
> wdunlap tibco.com
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
-- 
Luke Tierney
Chair, Statistics and Actuarial Science
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 at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu