[Rd] reference counting bug: overwriting for loop 'seq' variable

[luke at stat.uiowa.edu]

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
>
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> R-devel at r-project.org mailing list
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>

-- 
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
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