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

From: Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk_at_idi.ntnu.no>
Date: Tue, 02 Jun 2009 10:04:54 +0200

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



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