[Rd] reference counting bug: overwriting for loop 'seq' variable
Wacek Kusnierczyk
Waclaw.Marcin.Kusnierczyk at idi.ntnu.no
Tue Jun 2 10:04:54 CEST 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.
>
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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