R devel - Sep 2006 - Strange behaviour of the [[ operator (PR#9263)

On 9/29/2006 9:06 PM, Herve Pages wrote:
> Hi,
> 
> This looks like a bug:
> 
>     > a <- list(b=5)
>     > a[['b']]
>     [1] 5
>     > a[[t<-'b']]
> 
> Nothing gets printed!
> 
> I need to use parenthesis to see the expected result:
> 
>     > a[[(t<-'b')]]
>     [1] 5
Yes, I agree it looks like a bug, and it's still there in R-devel.  The 
problem is that the assignment sets the R_Visible flag to false, and the 
indexing never sets it to true.  The same problem happens with a[t <- 
'b'], whether a is a list or not:

 > a <- c(b=5, c=6, d=7)
 > a[t <- 'b']
 > .Last.value
b
5

Duncan Murdoch

Hi Duncan,

This is even more confusing with a matrix:

    > x <- matrix(5:-6, 3)
    > x
         [,1] [,2] [,3] [,4]
    [1,]    5    2   -1   -4
    [2,]    4    1   -2   -5
    [3,]    3    0   -3   -6
    > x[invisible(2),3]
    [1] -2
    > x[2,invisible(3)]
    > .Last.value
    [1] -2

Thanks,
H.


Quoting murdoch at stats.uwo.ca:
> On 9/29/2006 9:06 PM, Herve Pages wrote:
> > Hi,
> > 
> > This looks like a bug:
> > 
> >     > a <- list(b=5)
> >     > a[['b']]
> >     [1] 5
> >     > a[[t<-'b']]
> > 
> > Nothing gets printed!
> > 
> > I need to use parenthesis to see the expected result:
> > 
> >     > a[[(t<-'b')]]
> >     [1] 5
> 
> Yes, I agree it looks like a bug, and it's still there in R-devel.  The
> problem is that the assignment sets the R_Visible flag to false, and the 
> indexing never sets it to true.  The same problem happens with a[t <- 
> 'b'], whether a is a list or not:
> 
>  > a <- c(b=5, c=6, d=7)
>  > a[t <- 'b']
>  > .Last.value
> b
> 5
> 
> Duncan Murdoch
> 
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