[Rd] [<-.POSIXlt changes order of attributes (PR#9197)

[Prof Brian Ripley]

Please point us to the documentation that says attributes are ordered.
E.g. R-lang.texi says

  All objects except @code{NULL} can have one or more attributes attached
  to them.  Attributes are stored as a list where all elements are named.

(although in fact they are stored in a pairlist).

I know of tens of functions that change the order of attributes, and 
did look at teaching identical() that they are not ordered.  But 
since they are stored as a pairlist, it would be quite expensive.
(Given that attributes seem to be growing in use, another internal storage 
mechanism is becoming more appropriate.)

BTW, all.equal() does not assume attributes are ordered.


On Fri, 1 Sep 2006, gregor.gorjanc at bfro.uni-lj.si wrote:

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> 
> Hello!
> 
> I was doing some tests with identical() and found out that [<-.POSIXlt
> method changes order of attributes. This example shows that:
> 
> x <- strptime("1900-1-1", format="%Y-%m-%d")
> x <- c(x)
> y <- c(x, x+1)
> x1 <- x
> y1 <- y
> 
> attributes(x)
> $names
> [1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"  "isdst"
> 
> $class
> [1] "POSIXt"  "POSIXlt"
> 
> $tzone
> [1] ""     "CET"  "CEST"
> 
> identical(attributes(x), attributes(y))
> TRUE
> 
> x[1] <- NA
> attributes(x)
> $names
> [1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"  "isdst"
> 
> $tzone
> [1] ""     "CET"  "CEST"
> 
> $class
> [1] "POSIXt"  "POSIXlt"
> 
> y[1] <- NA
> attributes(y)
> $names
> [1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  "wday"  "yday"  "isdst"
> 
> $tzone
> [1] ""     "CET"  "CEST"
> 
> $class
> [1] "POSIXt"  "POSIXlt"
> 
> identical(attributes(x), attributes(x1))
> FALSE
> 
> identical(attributes(y), attributes(y1))
> FALSE
> 
> This can be solved with either:
> 
> Index: R/src/library/base/R/datetime.R
> ===================================================================
> --- R/src/library/base/R/datetime.R     (revision 39045)
> +++ R/src/library/base/R/datetime.R     (working copy)
> @@ -713,10 +713,10 @@
>  {
>      if(!as.logical(length(value))) return(x)
>      value <- as.POSIXlt(value)
> -    cl <- oldClass(x)
> +    att <- attributes(x)
>      class(x) <- class(value) <- NULL
>      for(n in names(x)) x[[n]][i] <- value[[n]]
> -    class(x) <- cl
> +    attributes(x) <- att
>      x
>  }
> 
> or in the same way as it is done in [.<-POSIXct method
> 
> Index: R/src/library/base/R/datetime.R
> ===================================================================
> --- R/src/library/base/R/datetime.R     (revision 39045)
> +++ R/src/library/base/R/datetime.R     (working copy)
> @@ -714,9 +714,11 @@
>      if(!as.logical(length(value))) return(x)
>      value <- as.POSIXlt(value)
>      cl <- oldClass(x)
> +    tz <- attr(x, "tzone")
>      class(x) <- class(value) <- NULL
>      for(n in names(x)) x[[n]][i] <- value[[n]]
>      class(x) <- cl
> +    attr(x, "tzone") <- tz
>      x
>  }
> 
> I have checked both versions in r-devel with make check-all and all went
> fine.
> 
> Regards, Gregor
> 
> 
> 

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595