R devel - May 2008 - all.equal, classes, and order of arguments

> a <- list()
> b <- structure(list(), class=c("list", "a"))
> all.equal(a, b)
[1] "Attributes: < target is NULL, current is list
>"
> all.equal(b, a)
[1] "Attributes: < Modes: list, NULL >"
[2] "Attributes: < names for target but not for current >"
[3] "Attributes: < Length mismatch: comparison on first 0 components
>"

which does not reveal the simple difference between a and b.  I had
also expected that all.equal would (in some sense) be symmetric with
respect to its arguments, but I can see that this would be very hard
to guarantee in general.

Hadley

-- 
http://had.co.nz/

>>>>> "hw" == hadley wickham <h.wickham at
gmail.com>
>>>>>     on Thu, 1 May 2008 20:53:10 -0500 writes:
    >> a <- list()
    >> b <- structure(list(), class=c("list", "a"))
    >> all.equal(a, b)
    hw> [1] "Attributes: < target is NULL, current is list >"
    >> all.equal(b, a)
    hw> [1] "Attributes: < Modes: list, NULL >"
    hw> [2] "Attributes: < names for target but not for current
>"
    hw> [3] "Attributes: < Length mismatch: comparison on first 0
components >"

    hw> which does not reveal the simple difference between a and b.  I had
    hw> also expected that all.equal would (in some sense) be symmetric with
    hw> respect to its arguments, but I can see that this would be very hard
    hw> to guarantee in general.

    hw> Hadley

You can use  'check.attributes' :
> str(all.equal)
function (target, current, ...)  
> str(all.equal.list)
function (target, current, check.attributes = TRUE, ...)  

> a <- list(); b <- structure(a, class=c("list",
"a")); all.equal(a, b)
[1] "Attributes: < target is NULL, current is list
>"
> all.equal(a,b, check.attributes=FALSE)
[1] TRUE
> all.equal(b,a, check.attributes=FALSE)
[1] TRUE
> 
Martin Maechler, ETH Zurich