[R] Is simplify2array working for dimension > 2?

[Bert Gunter]

Jean-Claude:

Well, here's my "explanation". Caveat emptor!

Note that:
"simplify2array() is the utility called from sapply() when simplify is
not false"

and

> sapply(a, I, simplify = "array")
     [,1]   [,2]
[1,] list,2 list,2
[2,] list,2 list,2

So it seems that simplify2array() is not intended to operate in the
way that you expected, i.e. that recursive simplification is done.
And, indeed, if you check the code for the function, you will see that
that is the case. Perhaps the key phrase in the docs is in the
sapply() part that says:

"sapply is a user-friendly version and wrapper of lapply by default
returning a vector, matrix or, if simplify = "array", an array ***if
appropriate***, by applying simplify2array(). "   In other words,
recursive simplification is considered not "appropriate".

FWIW I also find this somewhat confusing and think that explicitly
saying that recursive simplification is not done might make it less
so. But writing docs that  address all our possible misconceptions is
pretty difficult (or impossible!), and maybe adding that explicit
caveat would confuse others even more... :-(

Cheers,
Bert









On Thu, Feb 8, 2024 at 12:12 AM Jean-Claude Arbaut <arbautjc using gmail.com> wrote:
>
> Reading the doc for ?simplify2array, I got the impression that with the
> 'higher = T' argument the function returns an array of dimension greater
> than 2 when it makes sense (the doc says "when appropriate", which is
> rather vague). I would expect
>
> a <- list(
>   list(list(1, 2), list(3, 4)),
>   list(list(5, 6), list(7, 8))
> )
> simplify2array(a, higher = T)
>
> to return the same (possibly up to a dimension permutation) as
> array(1:8, dim = c(2, 2, 2))
>
> However, in this case simplify2array returns a matrix (i.e. 2 dimensional
> array), whose elements are lists.
> It's the same as
> structure(list(list(1, 2), list(3, 4), list(5, 6), list(7, 8)), dim = c(2,
> 2))
>
> I get the same behavior with
> a <- list(
>   list(c(1, 2), c(3, 4)),
>   list(c(5, 6), c(7, 8))
> )
> but then the matrix elements are numeric vectors instead of lists.
>
> Did I miss something to get the result I expected with this function? Or is
> it a bug? Or maybe the function is not supposed to return a higher
> dimensional array, and I didn't understand the documentation correctly?
>
> There is a workaround, one can do for instance
> array(unlist(a), dim = c(2, 2, 2))
> and there may be better options (checking dimensions?).
>
> In case it's important: running R 4.3.2 on Debian 12.4.
>
> Best regards,
>
> Jean-Claude Arbaut
>
>         [[alternative HTML version deleted]]
>
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