[Rd] Inconsistency between row and nrow

[@vi@e@gross m@iii@g oii gm@ii@com]

It can be informative to look at what the actual functions being discussed do.

Dim is an internal, meaning written in some variant of C, perhaps:

> dim
function (x)  .Primitive("dim")

The function nrow, in my distribution, actually just calls dim() and throws away one dimension:

> nrow
function (x) 
dim(x)[1L]
<bytecode: 0x0000022df4d6f348>
<environment: namespace:base>

The function row is a bit related in calling dim in one of several ways:

> row
function (x, as.factor = FALSE) 
{
    if (as.factor) {
        labs <- rownames(x, do.NULL = FALSE, prefix = "")
        res <- factor(.Internal(row(dim(x))), labels = labs)
        dim(res) <- dim(x)
        res
    }
    else .Internal(row(dim(x)))
}
<bytecode: 0x0000022d87fbfb98>
<environment: namespace:base>

Does this shed any light on why the result may be inconsistent?

-----Original Message-----
From: R-devel <r-devel-bounces using r-project.org> On Behalf Of Marc Schwartz via R-devel
Sent: Sunday, September 8, 2024 8:27 AM
To: Gabor Grothendieck <ggrothendieck using gmail.com>; r-devel using r-project.org
Subject: Re: [Rd] Inconsistency between row and nrow

Hi Gabor,

In strictly reading the help files for both nrow() and row(), the 'x' argument in the former case is "a vector, array, data frame, or NULL.", whereas in the latter case it is "a matrix-like object, that is one with a two-dimensional dim.".

Thus, I would expect row() to fail on a >= 3-dimensional array, as your example shows.

In reading the help file for slice.index(), there is the following in the See Also section:

"row and col for determining row and column indexes; in fact, these are special cases of slice.index corresponding to MARGIN equal to 1 and 2, respectively when x is a matrix."

further differentiating the behavior of row() and col() as more specific implementations in the 2-dimensional case.

To my read then, the difference in behavior appears to be intentional and expected.

Regards,

Marc Schwartz


-----Original Message-----
From: R-devel <r-devel-bounces using r-project.org <mailto:r-devel-bounces using r-project.org>> on behalf of Gabor Grothendieck <ggrothendieck using gmail.com <mailto:ggrothendieck using gmail.com>>
Date: Sunday, September 8, 2024 at 7:31 AM
To: "r-devel using r-project.org <mailto:r-devel using r-project.org>" <r-devel using r-project.org <mailto:r-devel using r-project.org>>
Subject: [Rd] Inconsistency between row and nrow


In the following nrow provides the expected result but row gives an
error. I would have thought that they would both work or both fail.


aa <- array(dim = 5:3)


nrow(aa)
## [1] 5


row(aa)
## Error in row(aa) : a matrix-like object is required as argument to 'row'


# this does work:
slice.index(aa, 1)


-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com


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