fnrow() returns error for 0-column data frames

[NicChr]

Hi, I noticed that fnrow() throws an out of bounds error when called on a data frame with 0 columns.
See an example below.

> library(collapse)
collapse 1.9.0.9000, see ?`collapse-package` or ?`collapse-documentation`

Attaching package: ‘collapse’

The following object is masked from ‘package:stats’:

    D

data(iris)
iris2 <- iris[, NULL, drop = FALSE]
iris3 <- iris[NULL, , drop = FALSE]
iris4 <- iris[NULL, NULL, drop = FALSE]
nrow(iris2)
[1] 150
ncol(iris2)
[1] 0
nrow(iris3)
[1] 0
ncol(iris3)
[1] 5
nrow(iris4)
[1] 0
ncol(iris4)
[1] 0
fnrow(iris2)
Error in .subset2(X, 1L) : subscript out of bounds
fncol(iris2)
[1] 0
fnrow(iris3)
[1] 0
fncol(iris3)
[1] 5
fnrow(iris4)
Error in .subset2(X, 1L) : subscript out of bounds
fncol(iris4)
[1] 0

Many thanks

[SebKrantz]

Thanks, I however don't see how this could be solved efficiently in R. The function is currently defined as

fnrow <- function (X) if (is.list(X)) length(.subset2(X, 1L)) else dim(X)[1L]

so the error comes from base:::.subset2 which fetches the first column. I could of course do an additional check length(X) > 0 or something like that, but the issue is that length(X) has some overhead on a data.frame. Another option would be rewriting the function in C, but this would incur some .Call overhead. I'll think about the C rewrite, although it would be a bit annoying for such a small function. For the time being I recommend to just use nrow() in cases where the data frame could have zero rows.

[NicChr]

Thanks, I would add that I think it affects other functions such as seq_row() and fsubset() for the special case of 0-column data frames.

Would something like this maybe work?

fnrow <- function (X){
    if (inherits(X, "data.frame")){
        length(attr(X, "row.names", exact = TRUE))
    } else {
        NULL
    }
}

You can maybe use the fact that data.frames have a row.names attribute though it's a bit slower than collapse::fnrow() but still faster than nrow().

[SebKrantz]

Thanks. I have rewritten fnrow in C. It is in the main branch now.