Using names(obj) <- names(dt) could be problematic #5079
Description
Since set* and := set by reference, it seems to alter the names() character vector by reference too. Then the assignment like names(obj) <- names(dt) might lead to unexpected error.
Consider the following example:
library(data.table)
dt <- data.table(x = 1:10, y = 10:1)
dt
p <- list(1, 2)
names(p) <- names(dt)
p
dt[, z := 1:10]
p
names(p)
p <- list(1, 2, 3)
names(p) <- names(dt)
p
setcolorder(dt, c("z", "y", "x"))
p> library(data.table)
> dt <- data.table(x = 1:10, y = 10:1)
> dt
x y
<int> <int>
1: 1 10
2: 2 9
3: 3 8
4: 4 7
5: 5 6
6: 6 5
7: 7 4
8: 8 3
9: 9 2
10: 10 1
> p <- list(1, 2)
> names(p) <- names(dt)
> p
$x
[1] 1
$y
[1] 2
> dt[, z := 1:10]
'names' attribute [3] must be the same length as the vector [2]
> p
$x
[1] 1
$y
[1] 2
'names' attribute [3] must be the same length as the vector [2]
> names(p)
[1] "x" "y" "z"
'names' attribute [3] must be the same length as the vector [2]
> p <- list(1, 2, 3)
> names(p) <- names(dt)
> p
$x
[1] 1
$y
[1] 2
$z
[1] 3
> setcolorder(dt, c("z", "y", "x"))
> p
$z
[1] 1
$y
[1] 2
$x
[1] 3
I don't see a "don't do this" in wiki and the only thing about this is in the vignette Reference Semantics.
Should we somehow emphasize such problems more somewhere?