forward operations to vectorised attributes

Reproduce the data.table examples with the errors package, especially the x[...] subset operation. Start at the errors class's subsetting operator and trace how data.table applies row filters to the column; done means the selected values and their error metadata have matching lengths without warnings or corruption.

The units package stores unit metadata for vectors and arrays as an attribute, and it seems to work fine with data.table:

library(data.table)
library(units)
#> udunits system database from /usr/share/udunits

(DT <- data.table(x = set_units(1:5, m), y = set_units(2, s)))
#>      x   y
#> 1: 1 m 2 s
#> 2: 2 m 2 s
#> 3: 3 m 2 s
#> 4: 4 m 2 s
#> 5: 5 m 2 s
DT[, z := x/y]
DT
#>      x   y       z
#> 1: 1 m 2 s 0.5 m/s
#> 2: 2 m 2 s 1.0 m/s
#> 3: 3 m 2 s 1.5 m/s
#> 4: 4 m 2 s 2.0 m/s
#> 5: 5 m 2 s 2.5 m/s
DT[x > set_units(3, m)]
#>      x   y       z
#> 1: 4 m 2 s 2.0 m/s
#> 2: 5 m 2 s 2.5 m/s

Similarly, the errors package stores uncertainty metadata for vectors and arrays, but this time the attribute is a vector of errors. Some operations are correctly forwarded:

library(errors)

(DT <- data.table(x = set_errors(1:5, 1:5/10), y = set_errors(2, 0.1)))
#>         x      y
#> 1: 1.0(1) 2.0(1)
#> 2: 2.0(2) 2.0(1)
#> 3: 3.0(3) 2.0(1)
#> 4: 4.0(4) 2.0(1)
#> 5: 5.0(5) 2.0(1)
DT[, z := x/y]
DT
#>         x      y       z
#> 1: 1.0(1) 2.0(1) 0.50(6)
#> 2: 2.0(2) 2.0(1)  1.0(1)
#> 3: 3.0(3) 2.0(1)  1.5(2)
#> 4: 4.0(4) 2.0(1)  2.0(2)
#> 5: 5.0(5) 2.0(1)  2.5(3)

As you can see above, errors are propagated for the division. But others not:

(DT.subset <- DT[x > set_errors(3)])
#> Warning: In '>' : boolean operators not defined for 'errors' objects,
#> errors dropped
#> Warning in exponent + value_digits: longer object length is not a multiple
#> of shorter object length
#> Warning in (scientific | (exponent > 4 + scipen | exponent < -3 - scipen))
#> & : longer object length is not a multiple of shorter object length
#> Warning in exponent + value_digits: longer object length is not a multiple
#> of shorter object length
#> Warning in (scientific | (exponent > 4 + scipen | exponent < -3 - scipen))
#> & : longer object length is not a multiple of shorter object length
#> Warning in exponent + value_digits: longer object length is not a multiple
#> of shorter object length
#> Warning in (scientific | (exponent > 4 + scipen | exponent < -3 - scipen))
#> & : longer object length is not a multiple of shorter object length
#> Error in dimnames(x) <- dn: length of 'dimnames' [1] not equal to array extent

The errors class does implement the subsetting operator, but it's not getting called, so the resulting errors object is corrupted (more errors than values):

DT$x
#> Errors: 0.1 0.2 0.3 0.4 0.5
#> [1] 1 2 3 4 5
DT.subset$x
#> Errors: 0.1 0.2 0.3 0.4 0.5
#> [1] 4 5