How to assign an attribute to a variable stored in a data.table

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What is an efficient way to modify a variable or a series of variables in a data.table to add or change attributes? For example, the R Hmisc package label function will add a label attribute to a variable, and make it class "labelled" so that [. and other operators will preserve the label (the package defines [.labelled. Likewise there is Hmisc::units to specify units of measurement. I typically add labels and units by using for example

label(age) <- 'Age at study entry'               # or:
label(mydataframe$age) <- 'Age at study entry'   # or:
mydataframe <- upData(mydataframe, labels=c(age='Age at study entry'), units='year')

data.table is correctly preserving attributes for simple operations:

require(Hmisc)
x1 <- runif(10)
label(x1) <- 'This is a label'
units(x1) <- 'xunits'
x2 <- rnorm(10)
d <- data.table(x1, x2)
label(d$x1)      # label there
label(d[,'x1'])  # label gone
label(d[1:3,]$x1)# label there
label(d[,x1])    # label there
units(d[1:2,x1]) # yes

so the primary question is how to insert attributes into an already existing data.table object with a minimum of memory use/execution time.

data.table r
1 Answers

Setting variable attributes (including labels) in data.table:

To easily and efficiently (by reference) set the label you can use: setattr().

Example:

library(data.table)
iDT <- data.table(iris)

setattr(iDT$Species, "label", "Know the species")
attributes(iDT$Species)

# $levels
# [1] "setosa"     "versicolor" "virginica" 
# 
# $class
# [1] "factor"
# 
# $label
# [1] "Know the species"
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