Assignment to empty index (empty square brackets x[]<-) on LHS

Question

While looking at an answer posted recently on SO, I noticed an unfamiliar assignment statement. Instead of the usual form of myVar<- myValue, it used the fo

Answer 1

This is an intentional and documented feature. As joran mentioned, the documentation page "Extract" includes this in the "Atomic Vectors" section:

An empty index selects all values: this is most often used to replace all the entries but keep the attributes.

However, in the case of recursive objects (data.frames or lists, for example), the attributes are only kept for the subsetted object. Its parts don't get such protection.

Here's an example:

animals <- factor(c('cat', 'dog', 'fish'))
df_factor <- data.frame(x = animals)
rownames(df_factor) <- c('meow', 'bark', 'blub')
str(df_factor)
# 'data.frame': 3 obs. of  1 variable:
#   $ x: Factor w/ 3 levels "cat","dog","fish": 1 2 3

df_factor[] <- 'cat'
str(df_factor)
# 'data.frame': 3 obs. of  1 variable:
#   $ x: chr  "cat" "cat" "cat"
rownames(df_factor)
# [1] "meow" "bark" "blub"

df_factor kept its rownames attribute, but the x column is just the character vector used in the assignment instead of a factor. We can keep the class and levels of x by specifically replacing its values:

df_factor <- data.frame(x = animals)
df_factor$x[] <- 'cat'
str(df_factor)
# 'data.frame': 3 obs. of  1 variable:
#   $ x: Factor w/ 3 levels "cat","dog","fish": 1 1 1

So replacement with empty subsetting is very safe for vectors, matrices, and arrays, because their elements can't have their own attributes. But it requires some care when dealing with list-like objects.