Data frames have several extract operators for subsetting data.

Question: I found on SO that the $ operator cannot work with variables that are not static for subsetting . I would like a more in depth answer.

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$ is commonly used to subset named lists and data frames. They are ailke except that data frames have a tabular shape. Take the following codes for example:

mtcars$"mpg"    # working
mtcars$mpg      # working
mtcars[["mpg"]] # working
mtcars[[mpg]]   # NOT working

my_list <- list(a = 1:3, b = c(TRUE, FALSE))
my_list$"a"     # working
my_list$a       # working
my_list[["a"]]  # working
my_list[[a]]    # NOT working

[[]] is an indexing operator which takes numbers (e.g. my_list[[1]]) or names of the elements (e.g. my_list[["a"]]), but the name must be passed with character strings. On the other hand, $ doesn’t have such limitation. As a result, if you do mtcars$poll, R will go find a column named “poll” in mtcars, which should be NULL (nothing), and that probably triggers errors in later part of your codes. But when you do holder[[poll]], R will read what poll is, take the string that returns, and do subsetting.

You can find more details in Len’s comprehensive article, Forms of the Extract Operator in R.