通过data.table R中的字符变量引用列

Question

I am using data.table and I want to filter a data.table within a function where I am passing the name of the column as a character vector.

For a simple reproducible example let's take the mtcars dataset of base R.

I can write using data.table syntax:

mtcars[am == 1, .N ]

But what if the name of the variable of interest -- ie am -- is stored as a character vector, ie "am"?

Your advice will be appreciated.

Answer 1

One option is to use get ( ?get search object by name):

mtcars[get('am') == 1, .N]
# [1] 13

Answer 2

Another option is to specify it in the .SDcols

mtcars[, sum(.SD==1) ,.SDcols = 'am']
#[1] 13

We can also include multiple variables

mtcars[, sum(Reduce(`&`, lapply(.SD, `==`, 1))), .SDcols = c('am', 'carb')]
#[1] 4