Removing multiple columns from R data.table with parameter for columns to remove

Question

I'm trying to manipulate a number of data.tables in similar ways, and would like to write a function to accomplish this. I would like to pass in a parameter containing a list of columns that would have the operations performed. This works fine when the vector declaration of columns is the left hand side of the := operator, but not if it is declared earlier (or passed into the function). The follow code shows the issue.

dt = data.table(a = letters, b = 1:2, c=1:13)
colsToDelete = c('b', 'c')
dt[,colsToDelete := NULL] # doesn't work but I don't understand why not.
dt[,c('b', 'c') := NULL] # works fine, but doesn't allow passing in of columns

The error is "Adding new column 'colsToDelete' then assigning NULL (deleting it)." So clearly, it's interpreting 'colsToDelete' as a new column name.

The same issue occurs when doing something along these lines

dt[, colNames := lapply(.SD, adjustValue, y=factor), .SDcols = colNames]

I new to R, but rather more experienced with some other languages, so this may be a silly question.

Answer 1

It's basically because we allow symbols on LHS of := to add new columns, for convenience: ex: DT[, col := val]. So, in order to distinguish col itself being the name from whatever is stored in col being the column names, we check if the LHS is a name or an expression.

If it's a name, it adds the column with the name as such on the LHS, and if expression, then it gets evaluated.

DT[, col := val] # col is the column name.

DT[, (col) := val]  # col gets evaluated and replaced with its value
DT[, c(col) := val] # same as above

The preferred idiom is: dt[, (colsToDelete) := NULL]

HTH