Select / assign to data.table when variable names are stored in a character vector

Question

How do you refer to variables in a data.table if the variable names are stored in a character vector? For instance, this works for a data.frame:

Answer 1

For multiple columns and a function applied on column values.

When updating the values from a function, the RHS must be a list object, so using a loop on .SD with lapply will do the trick.

The example below converts integer columns to numeric columns

a1 <- data.table(a=1:5, b=6:10, c1=letters[1:5])
sapply(a1, class)  # show classes of columns
#         a           b          c1 
# "integer"   "integer" "character" 

# column name character vector
nm <- c("a", "b")

# Convert columns a and b to numeric type
a1[, j = (nm) := lapply(.SD, as.numeric ), .SDcols = nm ]

sapply(a1, class)
#         a           b          c1 
# "numeric"   "numeric" "character"