How to reference column names that start with a number, in data.table

I think, this is what you're looking for, not sure. data.table is different from data.frame. Please have a look at the quick introduction, and then the FAQ (and also the reference manual if necessary).

require(data.table)
dt <- data.table("4PCS" = 1:3, y=3:1)
#    4PCS y
# 1:    1 3
# 2:    2 2
# 3:    3 1

# access column 4PCS
dt[, "4PCS"]

# returns a data.table
#    4PCS
# 1:    1
# 2:    2
# 3:    3

# to access multiple columns by name
dt[, c("4PCS", "y")]

Alternatively, if you need to access the column and not result in a data.table, rather a vector, then you can access using the $ notation:

dt$`4PCS` # notice the ` because the variable begins with a number
# [1] 1 2 3

# alternatively, as mnel mentioned under comments:
dt[, `4PCS`] 
# [1] 1 2 3

Or if you know the column number you can access using [[.]] as follows:

dt[[1]] # 4PCS is the first column here
# [1] 1 2 3

---

Edit:

Thanks @joran. I think you're looking for this:

dt[, `4PCS` + y]
# [1] 4 4 4

Fundamentally the issue is that 4CPS is not a valid variable name in R (try 4CPS <- 1, you'll get the same "Unexpected symbol" error). So to refer to it, we have to use backticks (compare`4CPS` <- 1)