Question

How can we select multiple columns using a vector of their numeric indices (position) in data.table?

This is how we would do with a data.frame:

df <- data.frame(a = 1, b = 2, c = 3)
df[ , 2:3]
#   b c
# 1 2 3

Answer 1

For versions of data.table >= 1.9.8, the following all just work:

library(data.table)
dt <- data.table(a = 1, b = 2, c = 3)

# select single column by index
dt[, 2]
#    b
# 1: 2

# select multiple columns by index
dt[, 2:3]
#    b c
# 1: 2 3

# select single column by name
dt[, "a"]
#    a
# 1: 1

# select multiple columns by name
dt[, c("a", "b")]
#    a b
# 1: 1 2

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For versions of data.table < 1.9.8 (for which numerical column selection required the use of with = FALSE), see this previous version of this answer. See also NEWS on v1.9.8, POTENTIALLY BREAKING CHANGES, point 3.