How to select columns programmatically in a data.table?

Some more possibilities:

DT[, .SD, .SDcols = keep]
DT[, mget(keep)]

This is covered in FAQ 1.1, 1.2 and 2.17.

Some possibilities:

DT[, keep, with = FALSE]
DT[, c('V1', 'V3'), with = FALSE]
DT[, c(1, 3), with = FALSE]
DT[, list(V1, V3)]

The reason DF[c('V1','V3')] works as it does for a data.frame is covered in ?`[.data.frame`

Data frames can be indexed in several modes. When [ and [[ are used with a single vector index (x[i] or x[[i]]), they index the data frame as if it were a list. In this usage a drop argument is ignored, with a warning.

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From data.table 1.10.2, you may use the .. prefix when subsetting columns programmatically:

When j is a symbol prefixed with .. it will be looked up in calling scope and its value taken to be column names or numbers [...] It is experimental.

Thus:

DT[ , ..keep]
#    V1 V3
# 1:  1  7
# 2:  2  8
# 3:  3  9