How to select columns programmatically in a data.table?

Question

I have the following data.table (DT): ``` DT <- data.table(V1 = 1:3, V2 = 4:6, V3 = 7:9) ``` I would like to select a subset of the variables programmatically (dynamically), by using an object where the relevant variable names are stored. For example, I want to select the two columns "V1" and "V3" stored in a variable "keep" ``` keep <- c("V1", "V3") ``` If we were to select the "keep" columns from a data.frame, the following would work: ``` DT[keep] ``` Unfortunately, this is not working when this is a data.table. I thought the data.frame and data.table are identical with this kind of behavior, but apperently they aren't. Anybody able to advise on the correct syntax?

Accepted Answer

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.

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
```

How to select columns programmatically in a data.table?

Solution

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