Select columns using a variable in R [duplicate]

Question

Given a data.table, how can I select a set of columns using a variable?

Example:

df[, 1:3]

is OK, but

idx <- 1:3
df[, idx]

is not OK: column named "idx" does not exist.

How can I use idx to select the columns in the simplest possible way?

Are you sure you have run idx <- 1:3 first? It should work. — bird
– bird, Commented Mar 22, 2021 at 17:28
Error in [.data.table(df, , idx) : j (the 2nd argument inside [...]) is a single symbol but column name 'idx' is not found. Perhaps you intended DT[, ..idx]. This difference to data.frame is deliberate and explained in FAQ 1.1. — quesadagranja
– quesadagranja, Commented Mar 22, 2021 at 17:28
If it is data.table. you need df[, ..idx] or df[, idx, with = FALSE] — akrun
– akrun, Commented Mar 22, 2021 at 17:44

akrun · Accepted Answer · 2021-03-22 17:45:22Z

2

We can use .. before the idx to select the columns in data.table or with = FALSE

library(data.table)
df[, ..idx]
df[, idx, with = FALSE]

answered Mar 22, 2021 at 17:45

akrun

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2 Comments

You know of any such function support natively by R?

@user1535147 if you are using base R, then df[idx] would work where df is a data.frame