Specific column selection from data.table in R

Question

I have a data.table in R. The table has column names. I have a vector named colnames with a few of the column names in the table.

colnames<-c("cost1", "cost2", "cost3")

I want to select the columns whose names are in the vector colnames from the table. The name of the table is dt.

I have tried doing the following:

selected_columns <- dt[,colnames]

But this, does not work, and I get an error.
However, when I try the following, it works:

selected_columns <- dt[,c("cost1", "cost2", "cost3")]

I want to use the vector variable (colnames) to access the columns and not the c("..") method.
How can I do so?

Could you send your dt? Because with a dt that I made, it works for me. — Carpa
– Carpa, Commented Mar 15, 2022 at 7:41
Does tis answer your question : stackoverflow.com/a/12392269/13513328 — Waldi
– Waldi, Commented Mar 15, 2022 at 10:05

Ray Stylee · Accepted Answer · 2022-03-16 00:50:45Z

2

You can try like this:

dt[, .SD, .SDcols = colnames]

Meanwhile, data.table gives an alternative choice in recent version:

dt[, ..colnames]

edited Mar 16, 2022 at 0:50

answered Mar 15, 2022 at 8:00

Ray Stylee

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Konrad · Accepted Answer · 2022-03-15 09:05:04Z

0

Another nice alternatives is leveraging select from tidyverse/dplyr universe. select gives you a lot of flexibility when selecting columns from a data frame / tibble or data table.

library("data.table")
library("tidyverse")
df <- data.table(mtcars)
columns_to_select <- c("cyl", "mpg")
select(df, columns_to_select)

You can also skip quoting column names if you wish

select(df, c(cyl, mpg))

or leverage ellipsis and pass multiple quoted or unquoted names

Here, comparing multiple objects.

objs <- list(select(df, c(cyl, mpg)),
             select(df, cyl, mpg),
             select(df, "cyl", "mpg"))
outer(objs, objs, Vectorize(all.equal))

You may want to have a further look at dtplyr, which provides a bridge between data table and dplyr, if you want to go this route.

edited Mar 15, 2022 at 9:05

answered Mar 15, 2022 at 8:55

Konrad

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