Select columns by class (e.g. numeric) from a data.table

Question

Why doesn't this work with data.table?

It works with data.frame. Is there a way to do this with a data table?

x <- data.table(v1=1:20,v2=1:20,v3=1:20,v4=letters[1:20])
y <- x[ , sapply(x, is.numeric)]

This returns:

v1    v2    v3    v4
TRUE  TRUE  TRUE FALSE

The data.table FAQ vignette covers this with FAQs 1.1 and 1.2 — mnel
– mnel, Commented Aug 5, 2014 at 3:34
Very nearly a duplicate of select multiple columns in data table — thelatemail
– thelatemail, Commented Aug 5, 2014 at 4:17
Possible duplicate of Selecting only numeric columns from a data frame — Aramis7d
– Aramis7d, Commented Apr 4, 2016 at 7:07

Henrik · Accepted Answer · 2021-07-30 18:13:09Z

44

From data.table 1.13.0 ".SDcols accepts a function which is used to select the columns of .SD". Thus, simply .SDcols = is.numeric:

x[ , .SD, .SDcols = is.numeric]

edited Jul 30, 2021 at 18:13

Henrik

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answered Sep 5, 2015 at 11:31

Artem Klevtsov

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2 Comments

Mathieu L Over a year ago

Hello Hello, I tried. dt_c[,.SD, .SDcols = sapply(dt_c, as.character)] It's not working. a solution

Artem Klevtsov Over a year ago

s/as.character/is.character/.

Mike.Gahan · Accepted Answer · 2014-08-05 03:23:43Z

15

data.table needs the with=FALSE to grab column numbers.

tokeep <- which(sapply(x,is.numeric))
x[ , tokeep, with=FALSE]

answered Aug 5, 2014 at 3:23

Mike.Gahan

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1 Comment

thelatemail Over a year ago

Is the which even needed? - x[,sapply(x,is.numeric),with=FALSE]

akrun · Accepted Answer · 2014-08-05 08:19:08Z

3

You may also try:

 x1 <- x[,Filter(is.numeric, .SD)]
 head(x1,3)
 #   v1 v2 v3
#1:  1  1  1
#2:  2  2  2
#3:  3  3  3

Although, I have to admit that it is slow for bigger datasets.

edited Aug 5, 2014 at 8:19

answered Aug 5, 2014 at 7:51

akrun

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Jeff Bezos · Accepted Answer · 2020-06-17 17:54:21Z

0

Similar to @akrun's answer

Filter(is.numeric, x)

answered Jun 17, 2020 at 17:54

Jeff Bezos

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Eyayaw · Accepted Answer · 2020-06-17 18:15:53Z

0

We can write a custom helper calledwhere(), and then we can subset a data.frame/data.table where f is satisfied:

where <- function(x, f) {
  colnames(x)[vapply(x, f, logical(1))]
}

df[, where(df, is.numeric), with = FALSE]

edited Jun 17, 2020 at 18:15

answered Jun 17, 2020 at 18:09

Eyayaw

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