Retrieving dynamic sets of columns from a data.table

Question

Assigning sets of columns whose names are stored in a character vector is well discussed, but I can't seem to find a solution to retrieve columns and operate on them.

cols = c("x", "y")
DT = data.table(x = 10:15, y = 20:25)

> DT
    x  y
1: 10 20
2: 11 21
3: 12 22
4: 13 23
5: 14 24
6: 15 25

Desired Output

> DT[,paste(x,y)]

[1] "10 20" "11 21" "12 22" "13 23" "14 24" "15 25"

My goal is to get a unique identifier for each row based on the columns in cols. I can do this by hard coding the column names but I'd rather reference cols

Here are some syntax I have tried and their output.

get()

> DT[,get(cols)]

[1] 10 11 12 13 14 15

Parentheses

> DT[,(cols)]

[1] "x" "y"

List

> DT[,as.list(cols)]
   V1 V2
1:  x  y

with=F

> DT[,cols, with=F]

    x  y
1: 10 20
2: 11 21
3: 12 22
4: 13 23
5: 14 24
6: 15 25

This works but I am unable to operate on the columns:

> DT[,paste(cols), with=F]

    x  y
1: 10 20
2: 11 21
3: 12 22
4: 13 23
5: 14 24
6: 15 25

Fyi, paste(cols) is the same as cols, and in the latest version of data.table, with=FALSE isn't necessary in some of these cases. — Frank
– Frank, Commented Dec 14, 2016 at 18:02

akrun · Accepted Answer · 2016-12-14 17:33:48Z

5

We can use paste with do.call on the Subset of Data.table (.SD)

DT[, do.call(paste, .SD)]
#[1] "10 20" "11 21" "12 22" "13 23" "14 24" "15 25"

If this needs to be done on a subset of columns, specify those columns in .SDcols and then do the above

DT[, do.call(paste, .SD), .SDcols = cols]
#[1] "10 20" "11 21" "12 22" "13 23" "14 24" "15 25"

Or use a modification of OP's solution with mget

DT[, do.call(paste, mget(cols))]

answered Dec 14, 2016 at 17:33

akrun

891k38 gold badges590 silver badges700 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

nate Over a year ago

This was a very illustrative answer. Thanks.

Collectives™ on Stack Overflow

Retrieving dynamic sets of columns from a data.table

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related