Applying a function to each row of a data.table

Question

I looking for a way to efficiently apply a function to each row of data.table. Let's consider the following data table:

library(data.table)

library(stringr)



x <- data.table(a = c(1:3, 1), b = c('12 13', '14 15', '16 17', '18 19'))

> x

   a     b

1: 1 12 13

2: 2 14 15

3: 3 16 17

4: 1 18 19

Let's say I want to split each element of column

b

by space (thus yielding two rows for each row in the original data) and join the resulting data tables. For the example above, I need the following result:

The following would work if column
a
has only unique values:

x[, list(str_split(b, ' ')[[1]]), by = a]

The following almost works (unless there are some identical rows in the original data table), but is ugly when

x

has many columns and copies column b to the result, which I would like to avoid.

>     x[, list(str_split(b, ' ')[[1]]), by = list(a,b)]

   a     b V1

1: 1 12 13 12

2: 1 12 13 13

3: 2 14 15 14

4: 2 14 15 15

5: 3 16 17 16

6: 3 16 17 17

7: 1 18 19 18

8: 1 18 19 19

What would be the most efficient and idiomatic way to solve this problem?

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Answer 1

How about :

x
   a     b
1: 1 12 13
2: 2 14 15
3: 3 16 17
4: 1 18 19

x[,list(a=rep(a,each=2), V1=unlist(strsplit(b," ")))]
   a V1
1: 1 12
2: 1 13
3: 2 14
4: 2 15
5: 3 16
6: 3 17
7: 1 18
8: 1 19

Generalized solution given comment :

x[,{s=strsplit(b," ");list(a=rep(a,sapply(s,length)), V1=unlist(s))}]

Applying a function to each row of a data.table

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