Victor K. - 1 month ago 5x

R 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`

`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

The following would work

`a`

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

The following

`x`

`> 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?

Answer

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))}]
```

Source (Stackoverflow)

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