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)         


        

Answer 1

The dplyr/tidyr approach also works with data tables.

library(dplyr)
library(tidyr)
x %>% 
  separate(b, into = c("b1", "b2")) %>% 
  gather(b, "V1", b1:b2) %>%
  arrange(V1) %>%
  select(a, V1)

Or, using the standard evaluation forms:

x %>% 
  separate_("b", into = c("b1", "b2")) %>% 
  gather_("b", "V1", c("b1", "b2")) %>%
  arrange_(~ V1) %>%
  select_(~ a, ~ V1)

The case of different numbers of values in the b column is only slightly more complicated.

library(stringr)

x2 <- data.table(
  a = c(1:3, 1), 
  b = c('12 13', '14', '15 16 17', '18 19')
)

n <- max(str_count(x2$b, " ")) + 1
b_cols <- paste0("b", seq_len(n))
x2 %>% 
  separate_("b", into = b_cols, extra = "drop") %>% 
  gather_("b", "V1", b_cols) %>%
  arrange_(~ V1) %>%
  select_(~ a, ~ V1)