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collapse text by 2 ID's in a row

I have a question similar to this topic: "Collapse text by group in data frame [duplicate]"

group text
a a1
a a2
a a3
b b1
b b2
c c1
c c2
c c3
c c4

I would like to collapse by two sequential ID's (not the whole ID group) 

group text
a a1a2
a a2a3
b b1b2
c c1c2
c c2c3
c c3c4
Solution

  • Alternative tidyverse answer:

    library(tidyverse)
dat %>%
  group_by(group) %>%
  mutate(text=paste0(lag(text),text)) %>% slice(-1)

    Using data.table:

    library(data.table)
setDT(dat)
dat[, paste0(shift(text,1), text)[-1], by=group]

#   group   V1
#1:     a a1a2
#2:     a a2a3
#3:     b b1b2
#4:     c c1c2
#5:     c c2c3
#6:     c c3c4