I would like to create a sequence of numbers within a group but starting from a specific string.

In this example, If the string matches UNP then sequence(seq coloumn) should start from the next row.

Only 1st instance of UNP should be considered not every instance on UNP for each group

CodePudding user response：

You can first create a column specifying the first occurrence of "UNP", then use cumsum() and lag() to calculate the Seq column.

library(dplyr)

df <- read.table(header = T, text = "
ColA    Colb    Seq
A   HM  0
A   RES 0
A   UNP 0
A   RES 1
A   RES 2
A   HM  3
B   HM  0
B   RES 0
B   UNP 0
B   RES 1
B   UNP 2
C   UNP 0") %>% 
  select(-Seq)

df %>% 
  group_by(ColA, Colb) %>% 
  mutate(seq_count = ifelse(first(Colb) == "UNP" & !duplicated(Colb), 1, 0)) %>% 
  group_by(ColA) %>% 
  mutate(Seq = lag(cumsum(cumsum(seq_count)), default = 0), .keep = "unused")
#> # A tibble: 12 × 3
#> # Groups:   ColA [3]
#>    ColA  Colb    Seq
#>    <chr> <chr> <dbl>
#>  1 A     HM        0
#>  2 A     RES       0
#>  3 A     UNP       0
#>  4 A     RES       1
#>  5 A     RES       2
#>  6 A     HM        3
#>  7 B     HM        0
#>  8 B     RES       0
#>  9 B     UNP       0
#> 10 B     RES       1
#> 11 B     UNP       2
#> 12 C     UNP       0

^{Created on 2022-03-31 by the reprex package (v2.0.1)}