group - Create sequence based on grouping variable in R -

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• add index contiguous runs of equal values 3 answers

i'm looking way create sequence of numbers ($c) ascend every time string changes in($a). contingent on grouping variable ($b). example:

a    b c a1   1 1 a1   1 1 a1   1 1 a10  1 2 a10  1 2 a2   1 3 a1   2 1 a20  2 2 a30  2 3 

or dplyr

df %>% group_by(b) %>% mutate(c = match(a, unique(a))) # source: local data frame [9 x 3] # groups: b #  #     b c # 1  a1 1 1 # 2  a1 1 1 # 3  a1 1 1 # 4 a10 1 2 # 5 a10 1 2 # 6  a2 1 3 # 7  a1 2 1 # 8 a20 2 2 # 9 a30 2 3 

with base r

df$c <- with(df, ave(as.character(a), b, fun=function(x) match(x, unique(x))))