Create sequence based on grouping variable in R

Question

I'm looking for a way to create a sequence of numbers ($C) that ascend every time a string changes in($A). This is contingent on a grouping variable ($B). Example:

score 4 · Answer 1 · answered Jul 14 '15 at 16:17

Using the devel version of data.table, could use the new rleid function

library(data.table) # v >= 1.9.5
setDT(df)[, C := rleid(A), by = B]
#      A 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

Pierre L · Accepted Answer · 2015-07-14T16:48:01.663

2

Or with dplyr

df %>% group_by(B) %>% mutate(C = match(A, unique(A)))
# Source: local data frame [9 x 3]
# Groups: B
# 
#     A 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))))

edited Jul 14 '15 at 16:48

answered Jul 14 '15 at 16:41

Pierre L

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This'll not work for example if `df$A[6]` can be `"a1"`. – Arun Jul 15 '15 at 00:10
It appears that the column has been grouped. If not, that step would have to become the first in this solution. – Pierre L Jul 15 '15 at 01:03

Create sequence based on grouping variable in R

2 Answers2

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