Adding an repeated index for factors in data frame

Question

I have a data frame in which I want to add an index e.g. 1...n for each factor in my data frame. Here is an example with some dummy data.

factor
a        
a          


        

Answer 1

In base R using sequence and table:

df$index <- sequence(table(df$factor))

   # factor index
# 1       a     1
# 2       a     2
# 3       a     3
# 4       a     4
# 5       a     5
# 6       b     1
# 7       b     2
# 8       b     3
# 9       b     4
# 10      b     5
# 11      c     1
# 12      c     2
# 13      c     3
# 14      c     4

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Data

df <- data.frame(factor=factor(rep(letters[1:3], times = c(5,5,4))))