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         
a         
a        
a        
b        
b        
b        
b        
b
c
c
c
c

I would like to add an additional column which adds an index 1 to n for each factor separately. The resulant data frame would look like:

factor  index
a        1
a        2 
a        3 
a        4
a        5
b        1
b        2
b        3
b        4
b        5 
c        1
c        2
c        3
c        4

Can anyone explain how to do so? Thanks in advance.

Answer 1

One way is:

unlist(lapply(split(x, x), seq_along))

where x is your factor as a vector.

R> x <- factor(rep(letters[1:3], times = c(5,5,4))) ## your data
R> data.frame(factor = x, index = unlist(lapply(split(x, x), seq_along), 
+             use.names = FALSE))
   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

Another way, on a similar theme is to use table() and seq_len():

unlist(sapply(table(x), seq_len), use.names = FALSE)

And another way is to use the run-length encoding via rle():

R> rle(as.character(x))$lengths
[1] 5 5 4

which we can plug into the sapply() code instead of the table() call:

R> unlist(sapply(rle(as.character(x))$lengths, seq_len), use.names = FALSE)
 [1] 1 2 3 4 5 1 2 3 4 5 1 2 3 4