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

Try the following function:

 facSeq <- function(x){
     x.l <-length(x)
     x.f.l <- length(levels(x))
     sapply(1:x.f.l,function(y) cumsum(as.integer(x)%in%y))[1:x.l+x.l*(as.integer(x)-1)]
 }

Testing:

fac1 <- factor(rep(letters[1:3],each=5))

> data.frame(fac1,index=facSeq(fac1))
   fac1 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
15    c     5

More interesting example:

fac2 <- factor(sample(letters[1:5],20,replace=T))

> data.frame(fac2,index=facSeq(fac2))
   fac2 index
1     a     1
2     a     2
3     d     1
4     b     1
5     a     3
6     e     1
7     e     2
8     a     4
9     c     1
10    e     3
11    b     2
12    d     2
13    b     3
14    e     4
15    e     5
16    d     3
17    c     2
18    e     6
19    b     4
20    d     4