How to conditionally append incremented suffix to vector in R -

i have character vector in r reoccurring values:

vec <- c('22','22','22','45','45','45','56','67','67','67','89','90') 

i want append _1, _2, _3 similar values in vector vector like:

vec <- c('22_1','22_2','22_3','45_1','45_2','45_3','56'_1,'67_1','67_2' ... 

my question how increment conditionally ('56' singular , not need appended value). have tried writing loop , store occurrence counts in list, verbose , suspect there easier way of doing within apply family.

we can use ave , sequence grouped 'vec', paste original vector.

 res <- paste(vec, ave(vec, vec, fun=seq_along), sep="_") 

in case, need replace _1 elements have frequency of 1, use table frequency, subset elements matches names of 'table' having frequency 1 'res' , remove _ followed 0 or more characters sub.

 tbl <- table(vec)  names(tbl)[tbl==1]  i1 <- vec %in% names(tbl)[tbl==1]  res[i1] <-  sub('_.*', '', res[i1])  res  #[1] "22_1" "22_2" "22_3" "45_1" "45_2" "45_3" "56"   "67_1" "67_2" "67_3"  #[11] "89"   "90"   

---

or can use make.unique

 make.unique(vec, sep="_")  #[1] "22"   "22_1" "22_2" "45"   "45_1" "45_2" "56"   "67"   "67_1" "67_2"  #[11] "89"   "90"