R - add column that counts sequentially within groups but repeats for duplicates

Question

I\'m looking for a solution to add the column \"desired_result\" preferably using dplyr and/or ave(). See the data frame here, where the group is \"section\" and the unique

Answer 1

If exact enumeration is necessary and you need the desired result to be consistent (so that a same exhibit in a different section will always have the same number), you can try:

library(dplyr)
df <- data.frame(section = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L),
                 exhibit = c('a', 'b', 'c', 'c', 'a', 'b', 'b', 'c'))
if (is.null(saveLevels <- levels(df$exhibit)))
    saveLevels <- sort(unique(df$exhibit)) ## or levels(factor(df$exhibit))
df %>%
    group_by(section) %>%
    mutate(answer = as.integer(factor(exhibit, levels = saveLevels)))
## Source: local data frame [8 x 3]
## Groups: section
##   section exhibit answer
## 1       1       a      1
## 2       1       b      2
## 3       1       c      3
## 4       1       c      3
## 5       2       a      1
## 6       2       b      2
## 7       2       b      2
## 8       2       c      3

If/when a new exhibit appears in subsequent sections, they should get newly enumerated results. (Notice the last exhibit is different.)

df2 <- data.frame(section = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L),
                  exhibit = c('a', 'b', 'c', 'c', 'a', 'b', 'b', 'd'))
if (is.null(saveLevels2 <- levels(df2$exhibit)))
    saveLevels2 <- sort(unique(df2$exhibit))
df2 %>%
    group_by(section) %>%
    mutate(answer = as.integer(factor(exhibit, levels = saveLevels2)))
## Source: local data frame [8 x 3]
## Groups: section
##   section exhibit answer
## 1       1       a      1
## 2       1       b      2
## 3       1       c      3
## 4       1       c      3
## 5       2       a      1
## 6       2       b      2
## 7       2       b      2
## 8       2       d      4