Find all sequences with the same column value

Question

I have the following data frame:

╔══════╦═════════╗
║ Code ║ Airline ║
╠══════╬═════════╣
║    1 ║ AF      ║
║    1 ║ KL      ║
║    8 ║ AR      ║
║    8 ║ A         


        

Answer 1

There is likely a more efficient route, but this should fly:

# example data
d <- data.frame(code = c(1,1,8,8,8),
     airline = c("AF","KL","AR","AZ","DL"),
     stringsAsFactors = FALSE)

# merge d to itself on the code column.  This isn't necessarily efficient
d2 <- merge(d, d, by = "code")

# prune d2 to remove occasions where
# airline.x and airline.y (from the merge) are equal
d2 <- d2[d2[["airline.x"]] != d2[["airline.y"]], ]
# construct the combinations for each airline using a split, apply, combine
# then, use stack to get a nice structure for merging
d2 <- stack(
      lapply(split(d2, d2[["airline.x"]]),
        function(ii) paste0(ii$airline.y, collapse = ",")))

# merge d and d2.  "ind" is a column produced by stack
merge(d, d2, by.x = "airline", by.y = "ind")
#  airline code values
#1      AF    1     KL
#2      AR    8  AZ,DL
#3      AZ    8  AR,DL
#4      DL    8  AR,AZ
#5      KL    1     AF