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

I think all you would need is a table

dat <- structure(list(Code = c(1L, 1L, 8L, 8L, 8L),Airline = structure(c(1L, 5L, 2L, 3L, 4L),.Label = c("AF", "AR", "AZ", "DL", "KL"),class = "factor")),.Names = c("Code", "Airline"),class = "data.frame", row.names = c(NA, -5L))

tbl <- crossprod(table(dat))
diag(tbl) <- 0

#        Airline
# Airline AF AR AZ DL KL
#      AF  0  0  0  0  1
#      AR  0  0  1  1  0
#      AZ  0  1  0  1  0
#      DL  0  1  1  0  0
#      KL  1  0  0  0  0

dd <- data.frame(Airline = colnames(tbl),
                 shared = apply(tbl, 1, function(x)
                   paste(names(x)[x > 0], collapse = ', ')))

merge(dat, dd)
#   Airline Code shared
# 1      AF    1     KL
# 2      AR    8 AZ, DL
# 3      AZ    8 AR, DL
# 4      DL    8 AR, AZ
# 5      KL    1     AF