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

Take the following as a comment that is posted as an answer just because this allows more convenient formatting.

for each code
  lookup all rows in the table where the value = code

ummm... sorry, I don't get how this psedudocode is related to your desired output

+--------------------+
| Airline SharedWith |
+--------------------+
| AF      "KL"       |
| KL      "AF"       |
| AR      "AZ","DL"  |
+--------------------+

The result of this pseudocode should rather be:

+---------------------+
+ Code  +  Airlines   +
+---------------------+
+  1    +  AF, KL     +
+  2    +  AR, AZ, DL +
+---------------------+

That is,

codes <- unique(dat$Code)
data.frame(Code=codes, Airlines = sapply(codes, function(x) paste(subset(dat, Code %in% x)$Airline, collapse=",")))