Nested loops in R from columns of airplane routes - r

Question

I am trying to make a double nested loop in R. The source is stored in routes and looks like this:
Airline AirlineID SourceAirport SourceAirportID DestinationAirport DestinationAirportID Codeshare Stops Equipment
Where every row is a flight. I am concerned with the SourceAirportID and the DestinationAirportID. My double nested loop should have the first index as the SourceAirportID to access a list of DestinationPortIDs. The nested loop needs to be of variable size because airports will not all have the same number of destinations. Here is my attempt:
graph <- list()
for (i in 1:11922) {
graph[i] <- list()
}
it <- 1
sid <- as.numeric(routes[1,4])
for (i in 1:length(routes$SourceAirportID)) {
if (sid != as.numeric((routes[i,4]))) {
sid <- as.numeric(routes[i,4])
it <- 1
}
else {
it <- it + 1
}
graph[sid][it] <- routes[sid,6]
}
Here are the first 4 rows of routes:
Airline AirlineID SourceAirport SourceAirportID DestinationAirport DestinationAirportID Codeshare Stops Equipment
17313 CG 1308 GKA 1 HGU 3 0 DH8 DHT
17314 CG 1308 GKA 1 LAE 4 0 DH8
17315 CG 1308 GKA 1 MAG 2 0 DH8
17316 CG 1308 GKA 1 POM 5 0 DH8
So I'm trying to get the list at graph[1] to contain 3 4 2 5. Instead graph[1] is null, graph[2] contains 4, graph[3] contains 2 and graph[3] contains 5. My code also throws over 50 warnings, so clearly I am doing something very wrong.
Appreciate the help!

Answer 1

I'm not sure if this is what you want:
#reproduciable data:
routes <- data.frame(SourceAirport=c('a','a','b','c','a','b','b','c'),
DestinationAirport=c('Q','W','D','D','Q','E','R','T'))
View(routes)
graph <- list()
o <- unique(routes$SourceAirport)
#DestinationAirports of routes that share the same SourceAirport will be saved within an element in graph
for (i in 1:length(o)) {
graph[[i]] <- routes$DestinationAirport[routes$SourceAirport==o[i]]
}