Create a list from a complex comparison of two lists

Question

I am working on market transaction data where each observation contains the value of the variable of the buyer's id, and the value of the variable of the seller's id. For each observation (i.e each transaction), I would like to create a variable equal to the number of other transactions the associated seller has done with a different buyer than the one involved in this transaction. As a consequence, in the following

data <- data.frame(Buyer_id = c("001","001","002","001"), Seller_id = c("021","022","022","021"))

I would like to obtain:

Result <- list(0,1,1,0)

I searched for already existing answers for similar problems than mine, usually involving the function mapply(), and tried to implement them, but it proved unsuccessful.

Thank you very much for helping me.

Answer 1

Are you looking for something like this? If yes, then you might want to change your reproducible example to have a c instead of list when you construct your data.frame.

data <- data.frame(Buyer_id = c("001","001","002","001"), 
                   Seller_id = c("021","022","022","021"))

data$n <- NA
for (i in seq_len(nrow(data))) {
  seller <- as.character(data[i, "Seller_id"])
  buyer <- as.character(data[i, "Buyer_id"])
  
  with.buyers <- as.character(data[data$Seller_id == seller, "Buyer_id"])
  with.buyers <- unique(with.buyers)
  
  diff.buyers <- with.buyers[!(with.buyers %in% buyer)]
  
  data[i, "n"] <- length(diff.buyers)
}

  Buyer_id Seller_id n
1      001       021 0
2      001       022 1
3      002       022 1
4      001       021 0