I'm trying to create append a table of quantiles in R for multiple subsets of data. Right now, I have a vector of ids (p_ids) in table DATA, which are not consecutive. For each value in p_ids, I am looking to list the quantile. So far, I've tried variations of: i <- 1 n <- 1 for (i in p_ids) { while(n <= nrow(data)) { quantiles[n] <- quantile(subset(alldata$variableA, alldata$variableB == i),probs = c(0,1,2,3)/3) n <- n + 1 } } I know my issue lies somewhere in the index, but I can't seem to get where the index should go. Suggestions?