I have a simple dataframe with two columns: df <- data.frame(x = c(1,1,2,2,3), y = c(rep(1:2,2),1), target = c('a','a','a','b','a')) I would like to compare the strings in the target column (find out whether they are equal or not, i.e., TRUE or FALSE) within every level of x (same number for x). First I would like to compare lines 1 and 2, then 3 and 4 ... My problem is that I am missing some comparisons, for example, line 5 has only one case instead of two - so it should turn out to be FALSE. Variable y indicates the first and second case within x. I played around with ddply doing something like: ddply(df, .(x), summarise, ifelse(as.character(df[df$y == '1',]$target), as.character(df[df$y == '2',]$target),0,1)) which is ugly ... and does not work ... Any insights how I could achieve this comparison? Thanks