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I have 2 lists of numbers (col1 & col2) below. I'd like to add 2 columns (col3 & col4) that do the following. col3 numbers col2 starting at 1 every time col2 changes (e.g. from b2 to b3). col4 is TRUE on the last occurrence for each value in col2. The data is sorted by col1, then col2 to begin. Note. values in col2 can occur for different values of col1. (i.e. I can have b1 for every value of col 1 (a, b, c)) I can get this working fine for ~5000 rows (~6 sec), but scaling to ~1 million rows it hangs up. Here is my code df$col3 <- 0 df$col4 <- FALSE stopHere <- nrow(df) c1 <- 'xxx' c2 <- 'xxx' for (i in 1:stopHere) { if (df[i, "col1"] != c1) { c2 <- 0 c3 <- 1 c1 <- df[i, "col1"] } if (df[i, "col2"] != c2) { df[i - 1, "col4"] <- TRUE c3 <- 1 c2 <- df[i, "col2"] } df[i, "col3"] <- c3 c3 <- c3 + 1 } This is my desired output. 1 a b1 1 FALSE 2 a b1 2 FALSE 3 a b1 3 TRUE 4 a b2 1 FALSE 5 a b2 2 TRUE 6 a b3 1 FALSE 7 a b3 2 FALSE 8 a b3 3 FALSE 9 a b3 4 FALSE 10 a b3 5 TRUE 11 b b1 1 FALSE 12 b b1 2 FALSE 13 b b1 3 FALSE 14 b b1 4 TRUE 15 b b2 1 FALSE 16 b b2 2 FALSE 17 b b2 3 FALSE 18 b b2 4 TRUE 19 c b1 1 TRUE 20 c b2 1 FALSE 21 c b2 2 FALSE 22 c b2 3 TRUE 23 c b3 1 FALSE 24 c b3 2 TRUE 25 c b4 1 FALSE 26 c b4 2 FALSE 27 c b4 3 FALSE 28 c b4 4 FALSE

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