R: Create a sequential counter for each unique value of a given column

Question

Let's say I have the following dataframe:

personid date measurement
1         x     23
1         x     32
2         y     21
3         x     23
3         z     23
3         y     23

I want to sort this dataframe by the measurement column, and then create a new column that is a sequence along the sorted measurement column, like so:

personid date measurement id
1         x     23         2
1         x     32         3
2         y     21         1
3         x     23         2
3         z     23         2
3         y     23         2

My first instinct was to do something like:

unique_measurements <- data.frame(unique(sort(df$measurement)))
unique_dates$counter <- 1:nrow(unique_dates)

Now I basically have a data-frame that represents a mapping from a given measurement to the correct counter. I recognize this is the wrong way of doing this, but (1) how would I actually use this mapping to achieve my goals; (2) what is the right way of doing this?

Answer 1

Here's a simpler way to do this:

df$id <- match(df$measurement, sort(unique(df$measurement)))
#   personid date measurement id
# 1        1    x          23  2
# 2        1    x          32  3
# 3        2    y          21  1
# 4        3    x          23  2
# 5        3    z          23  2
# 6        3    y          23  2

Answer 2

Using factor as an intermediate:

df$id = as.integer(factor(df$measurement))

---

If you want to use your method, just use merge (though it might mess up the row order, use dplyr::left_join or data.table::merge instead to preserve row order in the original).

unique_measurements <- data.frame(measurement = sort(unique(df$measurement)))
unique_dates$id <- 1:nrow(unique_dates)
merge(df, unique_dates)