Assign unique IDs by group, according to pattern

Question

I have a grouped dataset, but the groups don\'t have a unique identifier, like this:

direction <- c(\'N\',\'S\',\'W\',\'N\',\'N\',\'S\',\'W\',\'N\',\'S\',\'W\         


        

Answer 1

You can get this result with == to identify elements with "N" and cumsum to construct indices. Then pull out the values from the stored vector LETTERS as suggested in balter's answer.

Here, cumsum sums over a logical vector, coercing it to numeric binary (1s, and 0s). It thus repeats the same value and increments any time an "N" was encountered.

 x$ID <- LETTERS[cumsum(x$direction == "N")]

This returns

x
   direction measurement ID
1          N           4  A
2          S           6  A
3          W           1  A
4          N           7  B
5          N           2  C
6          S           4  C
7          W           7  C
8          N           4  D
9          S           1  D
10         W           4  D

Answer 2

I think this should work:

direction <- c('N','S','W','N','N','S','W','N','S','W')
measurement <- c(4,6,1,7,2,4,7,4,1,4)
x <- data.frame(direction, measurement)

inds <- which(direction == 'N')
diffs <- diff(c(inds, length(direction)+1))

groups <- rep(seq_along(inds),diffs)

x$ID <- LETTERS[groups]
x

Note that if you have more than 26 groups, LETTERS[] will start returning NA. You can always just use:

x$ID <- groups

If that happens.