Find index of change in a column

Question

Is there a way to find the indices of change of factors in a column with R? For example:

x <- c("aaa", "aaa", "aaa", "bbb", "bbb", "ccc", "ddd")

would return 3, 5, 6

Answer 1

You could try to compare shifted vectors, e.g.

which(x[-1] != x[-length(x)])
## [1] 3 5 6

This will work both on characters and factors

Answer 2

which(!!diff(as.numeric(x)))
[1] 3 5 6

The assumption is that you really have factors. They are saved internally with numerical values. So when the difference is taken, a one will result at every change. A second coercion is that zeroes are considered FALSE and other numbers TRUE. which locates the TRUE values aka non-zeroes.

Answer 3

rle can be used for this:

head(cumsum(rle(x)$lengths), -1)
[1] 3 5 6