Obtain index of the first element of the repeated value in a vector in R

Question

How can we in a, obtain the index of the first element of the repeated value (index of first 1, first 2, first 3 ...)?

a <- c(rep(1, 3), rep(2, 2), rep(3, 1), rep(4, 2))

desired.output <- c(1, 4, 6, 7)

Essentially answered here - https://stackoverflow.com/questions/5577727/is-there-an-r-function-for-finding-the-index-of-an-element-in-a-vector — thelatemail, Nov 19 '19 at 22:20
Also `sapply(unique(a), function(x) which.max(x == a))` but there are much better answers already. — Ronak Shah, Nov 19 '19 at 23:49

markus · Answer 1 · 2019-11-19T22:25:30.263

6

Another option is match

match(unique(a), a)
# [1] 1 4 6 7

From help('match')

match returns a vector of the positions of (first) matches of its first argument in its second.

edited Nov 19 '19 at 22:25

answered Nov 19 '19 at 22:19

markus

akrun · Accepted Answer · 2019-11-20T00:33:20.547

3

One option is tapply

as.vector(tapply(seq_along(a), a, FUN = `[`, 1))
#[1] 1 4 6 7

Or using

which(!duplicated(a))

Or with which and diff

which(c(TRUE, !!diff(a)))

Or when the vector is not numeric

which(c(TRUE, a[-1] != a[-length(a)]))

edited Nov 20 '19 at 00:33

answered Nov 19 '19 at 22:14

akrun

2 Answers2