Finding peaks in vector

Question

I\'m trying to find \"peaks\" in a vector, i.e. elements for which the nearest neighboring elements on both sides that do not have the same value have lower

Answer 1

This should work. The call to diff(sign(diff(x)) == -2 finds peaks by, in essence, testing for a negative second derivative at/around each of the unique values picked out by rle.

x <- c(0,1,1,2,3,3,3,2,3,4,5,6,5,7)

r <- rle(x)
which(rep(x = diff(sign(diff(c(-Inf, r$values, -Inf)))) == -2, 
          times = r$lengths))
# [1]  5  6  7 12 14

(I padded your vector with -Infs so that both elements 1 and 14 have the possibility of being matched, should the nearest different-valued element have a lower value. You can obviously adjust the end-element matching rule by instead setting one or both of these to Inf.)