R function for finding zero-crossing values in a vector. In the case of exact zeros, carries the previous sign forward. I.e. [+ + 0 -] has a zero-cross at position 4, but [+ + 0 +] has no zero-crosses.

find_0cross

find0cross <- function(x) {
  # x must be numeric
  stopifnot(is.numeric(x))

  # Length 0 and 1 can't have zero crossings
  if (length(x) < 2) {
    return(rep(FALSE, length(x)))
  }

  # Recursively find zero-crossing values
  is_cross <- function(y, s) {
    if (length(y) == 0) {
      return(logical(0))
    }
    s1 <- sign(y[1])
    yn <- y[-1]
    if (s1 == 0) {
      c(FALSE, is_cross(yn, s))
    } else {
      c(s1 * s < 0, is_cross(yn, s1))
    }
  }

  # Start looking at the first non-zero value
  if (all(x == 0)) {
    return(rep(FALSE, length(x)))
  }
  result <- logical(length(x))
  first_non0 <- which(x != 0)[1]
  result[1:first_non0] <- FALSE
  if (first_non0 == length(x)) {
    return(result)
  }
  s <- sign(x[first_non0])
  for (i in (first_non0 + 1):length(x)) {
    si <- sign(x[i])
    if (si == 0) {
      result[i] <- FALSE
    } else {
      result[i] <- s * si < 0
      s <- si
    }
  }
  result
}

x <- c(rep(5, 3), rep(0, 2), rep(-5, 3), 5, 5, 0, 5, -5)
# Should be 6, 9, 13
which(find0cross(x))