Last status in a vector equalling c(x, y, z)

# 'a' is a vector
# 'vals' is a vector
# returns a vector of same lengths as 'a' showing the last value in 'a' equal to any value in 'vals'
# 'fill' is the value given if none of the values in 'vals' has yet occurred in 'a'
# 'excl' is a vector specifying values of 'vals' that are to be excluded. Best used when vals is not specified (and defaults to unique(a))

last_status <- function(a, vals = unique(a), excl = NA, fill = NA) {
  if (!is.na(excl)) {vals <- setdiff(vals, excl)}
  i <- a %in% vals
  j <- a[i][cumsum(i)]
  c(rep(fill, length(a) - length(j)), j)
}

# example: use a sequence of prescribing events to determine whether a patient had a current prescription of citalopram at each time

pharma_history <- c('eucerin', 'benazepril_start', 'aspirin', 'aspirin', 'citalopram_start', NA, 'aspirin', 'eucerin', NA, 'citalopram_end', '%%%%', NA)
last_status(pharma_history, c('citalopram_start', 'citalopram_end')) # 'citalopram_start' indicates current prescription