Default missing values progressively in R

coalesce.r

# coalesce missing values
# e.g.
# > x = c(NA, 2, NA, 4)
# > y = c(1, NA, NA, 4)
# > z = c(1, NA, 3, 4)
# > coalesce(x, y, z)
coalesce = function(x, ...) {
  result = x
  for (y in list(...)) {
    mask = is.na(result)
    result = replace(result, mask, y[mask])
  }
  return(result)
}