apply() functions

lapply(<list>, <function>): similar to purrr::map()

• applies a function to each element of a list
• returns a list of same length

Variations on this:

• sapply() and vapply() (try to) simplify the result
  • similar to purrr::map_vec()
• tapply() is to generate summaries for groups (cf. group_by() & summarize())