Lists

• NULL can be used as a placeholder in lists but not in vectors

list(NULL, NULL, month.name)

## [[1]]
## NULL
## 
## [[2]]
## NULL
## 
## [[3]]
##  [1] "January"   "February"  "March"     "April"     "May"       "June"     
##  [7] "July"      "August"    "September" "October"   "November"  "December"

# vs. 

c(1, NA, 3, NULL, 5)

## [1]  1 NA  3  5