#---------------------------------------
#
# A list is the most general sort of structure in R. A
# list can have elements of different types and different
# lengths. Its elements can be named (more common) or not
# (less common). Here's one without names.
#
list (1:3, c("Yes", "No"), c(T, F, F, T), log)
#
# Notice the double-square brackets. Here's one with names.
(mylist <- list (Red = 1:3, Blue = letters[5:9], Black = c(F, F, T)))
#
# We get at the elements of a named list with dollar-sign 
# notation. We can also use square brackets (to get a sub-list)
# or double-square brackets (to get the contents)
#
length (mylist); lengths (mylist)
mylist$Red + 1    # We can do math on a numeric vector
mylist$R + 1      # We only need to uniquely identify the name
mylist$Bl + 1     # Ambiguous
mylist["Red"]     # Returns a list with one item (full name needed)
mylist["Red"] + 1 # We can't do math on a list
mylist[["Red"]]   # Returns the element itself (full name needed)
mylist[[1]]   + 1 # same thing
mylist[c(2, 1)]   # Usual extraction for a sublist
#
# Handy utility functions include length (# of items in list),
# lengths (sizes of each item), unlist
length (mylist)
lengths (mylist)
unlist (mylist)   # make list into vector
class (mylist); is.list (mylist)
#
#
# Add a new item
mylist$White <- c(1, 1, 2, 3, 5, 8, 13)
mylist
#
# Remove an item by assigning NULL to it. In this case
# you will need the full name
#
mylist$Blue <- NULL
mylist
#
#

#
# lapply() operates on each element of a list, returning a
# list; sapply() does the same but tries to return a vector
#
mylist2 <- list (A = 1:10, B = 65:80, C = 220:250)
lengths (mylist2)
sapply (mylist2, class) # what class is each element

# Suppose we have some NAs in there. This comes up often.
mylist2$A[c(4, 6)] <- NA; mylist2$C[22:25] <- NA

sapply (mylist2, function (x) sum (is.na (x))) # vector
lapply (mylist2, function (x) sum (is.na (x))) # list
#
# sapply() will return a list if it has to
#
sapply (mylist2, function (x) which (x > 247))