R - subscripting 2

• subscripting with positive and negative indices
• character subscripting (vectors and lists)
• logical subscripting

subsetting with character indices - vectors and lists

• extract elements of named vectors, lists

named.x <- 1:10  # produce a vector containing the numbers 1...10
named.x   #  has no names yet

 [1]  1  2  3  4  5  6  7  8  9 10

• if the object has no names, NAs are produced

named.x["one"]

[1] NA

• name the vector:

names(named.x) <- c("one","two","three","four","five","six","seven","eight","nine","ten")
named.x

  one   two three  four  five   six seven eight  nine   ten 
    1     2     3     4     5     6     7     8     9    10

• now it works

named.x["one"]

one 
  1

• if names are duplicated only the first match is returned (the value with the lowest index)

names(named.x)<-rep(c("one","two","three","four","five"),rep(2,5))
named.x

  one   one   two   two three three  four  four  five  five 
    1     2     3     4     5     6     7     8     9    10

named.x[c("one","two","three")]

  one   two three 
    1     3     5

• NA is returned if a name do not match any element

named.x["twenty"]

<NA> 
  NA

• NA is returned if a subscript is character NA

named.x[c("one",NA,"three")]

  one  <NA> three 
    1    NA     5

• to find all occurences one can use %in% and then subset with the logical vector (you can do this in one step)

named.x[names(named.x) %in% c("one","two")]

one one two two 
  1   2   3   4