R - subscripting

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

subsetting

• creating a list for examples

myl <- list(one=10,two=20,three=30)
myi <- "one"

one

• subsetting is carried out with three different operators:
  • dollar: $
  • square bracket: [
  • double square bracket: [[
• these are generic functions
• there are four basic types of subscript indices
  • positive integers
  • negative integers
  • logical vectors
  • character vectors
• the first (left-most) index moves the fastest, the right-most index is the slowest (so a matrix is filled column by column)

the square bracket

• the returned value is of the same type of the value it is applied to

myl[c(1,2)]

10 20

• evaluate its second argument

myl[myi]

10

• does not use partial matching when extracting named elements

myl["on"]

• cannot be used for subscripting environments

the double square bracket

• extract a single value

myl[[1]]

10

• evaluate its second argument

myl[[myi]]

10

• does not use partial matching when extracting named elements

myl[["on"]]

the dollar

• extract a single value

myl$two

20

• does not evaluate its second argument

myl$myi

• uses partial matching extracting named elements

myl$on

10

• cannot be used for subscripting atomic vectors

subsetting with positive indices

• vector of positive integer values to indicate the set to be extracted

myv <- 1:10 
myi <- c(2,4,7)
myv[myi]
myv[c(2,4,7)]

[1] 2 4 7
[1] 2 4 7

• if the subscript is larger than the length of the vector, NAs are produce

myv[c(1,11,3)]

[1]  1 NA  3

• NA as subscript produces also a NA

myv[c(1,11,NA)]

[1]  1 NA NA

• zero as subscript will be ignored

myv[c(1,2,0,3)]

[1] 1 2 3

• if the subscript vector has the length zero, the result is also of length zero

myi <- numeric()
myv[myi]

integer(0)

• example: select elements which have even numbered subscripts

v <- rnorm(20)
myi <- seq(2,length(v),by=2)
myi
v[myi]

 [1]  2  4  6  8 10 12 14 16 18 20
 [1]  0.35380115 -0.02156840 -1.51804278  0.38278037  0.03867578 -1.25803279
 [7]  0.62863255  0.07111270 -0.73416837  0.18966622

subsetting with negative indices

• subscript vector consists of negative integer values indicating the indices not to be extracted
• zero as subscript will be ignored

myv[-c(1,2,0,3)]

[1]  4  5  6  7  8  9 10

• NAs are not allowed

myv[-c(1,NA,3)]

Error in myv[-c(1, NA, 3)] : 
  nur Nullen dürfen mit negativen Indizes gemischt werden

• zero length subscript produces zero length result

myv <- 1:10 
myi <- numeric()
myv[-myi]

 integer(0)

• positive and negatives subscripts cannot be mixed

to be continued... (most parts taken from Gentleman, R Programming for Bioinformatics) …