Section 23 List Subsetting

23.1 Subset a list

Example list:

x <- c(1:20); y <- rep(c('R1','R2','R3'), each=4); z <- rep(c(T,F), length=4)

y <- list(Age=x, Region=y, Epi=z)

Index	Explanation	Example.
$	Get the elements of the list if the list has names	y$Age
[i]	Single square bracket with an integer to get the specific element of the list. The outcome is a LIST.	y[1]; y[3]
[[i]]	Double square bracket with an integer to get the specific element of the list. The outcome is the corresponding CLASS.	y[[1]]; y[[3]]
Positive integer	Select all elements corresponding to the integer value of the specific element	y[2]; y[[2]]; y[[2]][1]
Negative integer	Remove the element(s) corresponding to the integer value(s). Only valid with [i].	y[-2]
Zero	Select no element. Only valid with [i].	y[0]
Blank	Select all elements for the specific dimension. Only valid with [i].	y[]
Logical values	Select the element corresponding to the logical value TRUE. Only valid with [i].	y[c(T,T,F)]
Names	Select the element corresponding to the named value	y$Age; y['Age']; y[['Age']]

[i] indicates the single square bracket with an integer index value

23.2 Example

?list

x <- c(1:20)
y <- rep(c('R1','R2','R3'), each=4)
z <- rep(c(T,F), length=4)
y <- list(Age=x, Region=y, Epi=z)
y

y$Age
y[1]
y[3]
y[[1]]
y[[3]]
y[2]
y[[2]]
y[-2]
y[0]
y[]
y[c(T,T,F,F)]
y$Age
y['Age']
y[['Age']]



x <- c(1:20)
y <- rep(c('R1','R2','R3'), each=4)
z <- rep(c(T,F), length=4)
DF <- data.frame(ID=letters[1:10],
                 Pos=c(T,F),
                 Time=sample(1:100, size=10, replace=TRUE),
                 stringsAsFactors=FALSE)

y <- list(Age=x, Region=y, Epi=z, Samp=DF)
y

is.list(y)
names(y)

attributes(y)
str(y)

y[-1]
y[[-1]]  # Error
y[4]
y[[4]]
y[-1]
y[0]
y[]
y[c(T,T,F,T)]
y['Age']
y[c('Age','Samp')]
y[['Age']]
y[[c('Age','Samp')]]  # Error
y$Age
y$Age[3:5]
y$Age[-c(1:3)]

y[1]
y[1][1]   # Only one element, so additional [1] redundant
y[[1]][1]
y[[1]][-c(1:3)]

x <- c(1:20)
y <- rep(c('R1','R2','R3'), each=4)
z <- rep(c(T,F), length=4)
DF <- data.frame(ID=letters[1:10],
                 Pos=c(T,F),
                 Time=sample(1:100, size=10, replace=TRUE),
                 stringsAsFactors=FALSE)
lstZ <- list(num=c(1:10),
             char=LETTERS[1:5],
             logic=c(T,F,F,T),
             df=data.frame(X=c(1:10), Y=c('M','F'), Z=c(T,F,T,F,T)))

z <- list(Age=x, Region=y, Epi=z, Samp=DF, lstZ=lstZ)
z

names(z)
str(z)

z[[5]][4]
z[[5]][[4]]
z[[5]]$df
z[[5]][[4]][c('X','Y')]
z[[5]][[4]][,c(1,2)]

z[5][[1]][[4]]  # Note: str(z[5])
z[5][[2]][[4]]  # Error: z[5] is a list of 1

Note:

• Two operators to handle list: [] and [[]]

• If list is a train includes engine with compartments, then:

  • [] returns a new list (a new train): engine and selected compartment
  • [[]] returns the contents (only the compartment)

• Remember that the [] operator always returns an object of the same class as the original. Since the original object was a list, the [] operator returns a list.

• Most functions cannot work on list. The function needs access to the original class.

• R list can be subsetted using the combination of operators.

• Partial matching of names is allowed with [[]] and $ operator.