UC Business Analytics R Programming Guide

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Subsetting Lists

“If list x is a train carrying objects, then x[[5]] is the object in car 5; x[4:6] is a train of cars 4-6” - @RLangTip

To subset lists we can utilize the single bracket [ ], double brackets [[ ]], and dollar sign $ operators. Each approach provides a specific purpose and can be combined in different ways to achieve the following subsetting objectives:


Subset list and preserve output as a list

To extract one or more list items while preserving1 the output in list format use the [ ] operator:

# extract first list item
l2[1]
## $item1
## [1] 1 2 3

# same as above but using the item's name
l2["item1"]
## $item1
## [1] 1 2 3

# extract multiple list items
l2[c(1,3)]
## $item1
## [1] 1 2 3
## 
## $item3
## [1]  TRUE FALSE  TRUE  TRUE

# same as above but using the items' names
l2[c("item1", "item3")]
## $item1
## [1] 1 2 3
## 
## $item3
## [1]  TRUE FALSE  TRUE  TRUE


Subset list and simplify output

To extract one or more list items while simplifying1 the output use the [[ ]] or $ operator:

# extract first list item and simplify to a vector
l2[[1]]
## [1] 1 2 3

# same as above but using the item's name
l2[["item1"]]
## [1] 1 2 3

# same as above but using the `$` operator
l2$item1
## [1] 1 2 3

One thing that differentiates the [[ operator from the $ is that the [[ operator can be used with computed indices. The $ operator can only be used with literal names.


Subset list to get elements out of a list

To extract individual elements out of a specific list item combine the [[ (or $) operator with the [ operator:

# extract third element from the second list item
l2[[2]][3]
## [1] "c"

# same as above but using the item's name
l2[["item2"]][3]
## [1] "c"

# same as above but using the `$` operator
l2$item2[3]
## [1] "c"


Subset list with a nested list

If you have nested lists you can expand the ideas above to extract items and elements. We’ll use the following list l3 which has a nested list in item 2.

l3 <- list(item1 = 1:3, 
           item2 = list(item2a = letters[1:5], 
                        item3b = c(T, F, T, T)))
str(l3)
## List of 2
##  $ item1: int [1:3] 1 2 3
##  $ item2:List of 2
##   ..$ item2a: chr [1:5] "a" "b" "c" "d" ...
##   ..$ item3b: logi [1:4] TRUE FALSE TRUE TRUE

If the goal is to subset l3 to extract the nested list item item2a from item2, we can perform this multiple ways.

# preserve the output as a list
l3[[2]][1]
## $item2a
## [1] "a" "b" "c" "d" "e"

# same as above but simplify the output
l3[[2]][[1]]
## [1] "a" "b" "c" "d" "e"

# same as above with names
l3[["item2"]][["item2a"]]
## [1] "a" "b" "c" "d" "e"

# same as above with `$` operator
l3$item2$item2a
## [1] "a" "b" "c" "d" "e"

# extract individual element from a nested list item
l3[[2]][[1]][3]
## [1] "c"

  1. Its important to understand the difference between simplifying and preserving subsetting. Simplifying subsets returns the simplest possible data structure that can represent the output. Preserving subsets keeps the structure of the output the same as the input. See Hadley Wickham’s section on Simplifying vs. Preserving Subsetting to learn more. ↩ ↩2