Recursively apply a Function to a List in R Programming – rapply() function

rapply() function in R Language is used to recursively apply a function to a list.

Syntax:
rapply(object, f, classes = “ANY”, deflt = NULL, how = c(“unlist”, “replace”, “list”))

Parameters:

object: represents list or an expression
f: represents function to be applied recursively
classes: represents class name of the vector or “ANY” to match any of the class
deflt: represents default result when how is not “replace”
how: represents modes

The modes in rapply() function are of 2 basic types. If how = “replace”, each element of the list object which is not itself is a list and has a class included in classes then each element of the list is replaced by the resulting value of the function f applied to the element.

If how = “list” or how = “unlist”, list object is copied and all non-list elements which are included in classes are replaced by the resulting value of the function f applied to the element and all other are replaced by deflt.

Example 1: Using replace mode

Output:

Whole List:
$a
[1] 1 2 3 4 5

$b
 [1] 100 101 102 103 104 105 106 107 108 109 110

$c
[1] "a" "b" "c"

Using replace mode:
$a
[1] 3

$b
[1] 105

$c
[1] "a" "b" "c"

Example 2: Using list mode

Output:

Whole List: 
$a
[1] 1 2 3 4 5

$b
 [1] 100 101 102 103 104 105 106 107 108 109 110

$c
[1] "a" "b" "c"

Using list mode:
$a
[1] 3

$b
[1] 105

$c
NULL

Example 3: Using unlist mode

Output:

Whole List:
$a
[1] 1 2 3 4 5

$b
 [1] 100 101 102 103 104 105 106 107 108 109 110

$c
[1] "a" "b" "c"

Using unlist mode:
  a   b 
  3 105