How can I remove an element from a list?

Question

I have a list and I want to remove a single element from it. How can I do this?

I\'ve tried looking up what I think the obvious names for this function would be in

Answer 1

In the case of named lists I find those helper functions useful

member <- function(list,names){
    ## return the elements of the list with the input names
    member..names <- names(list)
    index <- which(member..names %in% names)
    list[index]    
}


exclude <- function(list,names){
     ## return the elements of the list not belonging to names
     member..names <- names(list)
     index <- which(!(member..names %in% names))
    list[index]    
}  
aa <- structure(list(a = 1:10, b = 4:5, fruits = c("apple", "orange"
)), .Names = c("a", "b", "fruits"))

> aa
## $a
##  [1]  1  2  3  4  5  6  7  8  9 10

## $b
## [1] 4 5

## $fruits
## [1] "apple"  "orange"


> member(aa,"fruits")
## $fruits
## [1] "apple"  "orange"


> exclude(aa,"fruits")
## $a
##  [1]  1  2  3  4  5  6  7  8  9 10

## $b
## [1] 4 5

Answer 2

If you have a named list and want to remove a specific element you can try:

lst <- list(a = 1:4, b = 4:8, c = 8:10)

if("b" %in% names(lst)) lst <- lst[ - which(names(lst) == "b")]

This will make a list lst with elements a, b, c. The second line removes element b after it checks that it exists (to avoid the problem @hjv mentioned).

or better:

lst$b <- NULL

This way it is not a problem to try to delete a non-existent element (e.g. lst$g <- NULL)

Answer 3

if you'd like to avoid numeric indices, you can use

a <- setdiff(names(a),c("name1", ..., "namen"))

to delete names namea...namen from a. this works for lists

> l <- list(a=1,b=2)
> l[setdiff(names(l),"a")]
$b
[1] 2

as well as for vectors

> v <- c(a=1,b=2)
> v[setdiff(names(v),"a")]
b 
2

Answer 4

Just wanted to quickly add (because I didn't see it in any of the answers) that, for a named list, you can also do l["name"] <- NULL. For example:

l <- list(a = 1, b = 2, cc = 3)
l['b'] <- NULL

Answer 5

Use - (Negative sign) along with position of element, example if 3rd element is to be removed use it as your_list[-3]

Input

my_list <- list(a = 3, b = 3, c = 4, d = "Hello", e = NA)
my_list
# $`a`
# [1] 3

# $b
# [1] 3

# $c
# [1] 4

# $d
# [1] "Hello"

# $e
# [1] NA

Remove single element from list

 my_list[-3]
 # $`a`
 # [1] 3

 # $b
 # [1] 3

 # $d
 # [1] "Hello"

 # $e
 [1] NA

Remove multiple elements from list

 my_list[c(-1,-3,-2)]
 # $`d`
 # [1] "Hello"

 # $e
 # [1] NA

---

 my_list[c(-3:-5)]
 # $`a`
 # [1] 3

 # $b
 # [1] 3

---

 my_list[-seq(1:2)]
 # $`c`
 # [1] 4

 # $d
 # [1] "Hello"

 # $e
 # [1] NA

Answer 6

You can also negatively index from a list using the extract function of the magrittr package to remove a list item.

a <- seq(1,5)
b <- seq(2,6)
c <- seq(3,7)
l <- list(a,b,c)

library(magrittr)

extract(l,-1) #simple one-function method
[[1]]
[1] 2 3 4 5 6

[[2]]
[1] 3 4 5 6 7