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

If you don't want to modify the list in-place (e.g. for passing the list with an element removed to a function), you can use indexing: negative indices mean "don't include this element".

x <- list("a", "b", "c", "d", "e"); # example list

x[-2];       # without 2nd element

x[-c(2, 3)]; # without 2nd and 3rd

Also, logical index vectors are useful:

x[x != "b"]; # without elements that are "b"

This works with dataframes, too:

df <- data.frame(number = 1:5, name = letters[1:5])

df[df$name != "b", ];     # rows without "b"

df[df$number %% 2 == 1, ] # rows with odd numbers only

Answer 2

How about this? Again, using indices

> m <- c(1:5)
> m
[1] 1 2 3 4 5

> m[1:length(m)-1]
[1] 1 2 3 4

or

> m[-(length(m))]
[1] 1 2 3 4

Answer 3

Removing Null elements from a list in single line :

x=x[-(which(sapply(x,is.null),arr.ind=TRUE))]

Cheers

Answer 4

Don't know if you still need an answer to this but I found from my limited (3 weeks worth of self-teaching R) experience with R that, using the NULL assignment is actually wrong or sub-optimal especially if you're dynamically updating a list in something like a for-loop.

To be more precise, using

myList[[5]] <- NULL

will throw the error

myList[[5]] <- NULL : replacement has length zero

or

more elements supplied than there are to replace

What I found to work more consistently is

myList <- myList[[-5]]

Answer 5

Here is how the remove the last element of a list in R:

x <- list("a", "b", "c", "d", "e")
x[length(x)] <- NULL

If x might be a vector then you would need to create a new object:

x <- c("a", "b", "c", "d", "e")
x <- x[-length(x)]

• Work for lists and vectors

Answer 6

I would like to add that if it's a named list you can simply use within.

l <- list(a = 1, b = 2)    
> within(l, rm(a))
$b
[1] 2

So you can overwrite the original list

l <- within(l, rm(a)) 

to remove element named a from list l.