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

There's the rlist package (http://cran.r-project.org/web/packages/rlist/index.html) to deal with various kinds of list operations.

Example (http://cran.r-project.org/web/packages/rlist/vignettes/Filtering.html):

library(rlist)
devs <- 
  list(
    p1=list(name="Ken",age=24,
      interest=c("reading","music","movies"),
      lang=list(r=2,csharp=4,python=3)),
    p2=list(name="James",age=25,
      interest=c("sports","music"),
      lang=list(r=3,java=2,cpp=5)),
    p3=list(name="Penny",age=24,
      interest=c("movies","reading"),
      lang=list(r=1,cpp=4,python=2)))

list.remove(devs, c("p1","p2"))

Results in:

# $p3
# $p3$name
# [1] "Penny"
# 
# $p3$age
# [1] 24
# 
# $p3$interest
# [1] "movies"  "reading"
# 
# $p3$lang
# $p3$lang$r
# [1] 1
# 
# $p3$lang$cpp
# [1] 4
# 
# $p3$lang$python
# [1] 2

Answer 2

I don't know R at all, but a bit of creative googling led me here: http://tolstoy.newcastle.edu.au/R/help/05/04/1919.html

The key quote from there:

I do not find explicit documentation for R on how to remove elements from lists, but trial and error tells me

myList[[5]] <- NULL

will remove the 5th element and then "close up" the hole caused by deletion of that element. That suffles the index values, So I have to be careful in dropping elements. I must work from the back of the list to the front.

A response to that post later in the thread states:

For deleting an element of a list, see R FAQ 7.1

And the relevant section of the R FAQ says:

... Do not set x[i] or x[[i]] to NULL, because this will remove the corresponding component from the list.

Which seems to tell you (in a somewhat backwards way) how to remove an element.

Hope that helps, or at least leads you in the right direction.

Answer 3

You can use which.

x<-c(1:5)
x
#[1] 1 2 3 4 5
x<-x[-which(x==4)]
x
#[1] 1 2 3 5

Answer 4

Using lapply and grep:

lst <- list(a = 1:4, b = 4:8, c = 8:10)
# say you want to remove a and c
toremove<-c("a","c")
lstnew<-lst[-unlist(lapply(toremove, function(x) grep(x, names(lst)) ) ) ]
#or
pattern<-"a|c"
lstnew<-lst[-grep(pattern, names(lst))]