CODEWITHSUNDEEP
rvector
rlpatrao
rlpatrao

Reputation: 575

How to extract elements from list of lists

I am a newbie to R. I have a list t1 in R which looks like 

[[1]]
[[1]][[1]]
[1] "a"       "control"


[[2]]
[[2]][[1]]
[1] "a"        "disease1"


[[3]]
[[3]][[1]]
[1] "a"        "disease2"


[[4]]
[[4]][[1]]
[1] "b"       "control"


[[5]]
[[5]][[1]]
[1] "b"        "disease1"


[[6]]
[[6]][[1]]
[1] "b"        "disease2"

I need to get a unique list of first elements into a vector i.e ["a", "b"] from this vector t1. How can I do this?

Upvotes: 26

Views: 82978

Answers (7)

Jeff Bezos
Jeff Bezos

Reputation: 2273

Using base R:

t1 <- list(list("a", "control"), list("a", "disease"), list("b", "control"))
id <- sapply(t1, function(x) {x[[1]]})
unique(id)

Upvotes: 3

Joe
Joe

Reputation: 8631

As an update in 2020, this is done easily and intuitively with purrr. Using @Gago-Silva's test list:

library(purrr)
t1 %>% flatten() %>% map(1) %>% as_vector()

Sublists are flattened to character vectors, element 1 is extracted from these and this list of one-element character vectors converted to one vector.

Also note that you can get a tibble back directly from the list of lists with 

t1 %>% flatten_dfc()

Upvotes: 5

M Yuan
M Yuan

Reputation: 41

Use package rlist, i.e.

library(rlist)
yourlist %>>% list.map(.[1])

Upvotes: 4

Matthew Plourde
Matthew Plourde

Reputation: 44614

rapply offers yet another option:

unique(rapply(t1, function(x) head(x, 1)))

Upvotes: 21

agstudy
agstudy

Reputation: 121608

I tried to treat the general case when one or more of the sublists contain more than one element.

For example:

ll <- 
        list(list(c("a","control")),
             list(c("b","disease1")),
             list(c("c","disease2"),c("c","disease2bis")), # 2 elements
             list(c("d","disease3")),
             list(c("e","disease4"))
)

You can do something like this :

 unlist(lapply(ll,                                 ## for each element in the big list
        function(x) 
             sapply(1:length(x),                   ## for each element in the sublist
             function(y)do.call("[[",list(x,y))))) ## retrieve x[[y]]


[1] "a"           "control"     "b"           "disease1"    "c"         
     "disease2"    "c"           "disease2bis" "d"           "disease3"   
[11] "e"           "disease4"

Upvotes: 5

Paul Hiemstra
Paul Hiemstra

Reputation: 60984

I would use do.call and rbind to concatenate the list into a data.frame. Then you can use unique on the first column to get the unique items (using the example given by @A.R.):

spam = do.call("rbind", lapply(t1, "[[", 1))
> spam
     [,1] [,2]      
[1,] "a"  "control"                                                         
[2,] "b"  "disease1" 
> unique(spam[,1])
[1] "a" "b"

Upvotes: 15

Gago-Silva
Gago-Silva

Reputation: 1971

Another way is to use unlist:

> t1=list(list(c("a","control")),list(c("b","disease1")))
> t1
[[1]]
[[1]][[1]]
[1] "a"       "control"


[[2]]
[[2]][[1]]
[1] "b"        "disease1"

> matrix(unlist(t1),ncol=2,byrow=TRUE)
     [,1] [,2]      
[1,] "a"  "control" 
[2,] "b"  "disease1"

Upvotes: 20

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