R List Get First Item of Each Element

R list get first item of each element

sapply(d, "[[", 1) should do the trick.

A bit of explanation:

sapply: iterates over the elements in the list

[[: is the subset function. So we are asking sapply to use the subset function on each list element.

1 : is an argument passed to "[["

It turns out that "[" or "[[" can be called in a traditional manner which may help to illustrate the point:

x <- 10:1
"["(x, 3)
 # [1] 8

Select first element of nested list

Not much of a shortcut, but you can do this:

lapply(x, `[[`, 1)
# [[1]]
# [1] 1
#
# [[2]]
# [1] 3
#
# [[3]]
# [1] 5

Extract the first item of the list from nested lists

Try :

dt$col2 <-sapply(dt$col1 , "[", 1)

(The elements of your lists are not lists, just vectors, so one bracket is fine)

EDIT :

For a column inside a data.table :

 dt$col2 <- sapply(dt$col1,function(x){sapply(x,'[',1)})

extracting first value from a list

You need to subset the list, and then the vector in the list:

M[[1]][1]

In other words, M is a list of 1 element, a character vector of length 24.

You may want to use unlist M to convert it to just a vector.

M <- unlist(M)

Then you can just use M[1].

To remove the \" you can use sub:

sub("\"","",M[1])
[1] "  0.0337302"

Extract the first element for every vector in a list in R

You can check for number of rows in the matrix :

get_vec <- function(Data, n) {
  sapply(Data, function(x) if(nrow(x) >= n) x[n, 1] else NA)
}

vector1 <- get_vec(Data$Position, 1)
vector2 <- get_vec(Data$Position, 2)

Get access to the first row of a list in R

 A=list()
 A[[1]]=c(1,2)
 A[[2]]=c(3,4)
 A[[3]]=c(5,6)

 A
 # [[1]]
 # [1] 1 2

 # [[2]]
 # [1] 3 4

 # [[3]]
 # [1] 5 6

I. First Solution using sapply() function for just first elements

 sapply(A,'[[',1)
 # [1] 1 3 5

For Getting lets say 1st, 3rd, 4th elements of each nested list. Not applicable in this example

 sapply(A,`[`,c(1,3,4))

II. Second Solution using for loop

 for(i in 1:length(A)){
 print (A[[i]][1])
 }
 # [1] 1
 # [1] 3
 # [1] 5

How to get the nth element of each item of a list, which is itself a vector of unknown length

We can create a function using sapply

fun1 <- function(lst, n){
         sapply(lst, `[`, n)
   }
fun1(l, 1)
#[1] 1 3 5 6

fun1(l, 2)
#[1]  2  4 NA  7

Extract first Element of List in List

An option would be to use pluck

library(purrr)
map(lang_codes, pluck, 1)

---

Or in base R with lapply

sapply(lang_codes, `[[`, 1)
#[1] "en" "fr"

R - Filter List Using First Element of Each Item in the List

An option is keep to loop over the list of vectors and create a logical vector of length 1 with str_detect wrapped with any. Here the pattern checked is the character "K" or (|) "1" from the start (^) of the string

library(purrr)
library(stringr)
keep(l_vectors, ~ any(str_detect(.x, "^(K|1)")))
#[[1]]
#[1] "K"  "10" "20"

#[[2]]
#[1] "1"  "30" "40"

If we check only the first element, no need for any to be wrapped

keep(l_vectors, ~ str_detect(.x[1], "^(K|1)"))
#[[1]]
#[1] "K"  "10" "20"

#[[2]]
#[1] "1"  "30" "40"

If it is a fixed match, then as in @markus post, can use %in%

keep(l_vectors,  ~ .x[1] %in% c("K", "1"))

---

Related Topics