Layla Layla - 3 months ago 8
R Question

How to extract elements from a list in R

I have a list in R with the following elements:

[[812]]
[1] "" "668" "12345_s_at" "667" "4.899777748"
[6] "49.53333333" "10.10930207" "1.598228663" "5.087437057"

[[813]]
[1] "" "376" "6789_at" "375" "4.899655078"
[6] "136.3333333" "27.82508792" "2.20223398" "5.087437057"

[[814]]
[1] "" "19265" "12351_s_at" "19264" "4.897730912"
[6] "889.3666667" "181.5874908" "1.846451572" "5.087437057"


I know I can access them with something like
list_elem[[814]][3]
in case that I want to extract the third element of the position 814.
I need to extract the third element of all the list, for example
12345_s_at
, and I want to put them in a vector or list so I can compare their elements to another list later on. I have done the following code:

elem<-(c(listdata))
lp<-length(elem)
for (i in 1:lp)
{
newlist<-c(listdata[[i]][3]) ###maybe to put in a vector
print(newlist)
}


When I print the results I get the third element, but like this:

[1] "1417365_a_at"
[1] "1416336_s_at"
[1] "1416044_at"
[1] "1451201_s_at"


so I cannot traverse them with an index like
newlist[3]
, because it returns
NA

Where is my mistake?
Thanks

Answer

If you want to extrat the third element of each list element you can do:

List <- list(c(1:3), c(4:6), c(7:9))
lapply(List, '[[', 3)  # This retuns a list with only the third element
unlist(lapply(List, '[[', 3)) # This retuns a vector with the third elements

Using your example and taking into account @GSee comment you can do:

yourList <- list(c("","668","12345_s_at","667", "4.899777748","49.53333333",
       "10.10930207", "1.598228663","5.087437057"),
     c("","376", "6789_at",  "375",  "4.899655078","136.3333333",
       "27.82508792", "2.20223398",  "5.087437057"),
     c("", "19265", "12351_s_at", "19264", "4.897730912",
       "889.3666667", "181.5874908","1.846451572","5.087437057" ))

sapply(yourList, '[[', 3)
[1] "12345_s_at" "6789_at"    "12351_s_at"

Next time you can provide some data using dput on a portion of your dataset so that we can reproduce your problem easily.