lapply over vector, use values as output list names

Question

I was wondering what the best way is to use the lapply famliy (or plyr) family of functions to take a vector, apply a function to the elements, and return a list whose names are the same as the values supplied in the first argument vector. So, for example if i do:

lapply(letters[1:3], function(x) NULL)

[[1]]
NULL

[[2]]
NULL

[[3]]
NULL

I'd like to return, instead of [[1]], [[2]], [[3]] as the name (or rather just plain index) of the list, the letters "a", "b", and "c" as the names of the output list.

I know from here that I can do this (How to create a list with names but no entries in R/Splus?) to create the list beforehand with the right names, but I am wondering if I can do this without pre-creating the list with the right names.

Thanks, Matt

Peyton · Accepted Answer · 2013-09-17T15:31:29.800

12

sapply has a USE.NAMES argument that will do what you want:

sapply(letters[1:3], function(x) NULL, simplify=FALSE, USE.NAMES=TRUE)
# $a
# NULL

# $b
# NULL

# $c
# NULL

As Josh O'Brien notes in the comments--and as you've figured out--simplify=FALSE prevents the output from being reduced to a simpler data structure, keeping the results consistent with what you'd get with lapply (well, besides the names of course).

edited Sep 17 '13 at 15:31

answered Sep 17 '13 at 15:22

Peyton

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Possibly also set `simplify=FALSE` to make the behavior more consistently like `lapply()`'s. – Josh O'Brien Sep 17 '13 at 15:24
Josh O'Brien : I was going to respond with the problem of when I replace `Null` with, say, a string, it collapses from a list to a vector. Your comment addresses that, thanks. – mpettis Sep 17 '13 at 15:27
Yeah, I'll edit my answer. – Peyton Sep 17 '13 at 15:29

lapply over vector, use values as output list names

1 Answers1