Find top-level element that contains a sub-element in a nested list

Question

I would like to get a list of top level elements that contain a particular sub-element. Suppose I have a nested list:

v <- list(A=list('a', 'b', 'c'), B=list('c','d','e'), C=list('d'))

I am trying to get the list of all top-level elements that have a particular sub element:

NOTE: prefer a base-R solution but would be interesting to see others

Must it be a list-of-lists, rather than [`base` package which has set operations like `is.element`](https://stat.ethz.ch/R-manual/R-devel/library/base/html/sets.html) on vectors, or `dplyr::setops`, or `data.table::setops` ? — smci, Mar 21 '19 at 13:52
Related question: [Test if a vector contains a given element](https://stackoverflow.com/questions/1169248/test-if-a-vector-contains-a-given-element) — smci, Mar 21 '19 at 13:52

moodymudskipper · Accepted Answer · 2019-03-21T13:53:34.533

2

You can use Filter :

names(Filter(function(x) "c" %in% x,v))
# [1] "A" "B"
names(Filter(function(x) "d" %in% x,v))
# [1] "B" "C"

Or with library purrr :

names(purrr::keep(v, ~"c" %in% .))
# [1] "A" "B"
names(purrr::modify_if(v, ~!"c" %in% ., ~NULL))
# [1] "A" "B"

edited Mar 21 '19 at 13:53

answered Mar 21 '19 at 13:46

moodymudskipper

score 2 · Answer 2 · answered Mar 21 '19 at 13:48

2

sapply alternative to Filter

names(v)[sapply(v, function(x) 'c' %in% x)]

answered Mar 21 '19 at 13:48

drmariod

2 Answers2