How to look up specific indices of vectors in a list of vectors, where the indices are given in a ve-CodePudding

I would like to find an efficient operation to do the following look up in a list:

L = list(10:15,11:20)
a = c(3,7)
b = numeric()
for(i in 1:length(a)) b[i] = L[[i]][a[i]]

I think for loops are inefficient and I imagine this can be done faster using, for example, sapply. My main goal is to do this efficiently when L is long.

CodePudding user response：

Another apply method would be sapply().

sapply(1:length(a), function(x) L[[x]][a[x]])
[1] 12 17

CodePudding user response：

You could use Map or mapply. Since mapply can automatically simplify to a vector, we can could use that here to get b in one go:

b <- mapply(function(list_members, indices) list_members[indices],
       list_members = L, indices = a, SIMPLIFY = TRUE)

b
#> [1] 12 17

CodePudding user response：

We could use

library(dplyr)
stack(setNames(L, a)) %>%
   group_by(ind) %>% 
   summarise(out = values[[as.numeric(as.character(first(ind)))]]) %>%
   pull(out)
[1] 12 17

Or in base R using vapply which would be faster

vapply(seq_along(L), \(i) L[[i]][a[i]], numeric(1))
[1] 12 17

or use imap as a compact option

library(purrr)
imap_dbl(setNames(L, a), ~ .x[as.numeric(.y)])
 3  7 
12 17