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How is the name of the list and the name of the list as an item in the list?

I have a list of vectors (shown below). I would like to know in which elements of the list each element of vectors is. In other words, I would like to invert the list to create a new list whose names are taken from vectors.

What is the best way to do this?

 lst <- list(a=c(2, 3, 6, 10, 15, 17), b=c(4, 6, 9, 7, 6, 4, 3, 10), c=c(9, 2, 1, 4, 3), d=c(3, 6, 17)) lst $a [1] 2 3 6 10 15 17 $b [1] 4 6 9 7 6 4 3 10 $c [1] 9 2 1 4 3 $d [1] 3 6 17

I would like to receive the following answer.

 $`1` [1] "c" $`10` [1] "a" "b" $`15` [1] "a" $`17` [1] "a" "d" $`2` [1] "a" "c" $`3` [1] "a" "b" "c" "d" $`4` [1] "b" "b" "c" $`6` [1] "a" "b" "b" "d" $`7` [1] "b" $`9` [1] "b" "c"
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Here's the basic R path with stack and unstack :

 unstack(stack(lst), ind ~ values) # $`1` # [1] "c" # # $`2` # [1] "a" "c" # # $`3` # [1] "a" "b" "c" "d" # # $`4` # [1] "b" "b" "c" # # $`6` # [1] "a" "b" "b" "d" # # $`7` # [1] "b" # # $`9` # [1] "b" "c" # # $`10` # [1] "a" "b" # # $`15` # [1] "a" # # $`17` # [1] "a" "d"
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Here's an approach that uses split from R base after using melt from "reshape2":

 library(reshape2) x <- melt(lst) split(x$L1, x$value) # $`1` # [1] "c" # # $`2` # [1] "a" "c" # # $`3` # [1] "a" "b" "c" "d" # # $`4` # [1] "b" "b" "c" # # $`6` # [1] "a" "b" "b" "d" # # $`7` # [1] "b" # # $`9` # [1] "b" "c" # # $`10` # [1] "a" "b" # # $`15` # [1] "a" # # $`17` # [1] "a" "d"

Similarly, in the R database with stack :

 x <- stack(lapply(lst, c)) split(as.character(x$ind), x$values)

Or even more directly, if you are working with "lst" rather than "lst":

 x <- stack(lst) split(as.character(x$ind), x$values)

To clarify my comment, the more efficient way that I described was as follows:

 split(rep(names(lst), lapply(lst, nrow)), unlist(lst, use.names = FALSE))

For a much larger lst we get the following:

 fun1 <- function() split(rep(names(lst), lapply(lst, nrow)), unlist(lst, use.names = FALSE)) fun2 <- function() { x <- stack(lapply(lst, c)) ; split(as.character(x$ind), x$values) } fun3 <- function() { x <- melt(lst) ; split(x$L1, x$value) } fun4 <- function() unstack(stack(lapply(lst, as.vector)), ind ~ values) ## Make lst much bigger lst <- unlist(replicate(10000, lst, simplify = FALSE), recursive=FALSE) names(lst) <- make.unique(names(lst)) library(microbenchmark) system.time(fun3()) # user system elapsed # 48.338 0.000 47.643 microbenchmark(fun1(), fun2(), fun4(), times = 5) # Unit: milliseconds # expr min lq median uq max neval # fun1() 454.5913 456.6793 473.901 555.8954 574.4394 5 # fun2() 922.1282 1028.4972 1034.872 1068.4761 1150.8072 5 # fun4() 1222.5296 1300.0643 1323.253 1339.2037 1421.1546 5
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unlist list to get all numbers in vectors. Then use these numbers to split the vector of the names of the list elements.

 split( rep(names(lst),times=sapply(lst,length)), unlist(lst) ) $`1` [1] "c" $`2` [1] "a" "c" $`3` [1] "a" "b" "c" "d" $`4` [1] "b" "b" "c" $`6` [1] "a" "b" "b" "d" $`7` [1] "b" $`9` [1] "b" "c" $`10` [1] "a" "b" $`15` [1] "a" $`17` [1] "a" "d"
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