How to join same named sublist elements and add an ID variable

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Suppose we have a list like this:

l <- list()
l[[1]] <- list()
l[[2]] <- list()
l[[3]] <- list()

names(l) <- c("A", "B", "C")

l[[1]][[1]] <- data.frame(6)
l[[1]][[2]] <- data.frame(3)
l[[1]][[3]] <- data.frame(8)
l[[1]][[4]] <- data.frame(7)

l[[2]][[1]] <- data.frame(5)
l[[2]][[2]] <- data.frame(4)
l[[2]][[3]] <- data.frame(7)
l[[2]][[4]] <- data.frame(9)

l[[3]][[1]] <- data.frame(1)
l[[3]][[2]] <- data.frame(6)
l[[3]][[3]] <- data.frame(2)
l[[3]][[4]] <- data.frame(8)

names(l[[1]]) <- c("aa", "bb", "cc", "dd")
names(l[[2]]) <- c("aa", "bb", "cc", "dd")
names(l[[3]]) <- c("aa", "bb", "cc", "dd")

I want to create a list l2 which contains 4 elements: aa, bb, cc and dd. Each of these elements would be the dataframe which would contain the values of aa, bb, cc and dd from list l and also an ID variable which would indicate if the element came from the A, B or C element of list l. So if we built the end result from scratch, it would look like this:

l2 <- list()

l2[[1]] <- data.frame(Value = c(6, 5, 1), ID = c("A", "B", "C"))
l2[[2]] <- data.frame(Value = c(3, 4, 6), ID = c("A", "B", "C"))
l2[[3]] <- data.frame(Value = c(8, 7, 2), ID = c("A", "B", "C"))
l2[[4]] <- data.frame(Value = c(7, 9, 8), ID = c("A", "B", "C"))

names(l2) <- c("aa", "bb", "cc", "dd")

However, I cannot build it from scratch, but instead I must "reshape" l to l2. What is the best way to do this? Preferred solution is in purrr.

purrr r list
1 Answers

The key is transpose(). You could set .id = "ID" in the inner map_dfr() to create a new column ID recording the names of sub-lists where each value comes when row-binding each element together.

library(purrr)

l %>%
  transpose() %>%
  map(~ map_dfr(.x, set_names, "Value", .id = "ID"))
Output
$aa
  ID Value
1  A     6
2  B     5
3  C     1

$bb
  ID Value
1  A     3
2  B     4
3  C     6

$cc
  ID Value
1  A     8
2  B     7
3  C     2

$dd
  ID Value
1  A     7
2  B     9
3  C     8
user_100689850
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