Concatenate lists horizontally

Question

Consider a list of mixed classes like what returns from boxplot. I want to concatenate each list element, sort of stack each pair of elements horizontally.

(I clicked all of the "similar questions" and searched and am not aware of a base function to do this, modifyList being similar but not exactly what I want. I also looked quickly through the package rlist, but nothing struck me as similar. Also this question/answer is similar but only works for vectors)

f <- function(x) boxplot(mpg ~ vs, data = x, plot = FALSE)

(bp1 <- f(mtcars[mtcars$vs == 0, ]))
# $stats
#       [,1]
# [1,] 10.40
# [2,] 14.70
# [3,] 15.65
# [4,] 19.20
# [5,] 21.00
# 
# $n
# [1] 18
# 
# $conf
#          [,1]
# [1,] 13.97416
# [2,] 17.32584
# 
# $out
# [1] 26
# 
# $group
# [1] 1
# 
# $names
# [1] "0"


(bp2 <- f(mtcars[mtcars$vs == 1, ]))
# $stats
#      [,1]
# [1,] 17.8
# [2,] 21.4
# [3,] 22.8
# [4,] 30.4
# [5,] 33.9
# 
# $n
# [1] 14
# 
# $conf
#          [,1]
# [1,] 18.99955
# [2,] 26.60045
# 
# $out
# numeric(0)
# 
# $group
# numeric(0)
# 
# $names
# [1] "1"

The idea is to combine the two lists above into what one would get having simply done the following:

(bp  <- f(mtcars))
# $stats
#       [,1] [,2]
# [1,] 10.40 17.8
# [2,] 14.70 21.4
# [3,] 15.65 22.8
# [4,] 19.20 30.4
# [5,] 21.00 33.9
# 
# $n
# [1] 18 14
# 
# $conf
#          [,1]     [,2]
# [1,] 13.97416 18.99955
# [2,] 17.32584 26.60045
# 
# $out
# [1] 26
# 
# $group
# [1] 1
# 
# $names
# [1] "0" "1"

For the `mtcars` example, at least, you could just use `tmp <- lapply(1:length(bp1), function(x) cbind(bp1[[x]], bp2[[x]])); names(tmp) <- names(bp1)`. It's not super-robust, though. Flattening with `c(list, recursive = TRUE)` and reconstructing would work for more complicated structures, but would be a lot more work. [This question](https://xkcd.com/1205/), really. — alistaire, Jan 23 '16 at 08:38

rawr · Accepted Answer · 2017-03-19T17:17:21.500

This function seems to get the job done but is simple, so it can probably be broken easily.

cList <- function (x, y) {
  islist  <- function(x) inherits(x, 'list')
  get_fun <- function(x, y)
    switch(class(if (is.null(x)) y else x),
           matrix = cbind,
           data.frame = function(x, y)
             do.call('cbind.data.frame', Filter(Negate(is.null), list(x, y))),
           factor = function(...) unlist(list(...)), c)

  stopifnot(islist(x), islist(y))
  nn <- names(rapply(c(x, y), names, how = 'list'))
  if (is.null(nn) || any(!nzchar(nn)))
    stop('All non-NULL list elements should have unique names', domain = NA)

  nn <- unique(c(names(x), names(y)))
  z <- setNames(vector('list', length(nn)), nn)

  for (ii in nn)
    z[[ii]] <- if (islist(x[[ii]]) && islist(y[[ii]]))
      Recall(x[[ii]], y[[ii]]) else
        (get_fun(x[[ii]], y[[ii]]))(x[[ii]], y[[ii]])
  z
}

f <- function(x) boxplot(mpg ~ vs, data = x, plot = FALSE)
bp1 <- f(mtcars[mtcars$vs == 0, ])
bp2 <- f(mtcars[mtcars$vs == 1, ])
bp  <- f(mtcars)
identical(cList(bp1, bp2), bp)
# [1] TRUE

Also works on nested lists or lists not having the same elements in the same order, the caveat being the lists must be named, otherwise the function doesn't know which elements to concatenate.

l0 <- list(x = 1:5, y = matrix(1:4, 2), z = head(cars), l = list(1:5))
l1 <- list(x = factor(1:5), y = matrix(1:4, 2), z = head(cars), l = list(zz = 1:5))
l2 <- list(z = head(cbind(cars, cars)), x = factor('a'), l = list(zz = 6:10))

cList(l0, l2) ## should throw error
cList(l1, l2)

# $x
# [1] 1 2 3 4 5 a
# Levels: 1 2 3 4 5 a
# 
# $y
#      [,1] [,2]
# [1,]    1    3
# [2,]    2    4
# 
# $z
#   speed dist speed dist speed dist
# 1     4    2     4    2     4    2
# 2     4   10     4   10     4   10
# 3     7    4     7    4     7    4
# 4     7   22     7   22     7   22
# 5     8   16     8   16     8   16
# 6     9   10     9   10     9   10
# 
# $l
# $l$zz
# [1]  1  2  3  4  5  6  7  8  9 10

Update -- new version (approximately here) which can rbind or cbind rectangular objects (matrices, data frames)

cList <- function(x, y, how = c('cbind', 'rbind')) {
  if (missing(y))
    return(x)

  how <- match.arg(how)

  islist  <- function(x) inherits(x, 'list')
  get_fun <- function(x, y)
    switch(class(if (is.null(x)) y else x),
           matrix = match.fun(how),
           data.frame = function(x, y)
             do.call(sprintf('%s.data.frame', how),
                     Filter(Negate(is.null), list(x, y))),
           factor = function(...) unlist(list(...)), c)

  stopifnot(islist(x), islist(y))
  nn <- names(rapply(c(x, y), names, how = 'list'))

  if (is.null(nn) || any(!nzchar(nn)))
    stop('All non-NULL list elements should have unique names', domain = NA)

  nn <- unique(c(names(x), names(y)))
  z <- setNames(vector('list', length(nn)), nn)

  for (ii in nn)
    z[[ii]] <- if (islist(x[[ii]]) && islist(y[[ii]]))
      Recall(x[[ii]], y[[ii]]) else
        (get_fun(x[[ii]], y[[ii]]))(x[[ii]], y[[ii]])
  z
}

Not sure if it's worth the effort/time but, I think, a more flexible approach would be to keep the core of `cList` simple and use a -e.g.- `horiz_concat = function(...) UseMethod("horiz_concat")` + methods that does specific checking for each class of objects to be "concatenated horizontally". Then `cList` would need to worry only for the concatenation per se and let the respective function do the specific work. You could, also, make `cList` accept `...` arguments and, perhaps, add a check that if no "names" exist then do an elementwise concatenation? — alexis_laz, Jan 23 '16 at 17:21

Concatenate lists horizontally

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