transpose: Transpose a list.

In purrr: Functional Programming Tools

Transpose a list.

Description

transpose() turns a list-of-lists "inside-out"; it turns a pair of lists into a list of pairs, or a list of pairs into pair of lists. For example, if you had a list of length n where each component had values a and b, transpose() would make a list with elements a and b that contained lists of length n. It's called transpose because x[[1]][[2]] is equivalent to transpose(x)[[2]][[1]].

This function was superseded in purrr 1.0.0 because list_transpose() has a better name and can automatically simplify the output, as is commonly needed. Superseded functions will not go away, but will only receive critical bug fixes.

Usage

transpose(.l, .names = NULL)

Arguments

.l	A list of vectors to transpose. The first element is used as the template; you'll get a warning if a subsequent element has a different length.
.names	For efficiency, transpose() bases the return structure on the first component of .l by default. Specify .names to override this.

Value

A list with indexing transposed compared to .l.

transpose() is its own inverse, much like the transpose operation on a matrix. You can get back the original input by transposing it twice.

Examples

x <- map(1:5, \(i) list(x = runif(1), y = runif(5)))
# was
x |> transpose() |> str()
# now
x |> list_transpose(simplify = FALSE) |> str()

# transpose() is useful in conjunction with safely() & quietly()
x <- list("a", 1, 2)
y <- x |> map(safely(log))
# was
y |> transpose() |> str()
# now:
y |> list_transpose() |> str()

# Previously, output simplification required a call to another function
x <- list(list(a = 1, b = 2), list(a = 3, b = 4), list(a = 5, b = 6))
x |> transpose() |> simplify_all()
# Now can take advantage of automatic simplification
x |> list_transpose()

# Provide explicit component names to prevent loss of those that don't
# appear in first component
ll <- list(
  list(x = 1, y = "one"),
  list(z = "deux", x = 2)
)
ll |> transpose()
nms <- ll |> map(names) |> reduce(union)
# was
ll |> transpose(.names = nms)
# now
ll |> list_transpose(template = nms)
# and can supply default value
ll |> list_transpose(template = nms, default = NA)

purrr documentation built on Aug. 10, 2023, 9:08 a.m.