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This function was superseded in purrr 1.0.0 because list_transpose()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 Superseded functions will not go away, but will only receive critical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.
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()
#> List of 2
#> $ x:List of 5
#> ..$ : num 0.0418
#> ..$ : num 0.67
#> ..$ : num 0.363
#> ..$ : num 0.163
#> ..$ : num 0.505
#> $ y:List of 5
#> ..$ : num [1:5] 0.405 0.864 0.134 0.942 0.627
#> ..$ : num [1:5] 0.509 0.392 0.625 0.731 0.166
#> ..$ : num [1:5] 0.924 0.772 0.161 0.15 0.654
#> ..$ : num [1:5] 0.6985 0.0753 0.7632 0.4615 0.4756
#> ..$ : num [1:5] 0.989 0.546 0.185 0.952 0.732
# now
x |> list_transpose(simplify = FALSE) |> str()
#> List of 2
#> $ x:List of 5
#> ..$ : num 0.0418
#> ..$ : num 0.67
#> ..$ : num 0.363
#> ..$ : num 0.163
#> ..$ : num 0.505
#> $ y:List of 5
#> ..$ : num [1:5] 0.405 0.864 0.134 0.942 0.627
#> ..$ : num [1:5] 0.509 0.392 0.625 0.731 0.166
#> ..$ : num [1:5] 0.924 0.772 0.161 0.15 0.654
#> ..$ : num [1:5] 0.6985 0.0753 0.7632 0.4615 0.4756
#> ..$ : num [1:5] 0.989 0.546 0.185 0.952 0.732
# transpose() is useful in conjunction with safely() & quietly()
x <- list("a", 1, 2)
y <- x |> map(safely(log))
# was
y |> transpose() |> str()
#> List of 2
#> $ result:List of 3
#> ..$ : NULL
#> ..$ : num 0
#> ..$ : num 0.693
#> $ error :List of 3
#> ..$ :List of 2
#> .. ..$ message: chr "non-numeric argument to mathematical function"
#> .. ..$ call : language .Primitive("log")(x, base)
#> .. ..- attr(*, "class")= chr [1:3] "simpleError" "error" "condition"
#> ..$ : NULL
#> ..$ : NULL
# now:
y |> list_transpose() |> str()
#> List of 2
#> $ result:List of 3
#> ..$ : NULL
#> ..$ : num 0
#> ..$ : num 0.693
#> $ error :List of 3
#> ..$ :List of 2
#> .. ..$ message: chr "non-numeric argument to mathematical function"
#> .. ..$ call : language .Primitive("log")(x, base)
#> .. ..- attr(*, "class")= chr [1:3] "simpleError" "error" "condition"
#> ..$ : NULL
#> ..$ : NULL
# 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()
#> $a
#> [1] 1 3 5
#>
#> $b
#> [1] 2 4 6
#>
# Now can take advantage of automatic simplification
x |> list_transpose()
#> $a
#> [1] 1 3 5
#>
#> $b
#> [1] 2 4 6
#>
# 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()
#> $x
#> $x[[1]]
#> [1] 1
#>
#> $x[[2]]
#> [1] 2
#>
#>
#> $y
#> $y[[1]]
#> [1] "one"
#>
#> $y[[2]]
#> NULL
#>
#>
nms <- ll |> map(names) |> reduce(union)
# was
ll |> transpose(.names = nms)
#> $x
#> $x[[1]]
#> [1] 1
#>
#> $x[[2]]
#> [1] 2
#>
#>
#> $y
#> $y[[1]]
#> [1] "one"
#>
#> $y[[2]]
#> NULL
#>
#>
#> $z
#> $z[[1]]
#> NULL
#>
#> $z[[2]]
#> [1] "deux"
#>
#>
# now
ll |> list_transpose(template = nms)
#> $x
#> [1] 1 2
#>
#> $y
#> $y[[1]]
#> [1] "one"
#>
#> $y[[2]]
#> NULL
#>
#>
#> $z
#> $z[[1]]
#> NULL
#>
#> $z[[2]]
#> [1] "deux"
#>
#>
# and can supply default value
ll |> list_transpose(template = nms, default = NA)
#> $x
#> [1] 1 2
#>
#> $y
#> [1] "one" NA
#>
#> $z
#> [1] NA "deux"
#>