purrr (version 0.2.3)

transpose: 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]].

Usage

transpose(.l, .names = NULL)

Arguments

.l

A list of vectors to zip. The first element is used as the template; you'll get a warning if a sub-list is not the same length as the first element.

.names

For efficiency, transpose() usually inspects the first component of .l to determine the structure. Use .names if you want to override this default.

Value

A list with indexing transposed compared to .l.

Details

Note that 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

# NOT RUN {
x <- rerun(5, x = runif(1), y = runif(5))
x %>% str()
x %>% transpose() %>% str()
# Back to where we started
x %>% transpose() %>% transpose() %>% str()

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

# Use simplify_all() to reduce to atomic vectors where possible
x <- list(list(a = 1, b = 2), list(a = 3, b = 4), list(a = 5, b = 6))
x %>% transpose()
x %>% transpose() %>% simplify_all()

# 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)
ll %>% transpose(.names = nms)
# }

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