vec_chop | R Documentation |
vec_chop()
provides an efficient method to repeatedly slice a vector. It
captures the pattern of map(indices, vec_slice, x = x)
. When no indices
are supplied, it is generally equivalent to as.list()
.
list_unchop()
combines a list of vectors into a single vector, placing
elements in the output according to the locations specified by indices
.
It is similar to vec_c()
, but gives greater control over how the elements
are combined. When no indices are supplied, it is identical to vec_c()
,
but typically a little faster.
If indices
selects every value in x
exactly once, in any order, then
list_unchop()
is the inverse of vec_chop()
and the following invariant
holds:
list_unchop(vec_chop(x, indices = indices), indices = indices) == x
vec_chop(x, ..., indices = NULL, sizes = NULL)
list_unchop(
x,
...,
indices = NULL,
ptype = NULL,
name_spec = NULL,
name_repair = c("minimal", "unique", "check_unique", "universal", "unique_quiet",
"universal_quiet"),
error_arg = "x",
error_call = current_env()
)
x |
A vector |
... |
These dots are for future extensions and must be empty. |
indices |
For For |
sizes |
An integer vector of non-negative sizes representing sequential
indices to slice For example,
|
ptype |
If |
name_spec |
A name specification for combining
inner and outer names. This is relevant for inputs passed with a
name, when these inputs are themselves named, like
See the name specification topic. |
name_repair |
How to repair names, see |
error_arg |
An argument name as a string. This argument will be mentioned in error messages as the input that is at the origin of a problem. |
error_call |
The execution environment of a currently
running function, e.g. |
vec_chop()
: A list where each element has the same type as x
. The size
of the list is equal to vec_size(indices)
, vec_size(sizes)
, or
vec_size(x)
depending on whether or not indices
or sizes
is provided.
list_unchop()
: A vector of type vec_ptype_common(!!!x)
, or ptype
, if
specified. The size is computed as vec_size_common(!!!indices)
unless
the indices are NULL
, in which case the size is vec_size_common(!!!x)
.
vec_chop()
vec_slice()
list_unchop()
vec_c()
vec_chop(1:5)
# These two are equivalent
vec_chop(1:5, indices = list(1:2, 3:5))
vec_chop(1:5, sizes = c(2, 3))
# Can also be used on data frames
vec_chop(mtcars, indices = list(1:3, 4:6))
# If `indices` selects every value in `x` exactly once,
# in any order, then `list_unchop()` inverts `vec_chop()`
x <- c("a", "b", "c", "d")
indices <- list(2, c(3, 1), 4)
vec_chop(x, indices = indices)
list_unchop(vec_chop(x, indices = indices), indices = indices)
# When unchopping, size 1 elements of `x` are recycled
# to the size of the corresponding index
list_unchop(list(1, 2:3), indices = list(c(1, 3, 5), c(2, 4)))
# Names are retained, and outer names can be combined with inner
# names through the use of a `name_spec`
lst <- list(x = c(a = 1, b = 2), y = 1)
list_unchop(lst, indices = list(c(3, 2), c(1, 4)), name_spec = "{outer}_{inner}")
# An alternative implementation of `ave()` can be constructed using
# `vec_chop()` and `list_unchop()` in combination with `vec_group_loc()`
ave2 <- function(.x, .by, .f, ...) {
indices <- vec_group_loc(.by)$loc
chopped <- vec_chop(.x, indices = indices)
out <- lapply(chopped, .f, ...)
list_unchop(out, indices = indices)
}
breaks <- warpbreaks$breaks
wool <- warpbreaks$wool
ave2(breaks, wool, mean)
identical(
ave2(breaks, wool, mean),
ave(breaks, wool, FUN = mean)
)
# If you know your input is sorted and you'd like to split on the groups,
# `vec_run_sizes()` can be efficiently combined with `sizes`
df <- data_frame(
g = c(2, 5, 5, 6, 6, 6, 6, 8, 9, 9),
x = 1:10
)
vec_chop(df, sizes = vec_run_sizes(df$g))
# If you have a list of homogeneous vectors, sometimes it can be useful to
# unchop, apply a function to the flattened vector, and then rechop according
# to the original indices. This can be done efficiently with `list_sizes()`.
x <- list(c(1, 2, 1), c(3, 1), 5, double())
x_flat <- list_unchop(x)
x_flat <- x_flat + max(x_flat)
vec_chop(x_flat, sizes = list_sizes(x))
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.