• 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().

• vec_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().

If indices selects every value in x exactly once, in any order, then vec_unchop() is the inverse of vec_chop() and the following invariant holds:

vec_unchop(vec_chop(x, indices), indices) == x

## Usage

vec_chop(x, indices = NULL)

vec_unchop(
x,
indices = NULL,
ptype = NULL,
name_spec = NULL,
name_repair = c("minimal", "unique", "check_unique", "universal")
)

## Arguments

x

A vector

indices

For vec_chop(), a list of positive integer vectors to slice x with, or NULL. If NULL, x is split into its individual elements, equivalent to using an indices of as.list(vec_seq_along(x)).

For vec_unchop(), a list of positive integer vectors specifying the locations to place elements of x in. Each element of x is recycled to the size of the corresponding index vector. The size of indices must match the size of x. If NULL, x is combined in the order it is provided in, which is equivalent to using vec_c().

ptype

If NULL, the default, the output type is determined by computing the common type across all elements of x. Alternatively, you can supply ptype to give the output a known type.

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 outer = c(inner = 1), or when they have length greater than 1: outer = 1:2. By default, these cases trigger an error. You can resolve the error by providing a specification that describes how to combine the names or the indices of the inner vector with the name of the input. This specification can be:

• A function of two arguments. The outer name is passed as a string to the first argument, and the inner names or positions are passed as second argument.

• An anonymous function as a purrr-style formula.

• A glue specification of the form "{outer}_{inner}".

• An rlang::zap() object, in which case both outer and inner names are ignored and the result is unnamed.

See the name specification topic.

name_repair

How to repair names, see repair options in vec_as_names().

## Value

• vec_chop(): A list of size vec_size(indices) or, if indices == NULL, vec_size(x).

• vec_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).

## Dependencies of vec_chop()

• vec_slice()

## Dependencies of vec_unchop()

• vec_c()

## Examples

vec_chop(1:5)
#> [[1]]
#> [1] 1
#>
#> [[2]]
#> [1] 2
#>
#> [[3]]
#> [1] 3
#>
#> [[4]]
#> [1] 4
#>
#> [[5]]
#> [1] 5
#>
vec_chop(1:5, list(1, 1:2))
#> [[1]]
#> [1] 1
#>
#> [[2]]
#> [1] 1 2
#>
vec_chop(mtcars, list(1:3, 4:6))
#> [[1]]
#>                mpg cyl disp  hp drat    wt  qsec vs am gear carb
#> Mazda RX4     21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
#> Mazda RX4 Wag 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
#> Datsun 710    22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
#>
#> [[2]]
#>                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
#> Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
#> Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
#> Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1
#>

# If indices selects every value in x exactly once,
# in any order, then vec_unchop() inverts vec_chop()
x <- c("a", "b", "c", "d")
indices <- list(2, c(3, 1), 4)
vec_chop(x, indices)
#> [[1]]
#> [1] "b"
#>
#> [[2]]
#> [1] "c" "a"
#>
#> [[3]]
#> [1] "d"
#>
vec_unchop(vec_chop(x, indices), indices)
#> [1] "a" "b" "c" "d"

# When unchopping, size 1 elements of x are recycled
# to the size of the corresponding index
vec_unchop(list(1, 2:3), list(c(1, 3, 5), c(2, 4)))
#> [1] 1 2 1 3 1

# 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)
vec_unchop(lst, list(c(3, 2), c(1, 4)), name_spec = "{outer}_{inner}")
#> y_1 x_b x_a y_2
#>   1   2   1   1

# An alternative implementation of ave() can be constructed using
# vec_chop() and vec_unchop() in combination with vec_group_loc()
ave2 <- function(.x, .by, .f, ...) {
indices <- vec_group_loc(.by)$loc chopped <- vec_chop(.x, indices) out <- lapply(chopped, .f, ...) vec_unchop(out, indices) } breaks <- warpbreaks$breaks
wool <- warpbreaks\$wool

ave2(breaks, wool, mean)
#>  [1] 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704
#>  [8] 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704
#> [15] 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704
#> [22] 31.03704 31.03704 31.03704 31.03704 31.03704 31.03704 25.25926
#> [29] 25.25926 25.25926 25.25926 25.25926 25.25926 25.25926 25.25926
#> [36] 25.25926 25.25926 25.25926 25.25926 25.25926 25.25926 25.25926
#> [43] 25.25926 25.25926 25.25926 25.25926 25.25926 25.25926 25.25926
#> [50] 25.25926 25.25926 25.25926 25.25926 25.25926

identical(
ave2(breaks, wool, mean),
ave(breaks, wool, FUN = mean)
)
#> [1] TRUE