R purrr pluck 安全地获取或设置嵌套数据结构深处的元素

pluck(.x, ..., .default = NULL)

pluck(.x, ...) <- value

pluck_exists(.x, ...)

attr_getter() 用于创建适合与 pluck() 和 chuck() 一起使用的属性获取器。 modify_in() 用于将函数应用于拾取位置。

# Let's create a list of data structures:
obj1 <- list("a", list(1, elt = "foo"))
obj2 <- list("b", list(2, elt = "bar"))
x <- list(obj1, obj2)

# pluck() provides a way of retrieving objects from such data
# structures using a combination of numeric positions, vector or
# list names, and accessor functions.

# Numeric positions index into the list by position, just like `[[`:
pluck(x, 1)
#> [[1]]
#> [1] "a"
#> 
#> [[2]]
#> [[2]][[1]]
#> [1] 1
#> 
#> [[2]]$elt
#> [1] "foo"
#> 
#> 
# same as x[[1]]

# Index from the back
pluck(x, -1)
#> [[1]]
#> [1] "b"
#> 
#> [[2]]
#> [[2]][[1]]
#> [1] 2
#> 
#> [[2]]$elt
#> [1] "bar"
#> 
#> 
# same as x[[2]]

pluck(x, 1, 2)
#> [[1]]
#> [1] 1
#> 
#> $elt
#> [1] "foo"
#> 
# same as x[[1]][[2]]

# Supply names to index into named vectors:
pluck(x, 1, 2, "elt")
#> [1] "foo"
# same as x[[1]][[2]][["elt"]]

# By default, pluck() consistently returns `NULL` when an element
# does not exist:
pluck(x, 10)
#> NULL
try(x[[10]])
#> Error in x[[10]] : subscript out of bounds

# You can also supply a default value for non-existing elements:
pluck(x, 10, .default = NA)
#> [1] NA

# The map() functions use pluck() by default to retrieve multiple
# values from a list:
map_chr(x, 1)
#> [1] "a" "b"
map_int(x, c(2, 1))
#> [1] 1 2

# pluck() also supports accessor functions:
my_element <- function(x) x[[2]]$elt
pluck(x, 1, my_element)
#> [1] "foo"
pluck(x, 2, my_element)
#> [1] "bar"

# Even for this simple data structure, this is more readable than
# the alternative form because it requires you to read both from
# right-to-left and from left-to-right in different parts of the
# expression:
my_element(x[[1]])
#> [1] "foo"

# If you have a list of accessors, you can splice those in with `!!!`:
idx <- list(1, my_element)
pluck(x, !!!idx)
#> [1] "foo"