R dplyr nth 从向量中提取第一个、最后一个或第 n 个值

nth(x, n, order_by = NULL, default = NULL, na_rm = FALSE)

first(x, order_by = NULL, default = NULL, na_rm = FALSE)

last(x, order_by = NULL, default = NULL, na_rm = FALSE)

x <- 1:10
y <- 10:1

first(x)
#> [1] 1
last(y)
#> [1] 1

nth(x, 1)
#> [1] 1
nth(x, 5)
#> [1] 5
nth(x, -2)
#> [1] 9

# `first()` and `last()` are often useful in `summarise()`
df <- tibble(x = x, y = y)
df %>%
  summarise(
    across(x:y, first, .names = "{col}_first"),
    y_last = last(y)
  )
#> # A tibble: 1 × 3
#>   x_first y_first y_last
#>     <int>   <int>  <int>
#> 1       1      10      1

# Selecting a position that is out of bounds returns a default value
nth(x, 11)
#> [1] NA
nth(x, 0)
#> [1] NA

# This out of bounds behavior also applies to empty vectors
first(integer())
#> [1] NA

# You can customize the default value with `default`
nth(x, 11, default = -1L)
#> [1] -1
first(integer(), default = 0L)
#> [1] 0

# `order_by` provides optional ordering
last(x)
#> [1] 10
last(x, order_by = y)
#> [1] 1

# `na_rm` removes missing values before extracting the value
z <- c(NA, NA, 1, 3, NA, 5, NA)
first(z)
#> [1] NA
first(z, na_rm = TRUE)
#> [1] 1
last(z, na_rm = TRUE)
#> [1] 5
nth(z, 3, na_rm = TRUE)
#> [1] 5

# For data frames, these select entire rows
df <- tibble(a = 1:5, b = 6:10)
first(df)
#> # A tibble: 1 × 2
#>       a     b
#>   <int> <int>
#> 1     1     6
nth(df, 4)
#> # A tibble: 1 × 2
#>       a     b
#>   <int> <int>
#> 1     4     9