Extract the first, last or nth value from a vector

These are straightforward wrappers around [[. The main advantage is that you can provide an optional secondary vector that defines the ordering, and provide a default value to use when the input is shorter than expected.

Usage

nth(x, n, order_by = NULL, default = default_missing(x))

first(x, order_by = NULL, default = default_missing(x))

last(x, order_by = NULL, default = default_missing(x))

Arguments

x

A vector

n

For nth(), a single integer specifying the position. Negative integers index from the end (i.e. -1L will return the last value in the vector).

If a double is supplied, it will be silently truncated.

order_by

An optional vector used to determine the order

default

A default value to use if the position does not exist in the input. This is guessed by default for base vectors, where a missing value of the appropriate type is returned, and for lists, where a NULL is return.

For more complicated objects, you'll need to supply this value. Make sure it is the same type as x.

Value

A single value. [[ is used to do the subsetting.

Examples

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
nth(x, 11)
#> [1] NA

last(x)
#> [1] 10
# Second argument provides optional ordering
last(x, y)
#> [1] 1

# These functions always return a single value
first(integer())
#> [1] NA