Extract The First, Last Or Nth Value From A Vector - Dplyr

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Extract the first, last, or nth value from a vector Source: R/nth-value.R nth.Rd

These are useful helpers for extracting a single value from a vector. They are guaranteed to return a meaningful value, even when the input is shorter than expected. You can also provide an optional secondary vector that defines the ordering.

Usage

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)

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).

order_by

An optional vector the same size as x used to determine the order.

default

A default value to use if the position does not exist in x.

If NULL, the default, a missing value is used.

If supplied, this must be a single value, which will be cast to the type of x.

When x is a list , default is allowed to be any value. There are no type or size restrictions in this case.

na_rm

Should missing values in x be removed before extracting the value?

Value

If x is a list, a single element from that list. Otherwise, a vector the same type as x with size 1.

Details

For most vector types, first(x), last(x), and nth(x, n) work like x[[1]], x[[length(x)], and x[[n]], respectively. The primary exception is data frames, where they instead retrieve rows, i.e. x[1, ], x[nrow(x), ], and x[n, ]. This is consistent with the tidyverse/vctrs principle which treats data frames as a vector of rows, rather than a vector of columns.

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 # `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

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