Add na_rm argument to nth(), first(), and last()

NEWS.md

@@ -2,6 +2,11 @@
 
 # dplyr 1.0.9
 
+
+
+* `nth()`, `first()`, and `last()` gained the `na_rm` argument since they are
+  summary functions (#6242).
+
 * New `rows_append()` which works like `rows_insert()` but ignores keys and
   allows you to insert arbitrary rows with a guarantee that the type of `x`
   won't change (#6249, thanks to @krlmlr for the implementation and @mgirlich

R/nth-value.R

@@ -19,6 +19,7 @@
 #'
 #'   For more complicated objects, you'll need to supply this value.
 #'   Make sure it is the same type as `x`.
+#' @param na_rm if `TRUE` missing values don't count
 #' @return A single value. `[[` is used to do the subsetting.
 #' @export
 #' @examples
@@ -39,12 +40,16 @@
 #'
 #' # These functions always return a single value
 #' first(integer())
-nth <- function(x, n, order_by = NULL, default = default_missing(x)) {
+nth <- function(x, n, order_by = NULL, default = default_missing(x), na_rm = FALSE) {
   if (length(n) != 1 || !is.numeric(n)) {
     abort("`n` must be a single integer.")
   }
   n <- trunc(n)
 
+  if (na_rm) {
+    x <- x[!is.na(x)]
+  }
+
   if (n == 0 || n > length(x) || n < -length(x)) {
     return(default)
   }
@@ -63,14 +68,14 @@ nth <- function(x, n, order_by = NULL, default = default_missing(x)) {
 
 #' @export
 #' @rdname nth
-first <- function(x, order_by = NULL, default = default_missing(x)) {
-  nth(x, 1L, order_by = order_by, default = default)
+first <- function(x, order_by = NULL, default = default_missing(x), na_rm = FALSE) {
+  nth(x, 1L, order_by = order_by, default = default, na_rm = na_rm)
 }
 
 #' @export
 #' @rdname nth
-last <- function(x, order_by = NULL, default = default_missing(x)) {
-  nth(x, -1L, order_by = order_by, default = default)
+last <- function(x, order_by = NULL, default = default_missing(x), na_rm = FALSE) {
+  nth(x, -1L, order_by = order_by, default = default, na_rm = na_rm)
 }
 
 default_missing <- function(x) {

tests/testthat/test-nth-value.R

@@ -42,3 +42,15 @@ test_that("nth() gives meaningful error message (#5466)", {
     (expect_error(nth(1:10, "x")))
   })
 })
+
+test_that("nth() works with NA values when na_rm = TRUE", {
+  expect_equal(nth(c(as.numeric(NA), 1:3), 1, na_rm = TRUE), 1)
+  expect_equal(nth(c(1, as.numeric(NA), 2, 3), 2, na_rm = TRUE), 2)
+  expect_equal(nth(c(1:3, as.numeric(NA)), -1, na_rm = TRUE), 3)
+})
+
+test_that("nth() uses default value when na_rm = TRUE and all NA values", {
+  expect_equal(nth(rep(as.numeric(NA), 3), 1, na_rm = TRUE), as.numeric(NA))
+  expect_equal(nth(rep(as.numeric(NA), 3), 2, na_rm = TRUE), as.numeric(NA))
+  expect_equal(nth(rep(as.numeric(NA), 3), -1, na_rm = TRUE), as.numeric(NA))
+})