docs, test, examples, xref

R/data_summary.R

@@ -17,6 +17,8 @@
 #' as a character string, e.g. `"mean_sepal_width = mean(Sepal.Width)"`. The
 #' summary function `n()` can be used to count the number of observations.
 #'
+#' @seealso `value_at()`, which can be used inside a `data_summary()` call.
+#'
 #' @return A data frame with the requested summary statistics.
 #'
 #' @examples
@@ -42,6 +44,14 @@
 #'
 #' # count observations within groups
 #' data_summary(mtcars, observations = n(), by = c("am", "gear"))
+#'
+#' # first and last observations of "mpg" within groups
+#' data_summary(
+#'   mtcars,
+#'   first = value_at(mpg),
+#'   last = value_at(mpg, -1),
+#'   by = c("am", "gear")
+#' )
 #' @export
 data_summary <- function(x, ...) {
   UseMethod("data_summary")

R/value_at.R

@@ -1,8 +1,8 @@
-#' @title Find the value at a specific position in a variable
+#' @title Find the value(s) at a specific position in a variable
 #' @name value_at
 #'
-#' @description This function can be used to compute summary statistics for a
-#' data frame or a matrix.
+#' @description This function can be used to extract one or more values at a
+#' specific position in a variable.
 #'
 #' @param x A vector or factor.
 #' @param position An integer or a vector of integers, indicating the position(s)
@@ -13,6 +13,8 @@
 #' computation.
 #' @param default The value to be returned if the position is out of range.
 #'
+#' @seealso `data_summary()` to use `value_at()` inside a `data_summary()` call.
+#'
 #' @return A vector with the value(s) at the specified position(s).
 #'
 #' @examples

tests/testthat/test-value_at.R

@@ -6,4 +6,7 @@ test_that("value_at", {
   expect_equal(value_at(efc$c12hour, 5:7), efc$c12hour[5:7], ignore_attr = TRUE)
   expect_equal(value_at(efc$e42dep, 123456, default = 55), 55, ignore_attr = TRUE)
   expect_null(value_at(efc$e42dep, 123456))
+  expect_null(value_at(efc$e42dep, NULL))
+  expect_error(value_at(efc$e42dep, NA), regex = "`position` can't")
+  expect_error(value_at(efc$e42dep, c(3, NA)), regex = "`position` can't")
 })