dplyr: A Grammar of Data Manipulation

context("Sample")

# Basic behaviour -------------------------------------------------------------

test_that("sample preserves class", {
  expect_is(sample_n(mtcars, 1), "data.frame")
  expect_is(sample_n(tbl_df(mtcars), 1), "tbl_df")

  expect_is(sample_frac(mtcars, 1), "data.frame")
  expect_is(sample_frac(tbl_df(mtcars), 1), "tbl_df")
})

# Ungrouped --------------------------------------------------------------------

df <- data.frame(
  x = 1:2,
  y = c(0, 1)
)

test_that("sample respects weight", {
  # error message from base R
  expect_error(sample_n(df, 2, weight = y))
  expect_equal(sample_n(df, 1, weight = y)$x, 2)

  expect_error(
    sample_frac(df, 2),
    "`size` of sampled fraction must be less or equal to one, set `replace` = TRUE to use sampling with replacement",
    fixed = TRUE
  )
  expect_error(
    sample_frac(df %>% group_by(y), 2),
    "`size` of sampled fraction must be less or equal to one, set `replace` = TRUE to use sampling with replacement",
    fixed = TRUE
  )
  # error message from base R
  expect_error(sample_frac(df, 1, weight = y))
  expect_equal(sample_frac(df, 0.5, weight = y)$x, 2)
})

test_that("sample_* error message", {
  expect_error(
    check_weight(letters[1:2], 2),
    "`weight` must be a numeric, not character",
    fixed = TRUE
  )
  expect_error(
    check_weight(-1:-2, 2),
    "`weight` must be a vector with all values nonnegative, not -1",
    fixed = TRUE
  )
  expect_error(
    check_weight(letters, 2),
    "`weight` must be a numeric, not character"
  )
})

test_that("sample gives informative error for unknown type", {
  expect_error(
    sample_n(list()),
    "`tbl` must be a data frame, not list",
    fixed = TRUE
  )

  expect_error(
    sample_frac(list()),
    "`tbl` must be a data frame, not list",
    fixed = TRUE
  )
})

# Grouped ----------------------------------------------------------------------

test_that("sampling grouped tbl samples each group", {
  sampled <- mtcars %>% group_by(cyl) %>% sample_n(2)
  expect_is(sampled, "grouped_df")
  expect_groups(sampled, "cyl")
  expect_equal(nrow(sampled), 6)
  expect_equal(sampled$cyl, rep(c(4, 6, 8), each = 2))
})

test_that("can't sample more values than obs (without replacement)", {
  by_cyl <- mtcars %>% group_by(cyl)
  expect_error(
    sample_n(by_cyl, 10),
    "`size` must be less or equal than 7 (size of data), set `replace` = TRUE to use sampling with replacement",
    fixed = TRUE
  )
})

df2 <- data.frame(
  x = rep(1:2, 2),
  y = rep(c(0, 1), 2),
  g = rep(1:2, each = 2)
)


test_that("grouped sample respects weight", {
  grp <- df2 %>% group_by(g)

  # error message from base R
  expect_error(sample_n(grp, 2, weight = y))
  expect_equal(sample_n(grp, 1, weight = y)$x, c(2, 2))

  # error message from base R
  expect_error(sample_frac(grp, 1, weight = y))
  expect_equal(sample_frac(grp, 0.5, weight = y)$x, c(2, 2))
})