data <- data.frame(
  day = c(1, 1, 1, 2, 2, 2),
  beep = c(1, 2, 3, 1, 2, 4),
  mood = c(1, 2, 3, 1, 2, 3),
  mood2 = c(1, 2, 3, 1, 2, 3)
)

dayvar <- "day"
beepvar <- "beep"

var <- c("mood", "mood2")

test_that("correct indexes are found by `find_index()`", {
  expect_identical(
    find_index(data, dayvar, beepvar),
    list(c(1L, 2L, 4L), c(2L, 3L, 5L))
  )
})