Summary  
This chapter explains how to perform integer division and compute remainders using dedicated arithmetic operators—%/% for the quotient and %% for the remainder.  

General domain of usage  
Resource allocation

**Integer division** calculates how many times one number fits into another, while the **remainder** shows what is left over.

Definition

For example, dividing 16 candies between 3 friends results in 5 candies for each friend and a remainder of 1, since `16 = 5 * 3 + 1`.

## Arithmetic Operators
In R, these operations are performed using:
- `%/%` for integer division;
- `%%` for the remainder of a division.

# Number of candies for each friend
16 %/% 3
# Number of candies remaining
16 %% 3

# =====================================================================
# tests/testthat/test-task.R
# =====================================================================

library(testthat)

test_that("Runs user_code.R without errors", {
  expect_error(run_user_code(file = "user_code.R"), NA)
})

test_that("Prints correct results for 435 hours", {
  res <- run_user_code(file = "user_code.R", echo = FALSE, print.eval = TRUE)
  out <- paste(res$logs, collapse = "\n")

  # With print.eval=TRUE and echo=FALSE, R prints RESULTS of expressions:
  # 435 %/% 24 -> 18
  # 435 %%  24 -> 3

  dynamic_test(
    grepl("18", out),
    success = "Correct full days printed (18)",
    failure = paste0("Expected to see 18 in output, got:\n", out)
  )

  dynamic_test(
    grepl("3", out),
    success = "Correct remaining hours printed (3)",
    failure = paste0("Expected to see 3 in output, got:\n", out)
  )
})

test_that("Uses %/% and %% operators", {
  code <- paste(readLines("user_code.R", warn = FALSE), collapse = "\n")

  dynamic_test(
    grepl("%/%", code, fixed = TRUE),
    success = "Uses integer division operator %/%",
    failure = "Expected %/% to be used in user_code.R"
  )

  dynamic_test(
    grepl("%%", code, fixed = TRUE),
    success = "Uses modulo operator %%",
    failure = "Expected %% to be used in user_code.R"
  )
})


test_hidden.R

# =====================================================================
# helper-run.R
# =====================================================================

run_user_code <- function(
  file = "user_code.R",
  capture = character(0),
  inject = list(),
  echo = FALSE,
  print.eval = TRUE
) {
  stopifnot(file.exists(file))

  env <- new.env(parent = baseenv())
  if (length(inject)) list2env(inject, envir = env)

  logs <- utils::capture.output(
    base::source(file, local = env, echo = echo, print.eval = print.eval)
  )

  values <- setNames(vector("list", length(capture)), capture)
  for (nm in capture) {
    values[[nm]] <- if (exists(nm, envir = env, inherits = FALSE)) get(nm, envir = env) else NULL
  }

  list(values = values, logs = logs, env = env)
}

dynamic_test <- function(condition, success, failure) {
  testthat::test_that(
    if (isTRUE(condition)) success else failure,
    {
      if (isTRUE(condition)) {
        testthat::expect_true(TRUE)
      } else {
        testthat::fail(failure)
      }
    }
  )
}


helper-run_user_code.R

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Integer Division and Remainders

Arithmetic Operators

Example

Solution

Integer Division and Remainders

Arithmetic Operators

Example

Solution