tdd

/tdd - Test-Driven Development for R

Follow the TDD workflow: write tests first, then implement code to make them pass.

Core Methodology

RED → GREEN → REFACTOR

1. RED - Write a failing test
2. GREEN - Write minimal code to pass
3. REFACTOR - Improve while keeping tests green

When to Use

• New features or functions
• Bug fixes (write test that reproduces bug first)
• Refactoring existing code
• Adding new model types or algorithms
• Creating data processing pipelines

TDD Workflow Steps

Step 1: Define Expected Behavior

# What should the function do?
# rescale01: Rescale a numeric vector to [0, 1] range
# - Minimum value maps to 0
# - Maximum value maps to 1
# - Handle NA values appropriately

Step 2: Write Failing Tests

# tests/testthat/test-rescale.R
library(testthat)

test_that("rescale01 maps to [0, 1] range", {
  expect_equal(rescale01(c(0, 5, 10)), c(0, 0.5, 1))
  expect_equal(rescale01(c(-10, 0, 10)), c(0, 0.5, 1))
})

test_that("rescale01 handles edge cases", {
  expect_equal(rescale01(c(5, 5, 5)), c(NaN, NaN, NaN))
  expect_equal(rescale01(numeric(0)), numeric(0))
})

test_that("rescale01 handles NA values", {
  expect_equal(rescale01(c(0, NA, 10)), c(0, NA, 1))
})

Step 3: Run Tests (They Should Fail)

devtools::test()
# ✖ rescale01 maps to [0, 1] range
# ✖ rescale01 handles edge cases
# ✖ rescale01 handles NA values

Step 4: Implement Minimal Code

# R/rescale.R
rescale01 <- function(x) {
  rng <- range(x, na.rm = TRUE)
  (x - rng[1]) / (rng[2] - rng[1])
}

Step 5: Run Tests Again

devtools::test()
# ✔ rescale01 maps to [0, 1] range
# ✔ rescale01 handles edge cases
# ✔ rescale01 handles NA values

Step 6: Refactor

Improve code while keeping tests green:

rescale01 <- function(x, na.rm = TRUE) {
  rng <- range(x, na.rm = na.rm, finite = TRUE)
  (x - rng[1]) / (rng[2] - rng[1])
}

Step 7: Verify Coverage

covr::package_coverage()
# rescale01: 100% coverage