Getting Started with muttest

In muttest: Mutation Testing

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
source("engine.R")

This guide walks you through your first mutation test run, from setup to interpreting results.

Prerequisites

muttest works with R packages with testthat tests out of the box. If you're not using a package structure, or not using testthat, see ?TestStrategy for configuration options.

Step 1 — Pick a source file to mutate

Start small. Choose one file from R/ that contains meaningful logic — branching, arithmetic, comparisons. Avoid files that are mostly glue code or just call other functions.

Step 2 — Define a test plan

A test plan describes what to mutate and which mutations to apply:

library(muttest)

plan <- muttest_plan(
  source_files = "R/is_adult.R",
  mutators = comparison_operators()
)

comparison_operators() is a preset that generates mutants by swapping each comparison operator for related alternatives. For >= it produces two mutants: >= → > and >= → <=.

Step 3 — Run the tests

muttest::muttest(plan, "tests/testthat")

Step 4 — Read the output and improve your tests

Each column in the progress table means:

| Column | Meaning | | ------ | ----------------------------------------------- | | K | Killed — mutants your tests caught | | S | Survived — mutants your tests missed | | E | Errors — mutants that caused unexpected errors | | T | Total mutants for this mutator/file combination | | % | Mutation score for this row |

The mutation score is Killed / Total × 100%. A ✔ row means at least one mutant was killed; an x row means all mutants survived.

Here is a complete example showing a weak test, the live output, and the fix:

boundary

The feedback loop

1. Write tests
2. Run muttest
3. Find survivors
4. Strengthen tests
5. Repeat

Start with one file. Aim for a meaningful score improvement each iteration rather than chasing 100% immediately. A score of 80%+ on critical business logic can be a reasonable target to start from.

Next steps

• What is Mutation Testing? — conceptual background and when mutation testing pays off most
• Mutator Reference — all available mutators and how to pick the right ones
• CI Integration — run mutation tests automatically on every push

muttest documentation built on May 14, 2026, 5:10 p.m.