inst/examples/boundary/README.md

In muttest: Mutation Testing

is_adult — Missing Boundary Value

What this demonstrates

A test suite that checks only values clearly on one side of a threshold will let boundary-shift comparison mutants survive. The surviving mutant names the exact input your tests have never exercised. Adding a test at that precise boundary kills it.

The function

is_adult returns TRUE when age is 18 or older, using >=. Swapping to > would incorrectly classify an 18-year-old as a minor.

The weak test

The tests check age 25 (clearly adult) and age 10 (clearly minor). Both inputs return the same result whether the operator is >= or >, so the tests cannot tell the operators apart.

The surviving mutant

>= → > survives. Changing age >= 18 to age > 18 only affects one input — age = 18 exactly. The tests never pass 18, so the function behaves identically for 25 and 10 under both operators.

The fix

Add a test at the boundary value 18. With >= 18, the condition is TRUE and the function returns TRUE. With > 18, it is FALSE and returns FALSE. This difference kills the mutant.

Key rule

When a comparison mutant survives, find the boundary value implied by the operator and add a test that passes exactly that value.

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