inst/examples/shipping/README.md

In muttest: Mutation Testing

Shipping Cost — Missing Boundary Value

What this demonstrates

A test suite that checks only "clearly above" and "clearly below" cases will let a boundary-shift mutant survive. The surviving mutant names the exact input your tests have never exercised. Adding a test at that precise boundary value kills it.

The function

shipping_cost returns a flat rate based on whether the package is over or under 5 kg. The strict > operator means a 5 kg package pays the lower rate; >= would charge it the higher rate.

The weak test

The test confirms that a heavy package costs more than a light one. It passes inputs of 10 and 2 kg — both clearly on opposite sides of the boundary — so it never exercises the value 5 itself.

The surviving mutant

> → >= survives. Changing weight_kg > 5 to weight_kg >= 5 only affects input weight_kg = 5 exactly. For inputs 10 and 2, the function returns identical results under both operators, so the test cannot tell the operators apart.

In production, >= 5 instead of > 5 would route 5 kg shipments to the expensive tier and no test would catch the regression.

The fix

Add a test that passes the exact boundary value 5. With > 5, the condition is FALSE and the function returns 5.00. With >= 5, it is TRUE and returns 15.00. 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.