inst/examples/operators/README.md

In muttest: Mutation Testing

Calculate — Type Check Instead of Value Check

What this demonstrates

An assertion that only checks whether a result is numeric cannot detect arithmetic operator swaps. Different operators produce different numbers, but all of them are numeric. Asserting the exact expected value kills the mutant.

The function

calculate(x, y) computes x * y + x. It contains both * and +, giving arithmetic operator mutators two targets to swap.

The weak test

The test asserts only that the result of calculate(2, 3) is numeric. Any arithmetic expression on two numbers returns a number, so is.numeric() passes for the original and all mutants.

The surviving mutant

* → / produces x / y + x. For x = 2, y = 3: 2/3 + 2 ≈ 2.67 — numeric. + → - produces x * y - x. For the same inputs: 2*3 - 2 = 4 — also numeric. Both mutants satisfy is.numeric().

The fix

Assert the exact expected value. calculate(2, 3) should equal 2 * 3 + 2 = 8. The mutants return ≈ 2.67 and 4 respectively — neither equals 8, so both are killed.

Key rule

When an arithmetic mutant survives, replace type or property checks with expect_equal and an exact numeric value computed by hand.

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