inst/examples/conditions/README.md

In muttest: Mutation Testing

Classify — Single-Branch Testing

What this demonstrates

When tests only exercise one branch of a multi-branch condition, negating the condition can redirect execution to a different branch while still producing a result that passes the assertion. Testing all branches and asserting exact values kills condition-negation mutants.

The function

sign_of classifies a number as "positive", "negative", or "zero" using two chained conditions. clamp constrains a value to a range using two guards. Each if condition is a target for negate_condition().

The weak test

The tests check only that sign_of(5) returns a string (type check) and that clamp(7, 5, 10) returns a number (type check). A negated condition may take a different branch but still return a string or a number.

The surviving mutant

negate_condition() on if (x > 0) produces if (!(x > 0)). For x = 5: - Original: !(5 > 0) = FALSE → falls through to else if (x < 0) which is also FALSE → returns "zero". - Actually: original if (5 > 0) = TRUE → returns "positive". - Mutant: if (!(5 > 0)) = FALSE → checks else if (5 < 0) = FALSE → returns "zero".

is.character("zero") = TRUE — the type check passes for the mutated return value.

The fix

Assert the exact string returned for each branch. Covering all three cases ("positive", "negative", "zero") ensures that redirecting to a different branch fails the test.

Key rule

When a condition-negation mutant survives, add tests for every branch of the condition and assert the exact return value for each case.

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