Float Equality

ID

swift.float_equality

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Swift

Tags

reliability, suspicious-code

Description

Reports an == or != binary expression where at least one operand is a bare floating-point literal. Most decimal fractions cannot be represented exactly in binary, so a value that "should" equal the literal after arithmetic rarely does.

if x == 0.0 { ... }              // FLAW
if total != 1.0 { ... }          // FLAW

if abs(x) < .ulpOfOne { ... }    // OK — tolerance comparison
if n == 0 { ... }                // OK — integer literal

Rationale

0.1 + 0.2 == 0.3 is false in every IEEE-754 implementation. The problem is not Swift-specific, but the == operator on Double / Float happily compiles and gives the impression of a meaningful check. In practice the comparison is unreliable unless both sides come from the same source (a stored constant, a Decimal, etc.).

Remediation

Compare using a tolerance, or use Decimal if exact arithmetic is required:

let tolerance = 1.0e-9
if abs(x - target) < tolerance { ... }

// Or use Decimal for currency-like math:
let total: Decimal = 1.0
if amount == total { ... }                  // Decimal supports exact equality

The rule does not fire on integer literals (n == 0) and does not fire on comparisons where the literal appears only inside a deeper expression (e.g. inside a function call argument).

References