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The %= (remainder assignment) operator is a compound assignment operator that calculates the remainder of dividing a left-hand operand by a right-hand operand, and assigns that resulting value back to the left-hand operand.

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Syntax and Evaluation Semantics

lhs %= rhs
While logically similar to the expanded expression lhs = lhs % rhs, the %= operator evaluates the left-hand side expression exactly once. If lhs is a complex expression with side effects—such as a subscript that calls a function (e.g., array[computeIndex()] %= 2)—the compound assignment guarantees that computeIndex() is executed only once. The expanded form would evaluate the left-hand expression twice, making the two forms operationally distinct.

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Technical Mechanics

  • Mutability: The left-hand operand (lhs) must be a mutable variable (declared with var), as the operator performs an in-place mutation.
  • Type Constraints: Both operands must conform to the BinaryInteger protocol (e.g., Int, UInt8, Int64) and must be of the exact same type. Swift does not perform implicit type conversion; attempting to apply %= across mixed integer types (e.g., an Int and an Int8) will result in a compiler error.
  • Floating-Point Restriction: Swift does not support the % or %= operators for floating-point types (Double, Float). Floating-point remainder operations must instead use the truncatingRemainder(dividingBy:) method.
  • Zero Divisor: If the right-hand operand (rhs) is 0, the operation will trigger a runtime crash (division by zero).

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Signage Behavior

When evaluating the remainder, Swift uses truncating division. The sign of the resulting remainder strictly follows the sign of the dividend (the left-hand operand). The sign of the divisor (the right-hand operand) is entirely ignored during the calculation. 
// Positive dividend, positive divisor
var a: Int = 10
a %= 3
print(a) // Output: 1

// Negative dividend, positive divisor
var b: Int = -10
b %= 3
print(b) // Output: -1

// Positive dividend, negative divisor
var c: Int = 10
c %= -3
print(c) // Output: 1
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