> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Swift Remainder Assignment
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.
## Syntax and Evaluation Semantics
```swift theme={"dark"}
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.
## 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).
## 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.
```swift theme={"dark"}
// 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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