---
title: "-=(_:_:)"
framework: swift
role: symbol
role_heading: Operator
path: "swift/uint64/-=(_:_:)"
---

# -=(_:_:)

Subtracts the second value from the first and stores the difference in the left-hand-side variable.

## Declaration

```swift
static func -= (lhs: inout UInt64, rhs: UInt64)
```

## Parameters

- `lhs`: A numeric value.
- `rhs`: The value to subtract from lhs.

## Discussion

Discussion The difference of the two arguments must be representable in the arguments’ type. In the following example, the result of 21 - 50 is less than zero, the minimum representable UInt8 value: var x: UInt8 = 21 x -= 50 // Overflow error note: Overflow checking is not performed in -Ounchecked builds.