<!--
{
  "availability" : [
    "iOS: 8.0.0 -",
    "iPadOS: 8.0.0 -",
    "macCatalyst: 13.0.0 -",
    "macOS: 10.10.0 -",
    "tvOS: 9.0.0 -",
    "visionOS: 1.0.0 -",
    "watchOS: 2.0.0 -"
  ],
  "documentType" : "symbol",
  "framework" : "Swift",
  "identifier" : "/documentation/Swift/UInt/-=(_:_:)",
  "metadataVersion" : "0.1.0",
  "role" : "Operator",
  "symbol" : {
    "kind" : "Operator",
    "modules" : [
      "Swift"
    ],
    "preciseIdentifier" : "s:Su2seoiyySuz_SutFZ"
  },
  "title" : "-=(_:_:)"
}
-->

# -=(_:_:)

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

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

## Parameters

`lhs`

A numeric value.

`rhs`

The value to subtract from `lhs`.

## 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.

---

Copyright &copy; 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)