String extension that modify self in swift

Question

I have written my fist extension in swift. Here is code: ``` extension String { // add padding // source: http://zh-wang.github.io/blog/2014/09/25/learning-swift-part-6/ func alignRight(count: Int, pad: Character) -> String { let amountToPad = count - countElements(self) if amountToPad < 1 { return self } let padString = String(pad) var string = self for _ in 1...amountToPad { string = padString + string } return string } } ``` This is working fine, I use it like this: ``` let a = "ddd" let b = a.alignRight(5, pad: "0") // b is 00ddd ``` I would like to know is it possible to do it like this ``` var a = "ddd" a.alignRight(5, pad: "0") // a is 00ddd ``` So I would like to make extension that is changing string, not making new one. I have a problem because I can not do `self = "something"` in my extension. Is this normal or I am missing something ?

Accepted Answer

You should use the `mutating` keyword.

```
mutating func alignRight(count: Int, pad: Character) {
// self = something works
}
```

See the extensions' documentation for more information.

String extension that modify self in swift

Solution

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