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iOS Functions in Swift Adding Power to Functions Function Scope

Manish Giri
Manish Giri
16,266 Points

Are function parameters immutable in Swift?

I was playing around with the code from the function scope video, and I got an errorr with this code -

var testArray: [Int] = [1,2,3,4,5]

func arrayModifier(arr: [Int]) -> [Int] {
    var firstArr = arr
    print("FirstArr = \(firstArr)")


    var secondArr = arr
    return secondArr

var resultArr = arrayModifier(arr: testArray)

The commented out part works fine(as in the video), but when I try to append a value to the function parameter arr, it failed. Here's the error -

Playground execution failed: error: Swift-Functions.playground:100:5: error: cannot use mutating member on immutable value: 'arr' is a 'let' constant

Does it mean that you cannot modify function parameters at all in Swift? Or is it only applicable to certain data types. Coming from a Java/JS background, this seems confusing.


Alex Koumparos
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi Manish,

Yes, function parameters are immutable in Swift (at least, as of Swift 3).

If you want to create a mutable value scoped to the function then declare a new variable equal to the parameter's value. If you want to a make the input parameter mutable and have the same scope as the input variable (so that changes in the function alter the value that was passed in when calling the function), then you can declare that parameter as inout.

Here is a link to Apple's documentation on inout: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html