Swift function that swaps two values - swift

Question

I need to write a function that swaps the contents of any two variables. This is what I have currently but I am getting the error "Cannot assign to value: 'a' is a 'let' constant".
func swap<T>(a: T, b: T) {
(a, b) = (b, a);
}
Can somebody please explain why this doesn't work and suggest how to fix it?
Thank you.

Answer 1

You need to make the parameters inout to be able to do that, like this:
func swap<T>(_ a: inout T,_ b: inout T) {
(a, b) = (b, a)
}
var a = 0, b = 1
swap(&a, &b)

Answer 2

The parameters inside your function is immutable so it cannot be swapped , like if you trying to swap two let value:
let x = 5
let y = 6
//if you try to swap these values Xcode will tell you to change their declaration to var
Here almost the same and to be able to change the values you have to pass inout declaration like below :
func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
(a, b) = (b, a)
}
// call it like this
var x = 1
var y = 2
swapTwoValues(&x, &y)
// Note that you cannot call it like this: swapTwoValues(&1, &2)

Answer 3

That is because the arguments of functions are constants and are treated as local properties, unless you mark them inout and then they are treated as sort of "pass-through" properties that aren't confined to the scope of the function.
var kiwi = "kiwi"
var mango = "mango"
func swap<T>(a: inout T, b: inout T) {
(a, b) = (b, a)
}
swap(&kiwi, &mango)

Answer 4

If having to put & everywhere or the verbosity of the function annoy you, you could always use a descriptive operator like this:
infix operator <->: AssignmentPrecedence
func <-> <A>(lhs: inout A, rhs: inout A) {
(lhs, rhs) = (rhs, lhs)
}
var kenobi = "Hello there!"
var helloThere = "Kenobi..."
helloThere <-> kenobi
print(helloThere, kenobi, separator: "\n")
// Hello there!
// Kenobi...
Which I feel looks much nicer. You don't need to use & in operators with inout parameters – that's how the += and -= operators work:
public extension AdditiveArithmetic {
static func +=(lhs: inout Self, rhs: Self) {
lhs = lhs + rhs
}
static func -=(lhs: inout Self, rhs: Self) {
lhs = lhs - rhs
}
}