How to get the Power of some Integer in Swift language?

Sometimes, casting an Int to a Double is not a viable solution. At some magnitudes there is a loss of precision in this conversion. For example, the following code does not return what you might intuitively expect.

Double(Int.max - 1) < Double(Int.max) // false!

If you need precision at high magnitudes and don't need to worry about negative exponents — which can't be generally solved with integers anyway — then this implementation of the tail-recursive exponentiation-by-squaring algorithm is your best bet. According to this SO answer, this is "the standard method for doing modular exponentiation for huge numbers in asymmetric cryptography."

// using Swift 5.0
func pow<T: BinaryInteger>(_ base: T, _ power: T) -> T {
    func expBySq(_ y: T, _ x: T, _ n: T) -> T {
        precondition(n >= 0)
        if n == 0 {
            return y
        } else if n == 1 {
            return y * x
        } else if n.isMultiple(of: 2) {
            return expBySq(y, x * x, n / 2)
        } else { // n is odd
            return expBySq(y * x, x * x, (n - 1) / 2)
        }
    }

    return expBySq(1, base, power) 
}

Note: in this example I've used a generic T: BinaryInteger. This is so you can use Int or UInt or any other integer-like type.

If you really want an 'Int only' implementation and don't want to coerce to/from Double, you'll need to implement it. Here is a trivial implementation; there are faster algorithms but this will work:

func pow (_ base:Int, _ power:UInt) -> Int {
  var answer : Int = 1
  for _ in 0..<power { answer *= base }
  return answer
}

> pow (2, 4)
$R3: Int = 16
> pow (2, 8)
$R4: Int = 256
> pow (3,3)
$R5: Int = 27

In a real implementation you'd probably want some error checking.

If you like, you could declare an infix operator to do it.

// Put this at file level anywhere in your project
infix operator ^^ { associativity left precedence 160 }
func ^^ (radix: Int, power: Int) -> Int {
    return Int(pow(Double(radix), Double(power)))
}

// ...
// Then you can do this...
let i = 2 ^^ 3
// ... or
println("2³ = \(2 ^^ 3)") // Prints 2³ = 8

I used two carets so you can still use the XOR operator.

Update for Swift 3

In Swift 3 the "magic number" precedence is replaced by precedencegroups:

precedencegroup PowerPrecedence { higherThan: MultiplicationPrecedence }
infix operator ^^ : PowerPrecedence
func ^^ (radix: Int, power: Int) -> Int {
    return Int(pow(Double(radix), Double(power)))
}

// ...
// Then you can do this...
let i2 = 2 ^^ 3
// ... or
print("2³ = \(2 ^^ 3)") // Prints 2³ = 8

Other than that your variable declarations have syntax errors, this works exactly how you expected it to. All you have to do is cast a and b to Double and pass the values to pow. Then, if you're working with 2 Ints and you want an Int back on the other side of the operation, just cast back to Int.

import Darwin 

let a: Int = 3
let b: Int = 3

let x: Int = Int(pow(Double(a),Double(b)))