swift How to cast from Int? to String

You can use string interpolation.

let x = 100
let str = "\(x)"

if x is an optional you can use optional binding

var str = ""
if let v = x {
   str = "\(v)"
}
println(str)

if you are sure that x will never be nil, you can do a forced unwrapping on an optional value.

var str = "\(x!)"

In a single statement you can try this

let str = x != nil ? "\(x!)" : ""

Based on @RealMae’s comment, you can further shorten this code using the nil coalescing operator (??)

let str = x ?? ""