Swift 3:如何根据不同的情况让常量？

let condition = true

if condition {
    let constant = "Value 1"
    print(constant)
} else {
    let constant = "Value 2"
    print(constant)
}

let value = 3

switch value {
case 1:
    let constant = "Value is 1"
    print(constant)
case 2:
    let constant = "Value is 2"
    print(constant)
default:
    let constant = "Value is neither 1 nor 2"
    print(constant)
}

let tuple = (1, 2)

if case (1, let y) = tuple {
    let constant = "Value is 1, y is \(y)"
    print(constant)
} else {
    let constant = "Value is not 1"
    print(constant)
}