Swift Array – 检查索引是否存在

我希望我能做这样的事情：

let arr: [String] = ["foo","bar"]
let str: String? = arr[1]
if let str2 = arr[2] as String? {
    // this wouldn't run
    println(str2)
} else {
    // this would be run
}

但我得到

Fatal error: Array index out of range