在swift 4中使用字节数组时出现“致命错误:索引超出范围”？

let byteArray: [UInt8] = [1, 2, 3, 4, 5]

let index = 3
if index < byteArray.count {
    let value = byteArray[index]
    print("Value at index \(index) is \(value)")
} else {
    print("Index out of range")
}

let byteArray: [UInt8] = [1, 2, 3, 4, 5]

if let value = byteArray.get(index: 3) {
    print("Value at index 3 is \(value)")
} else {
    print("Index out of range")
}

let byteArray: [UInt8] = [1, 2, 3, 4, 5]

func getValue(at index: Int) -> UInt8? {
    guard index < byteArray.count else { return nil }
    return byteArray[index]
}

if let value = getValue(at: 3) {
    print("Value at index 3 is \(value)")
} else {
    print("Index out of range")
}

let byteArray: [UInt8] = [1, 2, 3, 4, 5]

for i in byteArray.indices {
    let value = byteArray[i]
    print("Value at index \(i) is \(value)")
}