数字类型和文字
Swift 的内置数字类型是：
- 字大小（依赖于体系结构）签名的 Int 和 unsigned UInt 。
- 固定大小的有符号整数 Int8 ， Int16 ， Int32 ， Int64 和无符号整数 UInt8 ， UInt16 ， UInt32 ， UInt64 。
- 浮点类型 Float32 / Float ， Float64 / Double 和 Float80 （仅限 x86）。
字面
从上下文推断出数字文字的类型：
let x = 42 // x is Int by default
let y = 42.0 // y is Double by default
let z: UInt = 42 // z is UInt
let w: Float = -1 // w is Float
let q = 100 as Int8 // q is Int8
下划线（_）可用于分隔数字文字中的数字。前导零被忽略。
可以使用有效数和指数部分（ 对于十进制; 对于十六进制） 指定浮点文字。 «significand» e «exponent» 0x «significand» p «exponent»
整数文字语法
let decimal = 10 // ten
let decimal = -1000 // negative one thousand
let decimal = -1_000 // equivalent to -1000
let decimal = 42_42_42 // equivalent to 424242
let decimal = 0755 // equivalent to 755, NOT 493 as in some other languages
let decimal = 0123456789
let hexadecimal = 0x10 // equivalent to 16
let hexadecimal = 0x7FFFFFFF
let hexadecimal = 0xBadFace
let hexadecimal = 0x0123_4567_89ab_cdef
let octal = 0o10 // equivalent to 8
let octal = 0o755 // equivalent to 493
let octal = -0o0123_4567
let binary = -0b101010 // equivalent to -42
let binary = 0b111_101_101 // equivalent to 0o755
let binary = 0b1011_1010_1101 // equivalent to 0xB_A_D
浮点字面语法
let decimal = 0.0
let decimal = -42.0123456789
let decimal = 1_000.234_567_89
let decimal = 4.567e5 // equivalent to 4.567×10⁵, or 456_700.0
let decimal = -2E-4 // equivalent to -2×10⁻⁴, or -0.0002
let decimal = 1e+0 // equivalent to 1×10⁰, or 1.0
let hexadecimal = 0x1p0 // equivalent to 1×2⁰, or 1.0
let hexadecimal = 0x1p-2 // equivalent to 1×2⁻², or 0.25
let hexadecimal = 0xFEEDp+3 // equivalent to 65261×2³, or 522088.0
let hexadecimal = 0x1234.5P4 // equivalent to 0x12345, or 74565.0
let hexadecimal = 0x123.45P8 // equivalent to 0x12345, or 74565.0
let hexadecimal = 0x12.345P12 // equivalent to 0x12345, or 74565.0
let hexadecimal = 0x1.2345P16 // equivalent to 0x12345, or 74565.0
let hexadecimal = 0x0.12345P20 // equivalent to 0x12345, or 74565.0