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关于ios:Swift-Learning-Summary-Advanced-Operators

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Advanced Operators

Unlike arithmetic operators in C, arithmetic operators in Swift don’t overflow by default. If want to overflow by default, use the overflow operation begin with ampersand (&). For example, the overflow addition operator (&+).

It’s so free to define custom infix, prefix, postfix and assignment operators, precedence and associativity values.

Bitwise Operators

Here we use a function to pad 0 for the number’s print result.

func pad(num: UInt8, count: Int) -> String {var str = String(num, radix: 2)
    var res: String = str
    for _ in 0..<(count - str.count) {res = "0" + res}
    
    return  res
    
}

  • ~ NOT

    ~1 is 0

    ~0 is 1

  • & AND

    1 & 1 is 1

    1 & 0 is 0

    0 * 0 is 0

  • | OR

    1 | 1 is 1

    1 | 0 is 1

    0 | 0 is 0

  • ^ XOR

    1 ^ 1 is 0

    1 ^ 0 is 1

    0 ^ 0 is 0

Bitwise NOT Operator

Use ~ to do NOT operation.

var a: UInt8 = 0b11111100
var b = ~a
print(pad(num: b, count: 8))  // 00000011

UInt8 has 8 bits, and can store 0 to 255.

Bitwise AND Operator

Use & to do AND operation.

let a: UInt8 = 0b11111100
let b: UInt8 = 0b00011010
let c = a & b
print(pad(num: c, count: 8)) // 00011000

Bitwise OR Operator

Use | to do OR operator.

let a: UInt8 = 0b11111100
let b: UInt8 = 0b00011010
let c = a | b
print(pad(num: c, count: 8))  // 11111110

Bitwise XOR Operator

Return a new number whose bits are set to 1 where the input bits are different and are set to 0 where the input bits are the same.

let a: UInt8 = 0b11111100
let b: UInt8 = 0b00011010
let c = a ^ b
print(pad(num: c, count: 8))  // 11100110

Bitwise Left and Right Shift Operators

Left shift (<<) move all bits in a number to the left.

let a: UInt8 = 0b00111100
let b = a << 1
let c = a << 2
print(pad(num: b, count: 8))  // 01111000
print(pad(num: c, count: 8))  // 11110000

Shifting of the Unsigned Integer

It will place an zero at the left when right shift, and place an zero at the right when left shift.

Right shift (>>) move all bits in a number to the right.

let a: UInt8 = 0b00111100
let b = a >> 1
let c = a >> 2
print(pad(num: b, count: 8))  // 00011110
print(pad(num: c, count: 8))  // 00001111

Use bit shifting to encode and decode values within other data types:

let pink: UInt32 = 0xCC6699
let redComponent = (pink & 0xFF0000) >> 16
let greenComponent = (pink & 0x00FF00) >> 8
let blueComponent = pink & 0x0000FF
print(String(redComponent, radix: 16))   // cc
print(String(greenComponent, radix: 16)) // 66
print(String(blueComponent, radix: 16))  // 99

// By AND operation and shifting, we get the three part of the "pink" color: R G B

The CSS color value #CC6699 is written as 0xCC6699 in Swift’s hexadecimal number representation.

Shifting Behavior for Signed Integer

In the 8-bit signed integer example:

Signed Integer use its first bit to indicate whether the integer is positive or negative. (0 is positive, 1 is negative)

00000100 is 4

11111100 is -4, the first bit 1 indicate that the integer is negative, and the remain 7 bits is 124 means that 128 – 124 = -4

So the signed 8-bits can represent -128(10000000) to 127(01111111).

Two’s Complement Representation

The way it’s encoded above is known as a two’s complement representation. In this way it has several advantage.

  • Add -4 and -1, it add in binary rule and discard anything that overflow.

    (-4) + (-1) = 11

    11111100 + 11111111 = 11111011

  • arithmetic shift

    • left shift: an arithmetic shift = a logical shift. It will doubling the number.

    • right shift: when shift signed integers to the right, apply same rules as for unsigned integers, but fill any empty bits on the left with the sign bit, rather than with a zero.

      11111110 >> 1 will be 11111111
01111110 >> 1 will be 00111111

      In this way it ensures that the number will has the same sign after shifting. The shift will moves both positive and negative numbers closer to zero.

Overflow Operators

When the constant or variable can’t hold the value that too large or too small, Swift will raise an error.

// The num get the max value of Int8
var num = Int8.max

num += 1  // Here will a runtime-error.

If we specifically want an overflow condition to truncate the number of available bits, we can use the overflow operators that begin with an ampersand (&).

  • Overflow addition (&+)
  • Overflow subtraction (&-)
  • Overflow multiplication (&*)

Value Overflow

If we allow the number to overflow, it may overflows in both positive and negative direction.

var num: UInt8 = UInt8.max      // It's 11111111 (255)
num = num &+ 2

print(pad(num: num, count: 8))  // 00000001

var n: UInt8 = 1
n = n &- 2
print(pad(num: n, count: 8))    // 11111111

The overflow operation can be apply to signed integer.

var signedNum: Int8 = Int8.min     // -128
signedNum = signedNum &- 1
print(String(signedNum, radix: 2)) // 127

Precedence and Associativity

From high to low:

  • %
    • /
    • –
let a = 2 + 3 % 4 * 5
print(a)  // 17

Left associative: calculate from left to right.

  • *
  • /
  • %
  • &
  • &*
  • +
  • –
  • |
  • ^
  • &+
  • &-
  • is
  • as
  • as?
  • as!
  • &&
  • .&
  • ||
  • .|
  • .^

Right associative: calculate from right to left.

  • ??
  • :?
  • =
  • *=
  • /=
  • %=
  • +=
  • -=
  • <<=

  • =

  • &=
  • |=
  • ^=
  • &*=
  • &+=
  • &-=
  • &>>=
  • &<<=
  • .&=
  • .|=
  • .^=

Isn’t associative:

  • <<
  • &<<
  • &>>
  • <
  • <=

  • =

  • ==
  • !=
  • ===
  • !==
  • ~=
  • .<
  • .<=
  • .>
  • .>=
  • .==
  • .!=
  • ..<
  • …

Operator Methods

Classes and structures can overloading the existing operators.

struct Vector2D {var x = 0.0, y = 0.0}

extension Vector2D {static func + (left: Vector2D, right: Vector2D) -> Vector2D{return Vector2D(x: left.x + right.x, y: left.y + right.y)
    }
}

var a = Vector2D(x: 1.0, y: 2.0)
var b = Vector2D(x: 3.0, y: 3.0)
var c = a + b
print(c)  // Vector2D(x: 4.0, y: 5.0)

Prefix and Postfix Operators

extension Vector2D {static prefix func - (vector: Vector2D) -> Vector2D {return Vector2D(x: -vector.x, y: -vector.y)
    }  
}
var x = Vector2D(x: 1.0, y: 2.0)
var xx = -x
print(xx)   // Vector2D(x: -1.0, y: -2.0)

Compound Assignment Operators

+=

extension Vector2D {static func += (left: inout Vector2D, right: Vector2D){left = left + right}
}
var v = Vector2D(x: 1.0, y: 2.0)
var w = Vector2D(x: 4.0, y: -3.0)

v += w
print(v)  // Vector2D(x: 5.0, y: -1.0)

Equivalence Operators

==

One way to make == be valid for enumeration or structure: Conform to the Equalble , it has provided method to do that.

extension Vector2D: Equatable { }

var one = Vector2D(x: 1.0, y: 2.0)
var two = Vector2D(x: 1.0, y: 2.0)

print(one == two)  // true

Another way is to implement the method in the extension.

extension Vector2D: Equatable {static func == (left: Vector2D, right: Vector2D) -> Bool {return (left.x == right.x) && (left.y == right.y)
    }
}

var one = Vector2D(x: 1.0, y: 2.0)
var two = Vector2D(x: 1.0, y: 2.0)

print(one == two)  // true

Custom Operators

Prefix operator

prefix operator +++  
extension Vector2D {static prefix func +++ (vector: inout Vector2D) -> Vector2D {
        vector += vector
        return vector
    }
}

var some = Vector2D(x: 1.0, y: 2.0)
    +++some

print(some)   // Vector2D(x: 2.0, y: 4.0)

Precedence for Custom Infix Operators

Here we define a new custom infix operator call +-, which belongs to the precedence group AdditionPrecedence.

infix operator +-: AdditionPrecedence
exthe tension Vector2D {static func +- (left: Vector2D, right: Vector2D) -> Vector2D{return Vector2D(x: left.x + right.x, y: left.y - right.y)
    }
}

var a = Vector2D(x: 1.0, y: 2.0)
var b = Vector2D(x: 2.0, y: 3.0)
var c = a +- b

print(c) // Vector2D(x: 3.0, y: -1.0)

Don’t need to specify a precedence when defining a prefix or postfix operator. When apply both prefix and postfix to the same operand, the postfix operator is applied first.

Result Builders

If the code without result builder:

protocol Drawable {func draw() -> String
}
struct Line: Drawable {var elements: [Drawable]
    func draw() -> String{return  elements.map { $0.draw() }.joined(separator: "")
    }
}

struct Text: Drawable {
    var content: String
    init(_ content: String) {self.content = content}
    func draw() -> String {return content}
}

struct Space: Drawable {func draw() -> String {return  " "}
}

struct Star: Drawable {
    var length: Int
    func draw() -> String {return String(repeating: "*", count: length)
    }
}

struct AllCaps: Drawable {
    var content: Drawable
    func draw() -> String {return content.draw().uppercased()}
}

let name: String? = "Mike"
let drawing = Line(elements: [Star(length: 3),
    Text("Hello"),
    Space(),
    AllCaps(content: Text((name ?? "World") + "!")),
    Star(length: 3)
])

print(drawing.draw())  // ***Hello MIKE!***

The content in the AllCaps is awkward, if the content is complex, and have some extra conditions, it’s hard to write.

So we use the result builder to solve this problem.

@resultBuilder 
struct DrawingBuilder {
    // It combines several components into a line
    static func buildBlock(_ components: Drawable...) -> Drawable{return Line(elements: components)
    }
    
    static func buildEither(first: Drawable) -> Drawable{return first}
    
    static func buildEither(second: Drawable) -> Drawable {return second}
    
}

func draw(@DrawingBuilder content: () -> Drawable) -> Drawable {return content()
}

func caps(@DrawingBuilder content: () -> Drawable) -> Drawable {return AllCaps(content: content())
}

func makeGreeting(for name: String? = nil) -> Drawable {
    let greeting = draw {Star(length: 3)
        Text("Hello")
        Space()
        caps {
            if let name = name {Text(name + "!")
            } else {Text("World!")
            }
        }
        Star(length: 2)
    }
    return  greeting
}

let p = makeGreeting()
print(p.draw())        // ***Hello WORLD!**
let s = makeGreeting(for: "Amy")
print(s.draw())        // ***Hello AMY!**

The Swift will transform the call to caps(_:) into code like this:

let capsDrawing = caps {
    let partialDrawing: Drawable
    if let name = name {let text = Text(name + "!")
        partialDrawing = DrawingBuilder.buildEither(first: text)
    } else {let text = Text("World!")
        partialDrawing = DrawingBuilder.buildEither(second: text)
    }
    return partialDrawing
}

Add support for writing for loops

extension DrawingBuilder {static func buildArray(_ components: [Drawable]) -> Drawable {return Line(elements: components)
    }
}

let manyStars = draw {Text("Stars:")
    for length in 1...3 {Space()
        Stars(length: length)
    }
}

The result builder makes it easier and neater to organize the function blocks.

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