10个重要的面试问题 *

The len 变量等于5，也就是说 array1 has 5 elements, whereas array2 has 6 elements:

array1 = [1, 2, 3, 4, 5]
array2 = [1, 2, 3, 4, 5, 6]

When array1 is assigned to array2, a copy of array1 实际上是创建和分配的吗.

The reason is that swift arrays are value types (implemented as structs) and not reference types (i.e. classes). 将值类型赋值给变量时, 作为参数传递给函数或方法, or otherwise moved around, 它的副本实际上被创建、分配或传递. Note that swift dictionaries are also value types, implemented as structs.

Value types in swift are:

• structs (incl. arrays and dictionaries)
• enumerations
• 基本数据类型(布尔型、整数型、浮点型等).)

Swift没有在数据类型之间定义任何隐式强制转换, 即使它们在概念上几乎相同(比如 UInt and Int).

To fix the error, rather than casting, an explicit conversion is required. In the sample code, all expression operands must be converted to a common same type, which in this case is Double:

var result = Double(op1) + Double(op2) + op3

The second line uses the stringForKey method of NSUserDefaults, which returns an optional, 为了解释钥匙没找到的原因, or for the corresponding value not being convertible to a string.

During its execution, 如果找到键并且对应的值是字符串, 以上代码可以正常工作. But if the key doesn’t exist, 或者对应的值不是字符串, 应用程序崩溃，出现以下错误:

fatal error: unexpectedly found nil while unwrapping an Optional value

原因是强制展开操作符 ! 尝试强制从nil展开值是可选的吗. The forced unwrapping operator should be used only when an optional is known to contain a non-nil value.

The solution consists of making sure that the optional is not nil before force-unwrapping it:

let userPref = defaults.stringForKey("userPref")
if userPref != nil {
    printString(userPref!)
}

一个更好的方法是使用 optional binding:

if let userPref = defaults.stringForKey("userPref") {
    printString(userPref)
}

Swift字符串支持扩展字形集群. Each character stored in a string is a sequence of one or more unicode scalars that, when combined, 生成一个人类可读的字符. Since different characters can require different amounts of memory, and considering that an extreme grapheme cluster must be accessed sequentially in order to determine which character it represents, it’s not possible to know the number of characters contained in a string upfront, 不遍历整个字符串. 因为这个原因，复杂的 countElements function is O(n).

Raw values 用于将常量(文字)值关联到枚举情况. 该值类型是枚举类型的一部分, and each enum case must specify a unique raw value (duplicate values are not allowed).

下面的示例显示了具有类型为的原始值的枚举 Int:

enum IntEnum : Int {
    case ONE = 1
    case TWO = 2
    case THREE = 3
}

方法将枚举值转换为原始值 rawValue property:

var enumVar: intum = intum.TWO
var rawValue: Int = enumVar.rawValue

A raw value can be converted to an enum instance by using a dedicated initializer:

var enumVar: IntEnum? = IntEnum(rawValue: 1)

Associated values 用于将任意数据关联到特定的枚举大小写. Each enum case can have zero or more associated values, declared as a tuple in the case definition:

enum AssociatedEnum {
    case EMPTY
    case WITH_INT(value: Int)
    case WITH_TUPLE(value: Int, text: String, data: [Float])
}

Whereas the type(s) associated to a case are part of the enum declaration, 关联的值是特定于实例的, meaning that an enum case can have different associated values for different enum instances.

原因是swift自动桥接:

• number types to NSNumber
• strings to NSString
• arrays to NSArray
• dictionaries to NSDictionary

哪些都是引用类型.

There is a strong reference cycle between Master and Detail, with Master creating an instance of Detail 并存储对它的引用 Detail 类的实例的引用 Master creator. In both cases, references are strong, which means that none of the 2 instances will ever be deallocated, causing a memory leak.

To solve the problem it’s necessary to break at least one of the 2 strong relationships, by using either the weak or unowned modifier. 这两个修饰语的区别是:

• unowned: the reference is assumed to always have a value during its lifetime - as a consequence, 该属性必须为非可选类型.
• weak: at some point it’s possible for the reference to have no value - as a consequence, 属性必须是可选类型.

In the above code example, the proper solution is to define in Detail the reference to Master as unowned:

class Detail {
    unowned var master: Master
    ...
}

Structures are value types. By default, the properties of a value type cannot be modified from within its instance methods.

However, you can optionally allow such modification to occur by declaring the instance methods as ‘mutating’; e.g.:

struct IntStack {
  var items = [Int]()
  mutating func add(x: Int) {
    items.append(x) // All good!
  }
}

x is of type [String] and of value [“约翰”，“詹姆斯”，“路易斯”，“文森特”].

result1 is an array of strings, containing the list of the planet names result2 is a double, calculated as the sum of the distance of all planets

The map method of the Array struct type performs a transformation of the source array into an array of another type, whose values are obtained by executing the closure passed as parameter to each element of the array. 在上面的代码中，闭包返回 name property, so the map 方法返回行星名称数组.

给定一个初始值和一个闭包 reduce method of the Array struct type returns a single value obtained by recursively applying the closure to each element of the array. The closure takes the value calculated at the previous step (or the initial value if it’s the first iteration) and the current array element, and is expected to return a value of the same type of the initial value.

在上面的代码中，闭包返回 sum of what calculated at the previous step, plus the value of the distanceFromSun 属性设置当前元素的属性. 最后的结果是所有行星距离的总和.