Chapter12 Scala operator

1. Identifiers

• The variable name, function name, class name, and so on are collectively referred to as identifiers , and Scala can use any character as an identifier, for example （）！#%&×+-/:<>[email protected]\^|~ .

• Any sequence of characters can be used in anti-quotes .

    val √ = scala.math.sqrt _    √(2)    val `val42    println(`val`)

2. Central operator

• The operator is connected to two parameters, respectively, on either side of the operator.

• a 标识符 bEquivalent to a.标识符(b) .

    1to10    1.to(10)    1-> 10    1.->(10

3. Unary operators

• An operator with only one argument becomes a unary operator , and the position of the operator may be before or after the argument, all in the following two cases.

• a 标识符Equivalent to a.标识符() , 标识符 a equivalent to a.unary_标识符 .

    1 toString    1.toString()    a42    -a    a.unary_-

4. Assignment operators

• The assignment operator is in the form of an 操作符= expression a 操作符= b , which is equivalent to a = a 操作符 b .

    a1    aa1

• <=、>=、!=、==、===、=/=And so is not an assignment operator,

5. Priority level

• The precedence of the post operator is lower than the middle operator, which is a 中置操作符 b 后置操作符 equivalent to (a 中置操作符 b) 后置操作符 .

• The symbol priority is shown in the following table:

  operator	Priority Level
  Operators except for the following characters	1 (highest)
  * / %	2
  + -	3
  :	4
  < >	5
  !=	6
  &	7
  ^	8
  |	9
  Assignment operator	10 (lowest)

6. Adhesion

• Scala except the colon operator and the assignment operator are right-associative , and all other operators are left- associative .

    // 构造列表List    12Nil   // :: 右结合    1 :: (2Nil)    (12Nil// 错误

• The second parameter of the right combination is the method, for example, 2::Nil equivalent to Nil.::(2) .

7. Apply and Update methods

• The function or method is located to the left of the equal sign of an assignment statement , and is called by the Update method , otherwise called by the apply method .

    val scores = new scala.collection.mutable.HashMap[String, Int]    scores("Bob"100     等价于调用：scores.update("Bob"100)     val bobsScore = scores("Bob")    等价于调用：scores.apply("Bob")

8. Extractor unapply

• Extractor: an object with a unapply method that can be used as an inverse of the method used by the associated object.

• Unapply accepts an object from which to extract values.

     class fraction(n:int, D:int) {      Private ValNum:int =if(d = =0)1 ElseNPrivate ValDen:int =if(d = =0)0 ElseDdef* (other:fraction) =NewFraction (num * other.num, den * other.den)}//Unapply returns the two values that are constructed for the object     object fraction {      defApply (N:int, d:int) =NewFraction (n, D)defUnapply (input:fraction) =if(Input.den = =0) NoneElseSome ((Input.num, Input.den))} Object Main extends App {        varFraction (A, b) = fraction (3,4) * FRACTION (2,5) println (a) println (b)}

• Each sample class automatically has the Apply method and the Unapply method, which is explained in the following section of the sample class, as long as you understand it.

• The extractor can only test the input and not really extract its value, only returns a Boolean.

9. Unapplyseq method

• Use the Unapplyseq method to extract a sequence of values of any length , which returns a Option[seq[a]], and A is the type to be extracted .

    // Name 提取器可以产生所有组成部分的序列    // 关于 Option,Some,None后面章节会详细讲解，这里只需了解upapplySeq的作用就可以。    object Name {      def unapplySeq(input: String): Option[Seq[String]] =        if""Noneelse Some(input.trim.split("\\s+"))    }

adjourned

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Chapter12 Scala operator