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Fundamentals of scala (3): operators and process control

2022-07-19 11:46:00 【51CTO】

List of articles

One 、 Operator
Two 、 Process control

if - else
for
while and do......while
Cycle break

One 、 Operator

Scala Operators in and Java The operators in are basically the same .

Arithmetic operations

+ - * / % ,＋ and - Show positive and negative signs in the unary operation table , To express addition and subtraction in a binary operation .
/ To divide or divide , Keep only integer parts and discard decimal parts
besides ,+ It also means adding two strings

Relationship between operation

== != < > <= >=
stay Java in ,== Compare the values of the two variables themselves , The first address of two objects in memory ,equals Compare whether the contents contained in the string are the same .
Scala Medium == More similar to Java Medium equals, and eq() The comparison is the address

Logical operations

&& || !, The result of the operation is Boolean value
Scala Short circuit is also supported && ||

The assignment operation

= += -= *= /= %=
stay Scala There is no ++ and -- This grammar , adopt +=、-= To achieve the same effect

An operation

& | ^ ~
<< >> >>>, among << >> Is a signed shift left and right ,>>> unsigned right shift

stay Scala In fact, there is no operator in , All operators are method calls .

When an object's method is called , spot . It can be omitted
If there is only one function parameter , Or no parameters ,() It can be omitted
Operator priority ：

Take a chestnut ：

object Test {
  def main(args: Array[String]): Unit = {
    //   Standard addition operation 
    val i: Int = 1.+(1)
    //  When an object's method is called ,. It can be omitted 
    val n: Int = 1 + (1)
    //   If there is only one function parameter , Or no parameters ,() It can be omitted 
    val m: Int = 1 + 1

    println(i + " , " + n + " , " + m)
  }
}

     
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Two 、 Process control

Scala Process control in is the same as other programming languages , It also contains branch statements 、 Loop statement, etc .

if - else

Basic grammar ：

	if ( Conditional expression ) {
		 Code segment 
	}else if ( Conditional expression ) {
		 Code segment 
	}else {
		 Code segment 
	}

     
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Take a chestnut ：

object Test01_ifelse {
  def main(args: Array[String]): Unit = {
    println(" Please enter your age ：")
    val age: Int = StdIn.readInt();
    if (age >= 18 && age < 30) {
      println(" middle-aged ")
    } else if (age < 18) {
      println(" juvenile ")
    } else {
      println(" The elderly ")
    }
  }
}


     
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What's special ：

Different from other languages ,Scala Medium if else An expression actually has a return value , It can also be used as an expression , Defined as the return value of the last statement executed
Scala Inconsistent return value types in , Take their common ancestor type .
The return value can be Unit type , At this point, the value of the last expression is ignored , obtain ()
scala There is no ternary conditional operator in , It can be used if (a) b else c replace a ? b : c
Nested branches have the same characteristics .

Take a chestnut ：

object Test01_ifelse {
  def main(args: Array[String]): Unit = {
    println(" Please enter your age ：")
    
    //  The return value of the branch statement 
    val result: Unit = if (age >= 18) {
      println(" congratulations , Grown up ")
    } else if (age < 18) {
      println(" sorry , You're underage ")
    } else {
      println(" You're a man ")
    }
    println("result:" + result)

    val result2: String = if (age >= 18) {
      println(" congratulations , Grown up ")
      " congratulations , Grown up "
    } else if (age < 18) {
      println(" sorry , You're underage ")
      " sorry , You're underage "
    } else {
      println(" You're a man ")
      " You're a man "
    }
    println("result:" + result2)
    
	//  Return values of different types take the public parent class 
    val result3: Any = if (age >= 18) {
      println(" congratulations , Grown up ")
      " congratulations , Grown up "
    } else if (age < 18) {
      println(" sorry , You're underage ")
      age
    } else {
      println(" You're a man ")
      age
    }
    println("result:" + result3)

    // java The ternary operation in  a?b:c scala Use in  if (a) b else c
    val res: String = if(age>=18){
      " adult "
    }else{
      " A minor "
    }

    val res1 = if (age>= 18) " adult " else " A minor "
  }
}

     
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for

Scala Medium for The loop is called for Derivation

Range traversal ：

i Represents a cyclic variable
<- It is equivalent to assigning traversal value to variable
1 to 10RichInt A method call in , Return to one Range
Close back and forth

until It's also RichInt One of the methods in , return Range class
Before closed after opening

A range is also a collection , You can also traverse ordinary collections .

    for (i <- Array(10, 28, 9, 3, 2)) {
      println(i)
    }
    
    for (i <- List(10, 28, 9, 3, 2)) {
      println(i)
    }
    
    for (i <- Set(10, 28, 9, 3, 2)) {
      println(i)
    }

     
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Cycle step ：by + step

    for (i <- 1 to 10 by 2) {
      println(i)
    }
    
    for (i <- 30 to 10 by -2) {
      println(i)
    }


     
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Cycle guard ：

Cycle guard , That is, circulation protection （ Also known as conditional judgment , The guards ）. The protection type is true Then go inside the circulatory body , by false Then skip , Be similar to continue.

grammar ：

Equivalent to ：

Take a chestnut ：

    for (i <- 1 to 10) {
      if (i != 5) {
        println(i)
      }
    }

    for (i <- 1 to 10 if i != 5) {
      println(i)
    }

     
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A nested loop ：

Nested loops can combine conditions into one for in .

grammar ：

Equivalent to ：

Take a chestnut ： Print multiplication tables

object Test03_PracticeMulTable {
  def main(args: Array[String]): Unit = {
    for (i <- 1 to 9; j <- 1 to i) {
      print(s"$j * $i = ${i * j} \t")
      if (j == i) println()
    }
  }
}

     
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Loop introduces variables ：

The introduced variable in the loop , But not a cyclic variable .

Equivalent to ：

Loop return value ：

Scala in for The default return value of the loop is Unit, example ().

yield：Java One method of threading in is yield Indicates a thread concession
scala Is a keyword in Express ： At present for The loop generates a collection type as the return value , Then return .
Return the results processed during traversal to a new Vector Collection , Use yield keyword .

while and do…while

While and do…While The use of and Java The usage is the same in language , It is not recommended to use , The result type is Unit.

grammar ：

while ( The loop condition ) {
	  The loop body ( sentence )
	  Loop variable iteration 
}

do{
	  The loop body ( sentence )
	  Loop variable iteration 
 } while( The loop condition )

     
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Cycle break

Scala The built-in control structure specifically removes break and continue, In order to better adapt to functional programming , It is recommended to use the functional style to solve break and continue The function of , Not a keyword .Scala Use in breakable Control structure to achieve break and continue function .

Break the loop by throwing an exception

    try {
      for (i <- 0 until 5) {
        if (i == 3)
          throw new RuntimeException
        println(i)
      }
    } catch {
      case e: Exception => //  Exit loop 
    }

     
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Use scala Medium Breaks Medium break Method （ It is equivalent to encapsulating exception capture ）, Throw and catch exceptions .

import scala.util.control.Breaks

    Breaks.breakable(
      for (i <- 0 until 5) {
        if (i == 3)
          Breaks.break()
        println(i)
      }
    )

     
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This is the end of the sharing , I hope to learn from you Scala Language helps ！！！

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