Fundamentals of scala (3): operators and process control

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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 ：

(characters not shown below)
* / %
+ -
:
= !
< >
&
^
|
(all letters, $, _)

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)
  }
}

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 
	}

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 ")
    }
  }
}

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 "
  }
}

for

Scala Medium for The loop is called for Derivation

Range traversal ：

    for (i <- 1 to 10) {
    
      println(i + ":hello world")
    }

• 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

    for (i <- 1 until 10) {
    
      println(i + ":hello world")
    }

• 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)
    }

Cycle step ：by + step

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

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 ：

for(i <- collection if condition) {
    

}

Equivalent to ：

if (i <- collection) {
    
    if (condition) {
    
    }
}

Take a chestnut ：

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

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

A nested loop ：

Nested loops can combine conditions into one for in .

grammar ：

    for (i <- 1 to 3) {
    
      for (j <- 1 to 3) {
    
        println("i = " + i + ",j = " + j)
      }
    }

Equivalent to ：

    for (i <- 1 to 3; j <- 1 to 5) {
    
      println("i = " + i + ",j = " + j)
    }

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()
    }
  }
}

Loop introduces variables ：

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

for (i <- 1 to 10; j = 10 - i) {
    
    println("i = " + i + ", j = " + j)
}

Equivalent to ：

    for {
    
      i <- 1 to 10;
      j = 10 - i
    } {
    
      println("i = " + i + " , j = " + j)
    }

Loop return value ：

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

    val a: Unit = for (i <- 1 to 10) {
    
      println(i)
    }

• 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 .

val b: immutable.IndexedSeq[Int] = for (i <- 1 to 10) yield i * i
// default implementation is Vector, Vector(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)

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 )

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 
    }

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)
      }
    )

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