Kotlin-基础语法练习二

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每个 Kotlin 程序都是由两种部分组成的：

1、表达式（Expressions） ：用于计算值 的部分，比如 2 + 3、函数调用、变量赋值等，它们通常会返回一个结果。
2、语句（Statements） ：用于执行动作 的部分，比如 if 条件判断、for 循环、println() 打印语句等，它们主要是完成某种操作，而不一定返回值。

这些表达式和语句可以被组织在一起，形成所谓的 代码块（Blocks）。
代码块就是一组用大括号 {} 包起来的代码，通常用在函数、控制结构（如 if、when、for）等地方。

Example:

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fun sumOf(a:Int,b:Int): Int{
    return a+b
}

fun main(args: Array<String>){
    val a = 10
    val b = 5
    var sum = sumOf(a,b)
    var mul = a * b
    println(sum)
    println(mul)
}

Output:

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15
50

Kotlin if expression -Kotlin if表达式:

语法规则

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if(condition) 
       condition met! 
else 
       condition not met!

Example:

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fun main(args: Array<String>){
    val a = 1000
    val b = 999
    var c = 1122
    var max1 = if(a > b) a else b
    var max2 = if(c > a) c else a
    println("The maximum of ${a} and ${b} is $max1 " )
    println("The maximum of ${c} and ${a} is $max2 " )
}

Output:

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The maximum of 1000 and 999 is 1000 
The maximum of 1122 and 1000 is 1122

Kotlin Statement

Example:

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fun main(args: Array<String>){
    val sum: Int
    sum = 100
    
    // single statement
    println(sum)                             
    
    // Multiple statements
    println("Hello");println("Geeks!")       
}

Output:

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100
Hello
Geeks!

Kotlin Block

Example:

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// Start of main block or outer block
fun main(args: Array<String>) {              
    val array = intArrayOf(2, 4, 6, 8)
    
    // Start of inner block
    for (element in array) {                
       println(element)
    }                                   
    // End of inner block
    
}                                           
// End of main block

Output:

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Control Flow-控制语句

Kotlin if-else expression

if statement

语法规则

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	if(condition) {
	
	       // code to run if condition is true
	}

流程图

Example:

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fun main(args: Array<String>) {
    var a = 3
    if(a > 0){
        print("Yes,number is positive")
    }
}

Output:

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Yes, number is positive

if-else statement

语法规则

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if(condition) { 
        // code to run if condition is true
}
else { 
       // code to run if condition is false
}

流程图

Example:

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fun main(args: Array<String>) {
    var a = 5
    var b = 10
    if(a > b){
        print("Number 5 is larger than 10")
    }
    else{
        println("Number 10 is larger than 5")
    }
}

Output:

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	Number 10 is larger than 5

Example:

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fun main(args: Array<String>) {
    var a = 50
    var b = 40

    // here if-else returns a value which 
    // is to be stored in max variable
    var max = if(a > b){                  
        print("Greater number is: ")
        a
    }
    else{
        print("Greater number is:")
        b
    }
    print(max)
}

Output:

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Greater number is: 50

if-else-if ladder expression

语法规则

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if(Firstcondition) { 
    // code to run if condition is true
}
else if(Secondcondition) {
    // code to run if condition is true
}
else{
}

流程图

Example:

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import java.util.Scanner

fun main(args: Array<String>) {
   
    // create an object for scanner class
    val reader = Scanner(System.`in`)       
    print("Enter any number: ")

    // read the next Integer value
    var num = reader.nextInt()             
    var result  = if ( num > 0){
        "$num is positive number"
    }
    else if( num < 0){
        "$num is negative number"
    }
    else{
        "$num is equal to zero"
    }
    println(result)

}

Output:

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Enter any number: 12
12 is positive number

Enter any number: -11
-11 is negative number

Enter any number: 0
0 is zero

nested if expression

语法规则

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if(condition1){
            // code 1
      if(condition2){
                  // code2
      }
}

流程图

Example:

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import java.util.Scanner

fun main(args: Array<String>) {
 
    // create an object for scanner class
    val reader = Scanner(System.`in`)       
    print("Enter three numbers: ")

    var num1 = reader.nextInt()
    var num2 = reader.nextInt()
    var num3 = reader.nextInt()

    var max  = if ( num1 > num2) {
        if (num1 > num3) {
            "$num1 is the largest number"
        }
        else {
            "$num3 is the largest number"
        }
    }
    else if( num2 > num3){
        "$num2 is the largest number"
    }
    else{
        "$num3 is the largest number"
    }
    println(max)

}

Output:

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Enter three numbers: 123 231 321
321 is the largest number

Kotlin while loop

语法规则

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while(condition) {

           // code to run
}

流程图

Example:

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fun main(args: Array<String>) {
    var number = 1

    while(number <= 10) {
        println(number)
        number++;
    }
}

Output:

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Example:

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fun main(args: Array<String>) {
    var names = arrayOf("Praveen","Gaurav","Akash","Sidhant","Abhi","Mayank")
    var index = 0

    while(index < names.size) {
        println(names[index])
        index++
    }
}

Output:

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	Praveen
	Gaurav
	Akash
	Sidhant
	Abhi
	Mayank

Kotlin do-while loop

语法规则

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do {
      // code to run
}
while(condition)

流程图

Example:

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fun main(args: Array<String>) {
    var number = 6
    var factorial = 1
    do {
        factorial *= number
        number--
    }while(number > 0)
    println("Factorial of 6 is $factorial")
}

Output:

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Factorial of 6 is 720

Kotlin for loop

语法规则

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for(item in collection) {
       // code to execute
}

Range Using a for loop-使用for循环

Example1:

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fun main(args: Array<String>)
{
    for (i in 1..6) {
        print("$i ")
    }
}

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1 2 3 4 5 6

Example2:

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fun main(args: Array<String>)
{
    for (i in 1..10 step 3) {
        print("$i ")
    }
}

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1 4 7 10

Example3:

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fun main(args: Array<String>)
{
	    for (i in 5..1) {
	        print("$i ")
	    }
	    println("It prints nothing")
}

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It prints nothing

Example4:

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fun main(args: Array<String>)
{
    for (i in 5 downTo 1) {
        print("$i ")
    }
}

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5 4 3 2 1

Example5:

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fun main(args: Array<String>)
{
    for (i in 10 downTo 1 step 3) {
        print("$i ")
    }
}

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10 7 4 1

Array using for loop-使用for循环的数组
Example1:

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fun main(args: Array<String>) {
    var numbers = arrayOf(1,2,3,4,5,6,7,8,9,10)

    for (num in numbers){
        if(num%2 == 0){
            print("$num ")
        }
    }
}

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2 4 6 8 10

Example2:

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fun main(args: Array<String>) {
	
	    var planets = arrayOf("Earth", "Mars", "Venus", "Jupiter", "Saturn")
	
	    for (i in planets.indices) {
	        println(planets[i])
	    }
	}

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Earth
Mars
Venus
Jupiter
Saturn

Example3:

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fun main(args: Array<String>) {
    var planets = arrayOf("Earth", "Mars", "Venus", "Jupiter", "Saturn")

    for ((index,value) in planets.withIndex()) {
        println("Element at $index th index is $value")
    }
}

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Element at 0 th index is Earth
Element at 1 th index is Mars
Element at 2 th index is Venus
Element at 3 th index is Jupiter
Element at 4 th index is Saturn

String using a for loop-使用for循环的字符串
Example1:

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fun main(args: Array<String>) {
    var name = "Geeks"
    var name2 = "forGeeks"
    
    // traversing string without using index property
    for (alphabet in name)   print("$alphabet ")

    // traversing string with using index property
    for (i in name2.indices) print(name2[i]+" ")
    println(" ")
    
    // traversing string using withIndex() library function
    for ((index,value) in name.withIndex())
    println("Element at $index th index is $value")
}

Output:

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G e e k s f o r G e e k s  
Element at 0 th index is G
Element at 1 th index is e
Element at 2 th index is e
Element at 3 th index is k
Element at 4 th index is s

collection using for loop-使用for循环的集合
Example:

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fun main(args: Array<String>) {

    // read only, fix-size
    var collection = listOf(1,2,3,"listOf", "mapOf", "setOf")

    for (element in collection) {
        println(element)
    }
}

Output:

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1
2
3
listOf
mapOf
setOf

Kotlin when expression

when as a statement
Example1:

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fun main (args : Array<String>) {

    print("Enter the name of heavenly body: ")
    var name= readLine()!!.toString()
    
    when(name) {
    
        "Sun" -> print("Sun is a Star")
        "Moon" -> print("Moon is a Satellite")
        "Earth" -> print("Earth is a planet")
        else -> print("I don't know anything about it")
    }
}

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Enter the name of heavenly body: Sun
Sun is a Star
Enter the name of heavenly body: Mars
I don't know anything about it

Example2:

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fun main (args : Array<String>) {

    print("Enter the name of heavenly body: ")
    var name= readLine()!!.toString()
    
    when(name) {
    
        "Sun" -> print("Sun is a Star")
        "Moon" -> print("Moon is a Satellite")
        "Earth" -> print("Earth is a planet")
    }
}

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Enter the name of heavenly body: Mars
Process finished with exit code 0

when as an expression

Example1:
注意作为表达式else不能缺失，否则会报错：Kotlin: 'when' expression must be exhaustive, add necessary 'else' branch

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fun main(args : Array<String>) {

    print("Enter number of the Month: ")
    var monthOfYear  = readLine()!!.toInt()
    
    var month= when(monthOfYear) {
    
        1->"January"
        2->"February"
        3->"March"
        4->"April"
        5->"May"
        6->"June"
        7->"July"
        8->"August"
        9->"September"
        10->"October"
        11->"November"
        12->"December"
        else-> "Not a month of year"
    }
    
    print(month)
}

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Enter number of the Month: 8
August

Example2:

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fun main (args :Array<String>) {

    print("Enter name of the planet: ")
    var name=readLine()!!.toString()
    
    when(name) {
    
        "Mercury","Earth","Mars","Jupiter"
            ,"Neptune","Saturn","Venus","Uranus" -> print("This is a planet")
        else -> print("This not a planet")
    }
}

Output:

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Enter name of the planet: Earth
This is a Planet

Example3:

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fun main (args:Array<String>) {

    print("Enter the month number of year: ")
    var num= readLine()!!.toInt()
    
    when(num) {
    
        in 1..3 -> print("Spring season")
        in 4..6 -> print("Summer season")
        in 7..8 -> print("Rainy season")
        in 9..10 -> print("Autumn season")
        in 11..12 -> print("Winter season")
        !in 1..12 -> print("Enter valid month of the year")
    }
}

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Enter the month number of year: 5
Summer season
Enter the month number of year: 14
Enter valid month of the year

Example4:

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fun main(args: Array<String>) {

    var num: Any = "xx"
    
    when(num){
    
        is Int -> println("It is an Integer")
        is String -> println("It is a String")
        is Double -> println("It is a Double")
    }
}

Output:

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It is a String

Example5:

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// returns true if x is odd
	fun isOdd(x: Int) = x % 2 != 0
	// returns true if x is evevn
	fun isEven(x: Int) = x % 2 == 0
	
	fun main(args: Array<String>) {
	
	    var num = 8
	    
	    when{
	    
	        isOdd(num) ->println("Odd")
	        isEven(num) -> println("Even")
	        else -> println("Neither even nor odd")
	    }
	}

Output:

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Even

Example6:

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// Return s True if company start with "xx"
fun hasPrefix(company: Any):Boolean{
return when (company) {
    is String -> company.startsWith("xx")
    else -> false
  }
}

fun main(args: Array<String>) {

    var company = "xx is a computer science portal"
    var result = hasPrefix(company)
    
    if(result) {
    
        println("Yes, string started with xx")
    }
    else {
    
        println("No, String does not started with xx")
    }
}

Output:

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Yes, string started with xx