Kotlin print()、println()、readLine()、扫描仪、REPL

今天我们将学习如何使用Kotlin打印功能,以及如何从控制台获取和分析用户输入。

Kotlin 打印功能

要在屏幕上输出某些东西,使用以下两种方法:

打印()
打印()

印刷声明将其内部的所有内容打印到屏幕上。 println声明在输出的末尾附有新线条。印刷声明内部称为System.out.print。

 1fun main(args: Array<String>) {
 2var x = 5
 3print(x++)
 4println("Hello World")
 5print("Do dinasours still exist?\n")
 6print(false)
 7print("\nx is $x.")
 8println(" x Got Updated!!")
 9print("Is x equal to 6?: ${x == 6}\n")    
10}

To print a variable inside the print statement, we need to use the dollar symbol($) followed by the var/val name inside a double quoted string literal. To print the result of an expression we use ${ //expression goes here }. The output when the above code is run on the Kotlin Online Compiler is given below. kotlin print println functions

逃避字母和表达式

要逃避美元符号,假设把美元当作一个字符串,而不是计算它,我们可以逃避它。

 1fun main(args: Array<String>) {
 2val y = "\${2 == 5}"
 3println("y = ${y}")       
 4println("Do we use $ to get variables in Python or PHP? Example: ${'$'}x and ${'$'}y")
 5val z = 5
 6var str = "$z"
 7println("z is $str")
 8str = "\$z"
 9println("str is $str")
10}

kotlin print statements escaping literals Note a simple $ without any expression/variable set against it implicitly escapes it and treats it as a part of the string only.

印刷函数值

 1fun sumOfTwo(a: Int, b: Int) : Int{
 2    return a + b
 3}
 4
 5fun main(args: Array<String>) {
 6val a = 2
 7val b = 3
 8println("Value of ${'$'}a and ${'$'}b is : ${sumOfTwo(a,b)}") 
 9println(println("Printing Value of ${'$'}a and ${'$'}b is : ${sumOfTwo(a,b)}"))    
10}

The following output is printed: kotlin print statements printing functions Note: Passing a print inside another behaves like recursion. The innermost is printed first. print statement returns a Unit (equivalent of void in Java).

Kotlin 用户输入

为了获取用户输入,可以使用以下两种方法:

readLine()
[扫描器]( / 社区 / 教程 / 扫描器类 - 在 Java) 类

注意:用户输入需要命令行工具. 您可以使用 REPL 或 IntelliJ. 让我们在这里使用 IntelliJ。

使用 readLine( )

readLine() 返回 String? 类型的值,以处理当您阅读文件的结尾等时可能发生的 null 值。

1fun main(args: Array<String>) {
2    println("Enter your name:")
3    var name = readLine()
4    print("Length is ${name?.length}")
5}

正如您所看到的,我们需要解除可重置类型,以便在属性上使用 String 类型函数。使用 a!! 强迫将 String? 转换为 String,只有当您绝对确定该值不会为 null 否则它会崩溃。 ** 将输入转换为整数** 要将输入 String 转换为 Int,我们会做以下操作:

1fun main(args: Array<String>) {
2var number = readLine()
3try {
4        println("Number multiply by 5 is ${number?.toInt()?.times(5)}")
5    } catch (ex: NumberFormatException) {
6        println("Number not valid")
7    }
8}

再次,我们使用 ?. 运算器将可重置类型转换为使用 toInt() 的 Int。然后我们将其乘以 5. ** 连续阅读输入** 我们可以使用 do while loop 以连续阅读输入,如下所示。

 1do {
 2        line = readLine()
 3
 4        if (line == "quit") {
 5            println("Closing Program")
 6            break
 7        }
 8
 9        println("Echo $line")
10
11    } while (true)
12}

The output of the above in the IntelliJ Command Line is given below. kotlin print statements loop

使用 split 运算器读取多个值

我们可以读取由界限分开的多个值,并将其保存为下图所示的tuple形式。

 1fun readIntegers(separator: Char = ',')
 2        = readLine()!!.split(separator).map(String::toInt)
 3
 4fun main(args: Array<String>) {
 5    println("Enter your values:")
 6    try {
 7        val (a, b, c) = readLine()!!.split(' ')
 8        println("Values are $a $b and $c")
 9    } catch (ex: IndexOutOfBoundsException) {
10        println("Invalid. Missing values")
11    }
12
13    try {
14        val (x, y, z) = readIntegers()
15        println("x is $x y is $y z is $z")
16    } catch (ex: IndexOutOfBoundsException) {
17        println("Invalid. Missing values")
18    }
19    catch (ex: NumberFormatException) {
20        println("Number not valid")
21    }
22
23}

The split function takes in the character that'll be the delimiter. readIntegers() function uses a map on a split to convert each value to an Int. If you enter values lesser than the specified in the tuple, you'll get an IndexOutOfBoundsException. We've used try-catch in both the inputs. The output looks like this: kotlin print statements split and exceptions Alternatively, instead of tuples, we can use a list too as shown below.

1val ints: List<String>? = readLine()?.split("|".toRegex())
2println(ints)

科特林扫描仪类

要获取输入,我们可以使用扫描器(System.in)从标准输入键盘获取输入。

1fun main(args: Array<String>) {
2    val reader = Scanner(System.`in`)
3    print("Enter a number: ")
4
5    // nextInt() reads the next integer. next() reads the String
6    var integer:Int = reader.nextInt()
7
8    println("You entered: $integer")

reader.nextInt() 读取下一个整数. reader.next() 读取下一个字符串. reader.nextFloat() 读取下一个字符串等等. reader.nextLine() 将扫描仪传输到 nextLine,并清除缓冲器。

 1import java.util.*
 2
 3fun main(args: Array<String>) {
 4
 5    val reader = Scanner(System.`in`)
 6    print("Enter a number: ")
 7
 8    try {
 9        var integer: Int = reader.nextInt()
10        println("You entered: $integer")
11    } catch (ex: InputMismatchException) {
12        println("Enter valid number")
13    }
14    enterValues(reader)
15    //move scanner to next line else the buffered input would be read for the next here only.
16    reader.nextLine()
17    enterValues(reader)
18}
19
20fun enterValues(reader: Scanner) {
21    println("Enter a float/boolean :")
22    try {
23        print("Values: ${reader.nextFloat()}, ${reader.nextBoolean()}")
24    } catch (ex: InputMismatchException) {
25        println("First value should be a float, second should be a boolean. (Separated by enter key)")
26    }
27}

InputMismatchException is thrown when the input is of a different type from the one asked. The output is given below. kotlin scanner class

卡特勒 Repl

REPL 也被称为读-Eval-Print-Loop 用于直接在交互式壳中运行代码的一部分,W e 可以通过启动 kotlin 编译器在我们的终端/命令行中执行。

安装命令行编译器

我们可以在Mac/Windows/Ubuntu上安装命令行编译器,如下所示(https://kotlinlang.org/docs/tutorials/command-line.html).通常在Mac上,我们可以在我们的终端上使用HomeBrew来安装Kotlin编译器。

1brew update
2brew install kotlin

Once it is done, start the REPL by entering kotlinc in your terminal/cmd Following is my first code in the REPL. That's all for Kotlin print functions and quick introduction of Kotlin REPL.