在Kotlin中读取控制台输入 [英] Reading console input in Kotlin

问题描述

我正在尝试从Kotlin的控制台接受输入，但这很难，因为我不太确定其语法.

I am attempting to accept input from the console in Kotlin but it is difficult because I am not too sure about the syntax.

我从主体开始

fun main(args: Array<String>) {

}

此后我应该输入什么?我知道涉及到println()和readline()，但是我不知道如何构造它们.

WHAT should I enter after this? I am aware that the println() and readline() are involved but I do not know how to structure them.

目标:提示用户输入数字，输入的数字乘以6，程序将结果返回到控制台显示.

Objective: prompt user to enter a number, the number entered is multiplied by 6, program returns the result to the console display.

推荐答案

以下是Kotlin中从stdin中读取的A + B示例:

Here are A+B examples in Kotlin reading from stdin:

fun main() {
  val (a, b) = readLine()!!.split(' ')
  println(a.toInt() + b.toInt())
}

或

fun main(vararg args: String) {
  val (a, b) = readLine()!!.split(' ').map(String::toInt)
  println(a + b)
}

或

fun readInts() = readLine()!!.split(' ').map { it.toInt() }

fun main(vararg args: String) {
  val (a, b) = readInts()
  println(a + b)
}

或

import java.util.Scanner

fun main() {
  val input = Scanner(System.`in`)
  val a = input.nextInt()
  val b = input.nextInt()
  println(a + b)
}

或

with(Scanner(System.`in`)) {
    val a = nextInt()
    val b = nextInt()
    println(a + b)
}

---

竞争性编程

必读简介: https://kotlinlang.org/docs/tutorials/competitive -programming.html

这里是一堆(受本文启发)扩展了的辅助函数，用于读取所有可能的类型，列表，数组，2d数组等:

Here is an (inspired by the article) extended bunch of helper functions for reading all possible types, lists, arrays, 2d-arrays, etc:

private fun readln() = readLine()!!
private fun readlnByte() = readln().toByte()
private fun readlnShort() = readln().toShort()
private fun readlnInt() = readln().toInt()
private fun readlnLong() = readln().toLong()
private fun readlnFloat() = readln().toFloat()
private fun readlnDouble() = readln().toDouble()
private fun readlnBigInt(radix: Int = 10) = readln().toBigInteger(radix)
private fun readlnBigDecimal() = readln().toBigDecimal()

private fun lineSequence(limit: Int = Int.MAX_VALUE) = generateSequence { readLine() }.constrainOnce().take(limit)
private fun readlnStrings() = readln().split(' ')
private fun readlnBytes() = readlnStrings().map { it.toByte() }
private fun readlnShorts() = readlnStrings().map { it.toShort() }
private fun readlnInts() = readlnStrings().map { it.toInt() }
private fun readlnLongs() = readlnStrings().map { it.toLong() }
private fun readlnFloats() = readlnStrings().map { it.toFloat() }
private fun readlnDoubles() = readlnStrings().map { it.toDouble() }

private fun readByteArray() = readlnStrings().run { ByteArray(size) { get(it).toByte() } }
private fun readShortArray() = readlnStrings().run { ShortArray(size) { get(it).toShort() } }
private fun readIntArray() = readlnStrings().run { IntArray(size) { get(it).toInt() } }
private fun readLongArray() = readlnStrings().run { LongArray(size) { get(it).toLong() } }
private fun readFloatArray() = readlnStrings().run { FloatArray(size) { get(it).toFloat() } }
private fun readDoubleArray() = readlnStrings().run { DoubleArray(size) { get(it).toDouble() } }

private fun readlnByteArray(n: Int) = ByteArray(n) { readlnByte() }
private fun readlnShortArray(n: Int) = ShortArray(n) { readlnShort() }
private fun readlnIntArray(n: Int) = IntArray(n) { readlnInt() }
private fun readlnLongArray(n: Int) = LongArray(n) { readlnLong() }
private fun readlnFloatArray(n: Int) = FloatArray(n) { readlnFloat() }
private fun readlnDoubleArray(n: Int) = DoubleArray(n) { readlnDouble() }

private fun readByteArray2d(rows: Int, cols: Int) = Array(rows) { readByteArray().also { require(it.size == cols) } }
private fun readShortArray2d(rows: Int, cols: Int) = Array(rows) { readShortArray().also { require(it.size == cols) } }
private fun readLongArray2d(rows: Int, cols: Int) = Array(rows) { readLongArray().also { require(it.size == cols) } }
private fun readIntArray2d(rows: Int, cols: Int) = Array(rows) { readIntArray().also { require(it.size == cols) } }
private fun readFloatArray2d(rows: Int, cols: Int) = Array(rows) { readFloatArray().also { require(it.size == cols) } }
private fun readDoubleArray2d(rows: Int, cols: Int) = Array(rows) { readDoubleArray().also { require(it.size == cols) } }

private fun isWhiteSpace(c: Char) = c in " \r\n\t"

// JVM-only targeting code follows next

// readString() via sequence is still slightly faster than Scanner
private fun readString() = generateSequence { System.`in`.read().toChar() }
        .dropWhile { isWhiteSpace(it) }.takeWhile { !isWhiteSpace(it) }.joinToString("")
private fun readByte() = readString().toByte()
private fun readShort() = readString().toShort()
private fun readInt() = readString().toInt()
private fun readLong() = readString().toLong()
private fun readFloat() = readString().toFloat()
private fun readDouble() = readString().toDouble()
private fun readBigInt(radix: Int = 10) = readString().toBigInteger(radix)
private fun readBigDecimal() = readString().toBigDecimal()

private fun readBytes(n: Int) = generateSequence { readByte() }.take(n)
private fun readShorts(n: Int) = generateSequence { readShort() }.take(n)
private fun readInts(n: Int) = generateSequence { readInt() }.take(n)
private fun readLongs(n: Int) = generateSequence { readLong() }.take(n)
private fun readFloats(n: Int) = generateSequence { readFloat() }.take(n)
private fun readDoubles(n: Int) = generateSequence { readDouble() }.take(n)

请注意，扫描仪是有点慢.在某些情况下，例如竞争性编程，这可能很重要，因为只需替换 readLine 的"noreferrer>扫描仪.甚至我通过序列进行的次优readString()实现标记化操作，都快一点.与Kotlin内置的readLine()不同，它允许读取输入令牌直到下一个空白.

Beware that Scanner is somewhat slow. This may be important in some cases like competitive programming where program's execution on large inputs could be made up to two times faster just by replacing Scanner with plain readLine. Even my suboptimal readString() implementation tokenizing via sequence is slightly faster. It allows to read input tokens until any next whitespace unlike Kotlin's built-in readLine().

我希望有一天简洁，跨平台，高效，将在Kotlin stdlib中引入通用的控制台和文件输入解析支持.像readInt，readLong等全局和Reader扩展功能. 这不仅对竞争性编程非常有用，而且对于学习Kotlin作为第一语言也是非常有用的. 阅读数字的概念不需要首先解释集合，lambda和monad.

I hope someday a concise, crossplatform, performant, universal for both console and files input parsing support would be introduced in Kotlin stdlib. Like readInt, readLong, etc global and Reader extension functions. This would be very userful not only for competitive programming but also for learning Kotlin as first language. A concept of reading a number shouldn't require first explaining collections, lambdas and monads.

奖金

有时您从控制台输入/输出开始，但随后需要切换到文件. 在每个读取或写入调用前都加上文件流变量变得太麻烦了.

Sometimes you start with console input/output but then need to switch to files. It becomes too tedious to prepend every read or write call with file stream variable.

这里是Kotlin魔术的和平之处，它允许仅用几行包装未更改的控制台代码，以强制其对文件进行读写操作，并确保它们已正确关闭:

Here is a peace of Kotlin magic that allows to just wrap unchanged console code with a couple of lines to force it read and write to files also ensuring they are closed properly:

fun <T : Closeable, R> T.useWith(block: T.() -> R): R = use { with(it, block) }

File("a.in").bufferedReader().useWith {
    File("a.out").printWriter().useWith {
        val (a, b) = readLine()!!.split(' ').map(String::toInt)
        println(a + b)
    }
}

Scanner(File("b.in")).useWith {
    PrintWriter("b.out").useWith {
        val a = nextInt()
        val b = nextInt()
        println(a + b)
    }
}

在需要切换回控制台时，可以快速注释掉包装线.

Wrapping lines can be quickly commented out when happens a need to switch back to console.

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