Scala 中如何使用 continue 和 break 跳出循环-蒲公英云

Scala 中如何使用 continue 和 break 跳出循环

梦里梦外; 2023-02-25 14:24 76阅读 0赞

示例代码:

object Test {
  def main(args: Array[String]): Unit = {
    import scala.util.control.Breaks._
    println("=============== Continue ===================")
    for (i <- 1 to 10) {
      breakable {
        if (i % 2 == 0) {
          break
        }
        println(i)
      }
    }
    println("=============== Continue ===================")
    for (i <- 1 to 10 if i % 2 != 0) {
        println(i)
    }
    println("=============== Break ===================")
    breakable {
      for (i <- 1 to 10) {
        if (i % 2 == 0) {
          break
        }
        println(i)
      }
    }
    println("=============== Break ===================")
    var loop = true
    for (i <- 1 to 10 if loop == true) {
      if( i % 2 == 0){
        loop = false
      }
      println(i)
    }
  }
}

查看Breaks源码得知，breakable 与 break 方法组合用于控制循环的原理就是利用 break 方法抛出一个异常，然后 breakable 方法再捕获这个异常，从而结束整个 breakable 方法块内代码的执行，但是不影响 breakable 方法体外代码的执行，从而实现控制。
源码如下:

package scala
package util.control
/** A class that can be instantiated for the break control abstraction.
 *  Example usage:
 *  {
   {
   {
 *  val mybreaks = new Breaks
 *  import mybreaks.{break, breakable}
 *
 *  breakable {
 *    for (...) {
 *      if (...) break()
 *    }
 *  }
 *  }}}
 *  Calls to break from one instantiation of `Breaks` will never
 *  target breakable objects of some other instantiation.
 */
class Breaks {
  private val breakException = new BreakControl
  /**
   * A block from which one can exit with a `break`. The `break` may be
   * executed further down in the call stack provided that it is called on the
   * exact same instance of `Breaks`.
   */
  def breakable(op: => Unit) {
    try {
      op
    } catch {
      case ex: BreakControl =>
        if (ex ne breakException) throw ex
    }
  }
  sealed trait TryBlock[T] {
    def catchBreak(onBreak: =>T): T
  }
  /**
   * This variant enables the execution of a code block in case of a `break()`:
   * {
   {
   {
   * tryBreakable {
   *   for (...) {
   *     if (...) break()
   *   }
   * } catchBreak {
   *   doCleanup()
   * }
   * }}}
   */
  def tryBreakable[T](op: =>T) = new TryBlock[T] {
    def catchBreak(onBreak: =>T) = try {
      op
    } catch {
      case ex: BreakControl =>
        if (ex ne breakException) throw ex
        onBreak
    }
  }
  /**
   * Break from dynamically closest enclosing breakable block using this exact
   * `Breaks` instance.
   *
   * @note This might be different than the statically closest enclosing block!
   */
  def break(): Nothing = { throw breakException }
}
/** An object that can be used for the break control abstraction.
 *  Example usage:
 *  {
   {
   {
 *  import Breaks.{break, breakable}
 *
 *  breakable {
 *    for (...) {
 *      if (...) break
 *    }
 *  }
 *  }}}
 */
object Breaks extends Breaks
private class BreakControl extends ControlThrowable