块不能包含声明[重复]

Question

此问题已经在这里有了答案：

• Declare a variable without an initial value1个答案

我是Scala的新手，正在尝试创建UDF函数，该函数将返回由我的UDF函数标识的一串级联元组。

它看起来应该像这样，但是这里有一些问题，它不喜欢在块外定义变量“ fine”，并且不希望更改作为参数传递的数量。

val calculateFines: UserDefinedFunction = udf((ids: Array[Long], values: Array[Double], amount: Double, complain_id: Long) => {
       var fines
       ids.indices foreach { i => {
         val (id, value) = (ids(i), values(i))
         val penalty = if (value > amount)  amount else  value
         amount =  amount - penalty
         fines = fines + (amount, id, complain_id, penalty).toString()
         if (amount <= 0) 
           break
        }
       }
       return fines
     })

Answer 1

您可以通过少量修复使代码正常工作：

import scala.util.control.Breaks._ //we need this import to allow breaks since Scala doesn't support them out-of-box

val dysfunctional = udf((ids: Array[Long], values: Array[Double], amount: Double, complain_id: Long) => {
    var fines: String = "" //you need to initalize var
    var amountSum = amount //assign amount to var to allow to reassigment

    breakable {
      ids.indices foreach { i =>
        {
          val (id, value) = (ids(i), values(i))
          val penalty = if (value > amount) amount else value
          amountSum = amountSum - penalty
          fines = fines + (amount, id, complain_id, penalty)
          if (amount <= 0)
            break
        }
      }
    }
    fines
  })

这会行得通，但由于它是非功能性方法，因此许多人会对此感到不满意，Scala鼓励编写功能性代码。您可以尝试将其更改为如下所示：

val moreFunctional = (ids: Array[Long], values: Array[Double], amount: Double, complain_id: Long) => {

    val (_, fines) = (ids, values)
      .zipped // zip values and ids to single array of tuples
      .toStream //change it to stream to allow lazy computation
      .scanLeft((amount, "")) { //we pass tuple of amount and empty string as our initial state to scanLeft
        case ((amount, fines), (id, value)) => //second argument of scanLeft is function which receives previous state and currently processed element of array
          val penalty = if (value > amount) amount else value
          (amount, fines + (amount, id, complain_id, penalty).toString()) //here we passs next state for next iteration of scanLeft
      }
      .takeWhile { //we proceed with computations as long and amount is above zero
        case (amount, _) => amount > 0
      }
      .last //we're only interested in last state produced by scan left

    fines
  }