Scala中的聚合列表值 [英] Aggregate list values in Scala

问题描述

从包含名义和货币​​两个参数的对象列表开始，我该如何汇总每个货币的名义总数？

  //样本数据
 val t1 = new Trade（T150310，10000000， 英镑）; 
 val t2 =新交易（T150311，10000000，JPY）; 
 val t3 =新交易（T150312，10000000，USD）; 
 val t4 =新交易（T150313，100，JPY）; 
 val t5 =新交易（T150314，1000，GBP）; 
 val t6 =新交易（T150315，10000，USD）; 
 
 val trades = List（t1，t2，t3，t4，t5，t6）; 
  

解决方案

我写了一个简单的group-by操作 Groupable trait 带有从 Iterable 的隐式转换）将允许您按照货币将交易分组：

  trait Groupable [V]扩展Iterable [V] {
 def groupBy（f：V => K）：MultiMap [K，V] = {
 val m = new mutable.HashMap [K，Set [V ]]与mutable.MultiMap [K，V] 
 foreach {v => m add（f（v），v）} // add在MultiMap中定义
m 
 
 $ b隐含def it2groupable（it：Iterable [V]）：Groupable [ V] = new Groupable [V] {
 def elements = it.elements 
} 
  

Groupable 只是简单地提供一种从 Iterable 键的方法$ c>然后将所有这些具有相同键的项目分组。所以，在你的情况下：



  // mm是一个MultiMap [Currency，Trade] 
 val mm = trades groupBy {_.currency} 
  

您现在可以做一个非常简单的 mapElements （ mm 是一个 Map ）和一个 foldLeft （或 /： - 非常值得理解 foldLeft 操作符，因为它可以对集合进行非常简洁的聚合）获得总和：

$ $ $ $ $ $ $ $ $ $> $总和：Map [Currency，Int] = mm mapElements {ts =>
（0 /：ts）{（sum，t）=> sum + t.notional}
}


如果我犯了一些错误，最后一行。 ts 是 mm 的值，当然是 Iterable [Trade] 。

I am learning Scala and exploring some of the functional aspects of the language.

Starting with a list of objects containing two parameters notional and currency, how can I aggregate the total notional per currency?

//sample data
val t1 = new Trade("T150310", 10000000, "GBP");
val t2 = new Trade("T150311", 10000000, "JPY");
val t3 = new Trade("T150312", 10000000, "USD");
val t4 = new Trade("T150313", 100, "JPY");
val t5 = new Trade("T150314", 1000, "GBP");
val t6 = new Trade("T150315", 10000, "USD");

val trades = List(t1, t2, t3, t4, t5, t6);

解决方案

I wrote a simple group-by operation (actually a Groupable trait with an implicit conversion from an Iterable) which would allow you to group your trades by their currency:

trait Groupable[V] extends Iterable[V] {
  def groupBy(f: V => K): MultiMap[K, V] = {
    val m = new mutable.HashMap[K, Set[V]] with mutable.MultiMap[K, V]
    foreach { v => m add (f(v), v) } //add is defined in MultiMap
    m
  }
}
implicit def it2groupable(it: Iterable[V]): Groupable[V] = new Groupable[V] {
  def elements = it.elements
}

So Groupable is simply providing a way to extract a key from each item in an Iterable and then grouping all such items which have the same key. So, in your case:

//mm is a MultiMap[Currency, Trade]
val mm = trades groupBy { _.currency } 

You can now do a quite simple mapElements (mm is a Map) and a foldLeft (or /: - well worth understanding the foldLeft operator as it enables extremely concise aggregations over collections) to get the sum:

val sums: Map[Currency, Int] = mm mapElements { ts => 
    (0 /: ts) { (sum,t) => sum + t.notional } 
}

Apologies if I've made some mistakes in that last line. ts are the values of mm, which are (of course) Iterable[Trade].

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