在值上迭代两次（MapReduce）

问题描述

  public void reduce（Pair< String 
）
我收到一个迭代器作为参数，我想迭代两次值。 ，String>键，Iterator< IntWritable>值，
上下文上下文）
  

？怎么样 ？
签名由我使用的框架（即Hadoop）强加。

- 编辑 -

最后，真实签名 reduce 方法的方法是使用 iterable 。我被这个误导了（这实际上是唯一一个不被弃用（但是错误）的例子

解决方案

如果您想再次迭代，我们必须缓存来自迭代器的值。至少我们可以结合第一次迭代和缓存：

  Iterator< IntWritable> it = getIterator（）; 
列表< IntWritable> cache = new ArrayList< IntWritable>（）; 
 
 //第一次循环和缓存
 while（it.hasNext（））{
 IntWritable value = it.next（）; 
 doSomethingWithValue（）; 
 cache.add（value）; 
} 
 
 //第二个循环
（IntWritable value：cache）{
 doSomethingElseThatCantBeDoneInFirstLoop（value）; 
 
 $ / code> 

（仅仅为代码添加一个答案，知道你提到了这个解决方案你自己的评论;））



---

为什么没有缓存是不可能的： Iterator 是实现一个接口的东西，并没有一个单独的要求， Iterator 对象实际上存储了值。迭代两次，你必须重置迭代器（不可能）或克隆它（再次：不可能）。

给一个迭代器的例子，克隆/重置没有任何意义：

  public class Randoms实现Iterator< Double> {
 
 private int counter = 10; 
 
 @Override 
 public boolean hasNext（）{
 return counter> 0; 
} 
 
 @Override 
 public boolean next（）{
 count--; 
 return Math.random（）; 
} 
 
 @Override 
 public boolean remove（）{
 throw new UnsupportedOperationException（delete not supported）; 
} 
} 
  

I receive an iterator as argument and I would like to iterate on values twice.

public void reduce(Pair<String,String> key, Iterator<IntWritable> values,
                   Context context)

Is it possible ? How ?The signature is imposed by the framework I am using (namely Hadoop).

-- edit --
Finally the real signature of the reduce method is with an iterable. I was misled by this wiki page (which is actually the only non-deprecated (but wrong) example of wordcount I found).

解决方案

We have to cache the values from the iterator if you want to iterate again. At least we can combine the first iteration and the caching:

Iterator<IntWritable> it = getIterator();
List<IntWritable> cache = new ArrayList<IntWritable>();

// first loop and caching
while (it.hasNext()) {
   IntWritable value = it.next();
   doSomethingWithValue();
   cache.add(value);
}

// second loop
for(IntWritable value:cache) {
   doSomethingElseThatCantBeDoneInFirstLoop(value);
}

(just to add an answer with code, knowing that you mentioned this solution in your own comment ;) )

---

why it's impossible without caching: an Iterator is something that implements an interface and there is not a single requirement, that the Iterator object actually stores values. Do iterate twice you either have to reset the iterator (not possible) or clone it (again: not possible).

To give an example for an iterator where cloning/resetting wouldn't make any sense:

public class Randoms implements Iterator<Double> {

  private int counter = 10;

  @Override
  public boolean hasNext() {
     return counter > 0;
  }

  @Override
  public boolean next() {
     count--;
     return Math.random();
  }

  @Override
  public boolean remove() {
     throw new UnsupportedOperationException("delete not supported");
  }
}

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