大數據算法學習筆記(4)-數據流中的頻繁元素

問題

求數據流中最頻繁的元素，下面用MG算法近似解決求解這個問題。

代碼

Java

import java.util.*;

public class FrequentElement {
    public static void main(String[] args) {
        int k = 3;//計數器的個數
        Random random = new Random();
        Map<Integer, Integer> counters = new HashMap();
        int i = 0;//這個用於統計總的數量
        while (i < 10){//真實的數據流應該是源源不斷的，這裏簡化爲有限個了
            Integer element = random.nextInt(5);
            if (counters.size()<k){//模擬k個計數器
                System.out.println("正在計數");
                counters.put(element, counters.getOrDefault(element, 0)+1);
                System.out.println(counters);
            }else{//k個計數器都用上的時候，進行一輪減1
                System.out.println("開始一輪減1");
                System.out.println(counters);
                //只能用迭代器，不可用foreach，避免ConcurrentModificationException
                Iterator<Integer> iterator = counters.keySet().iterator();
                while (iterator.hasNext()){
                    Integer key = iterator.next();
                    if(counters.get(key)==1) iterator.remove();
                    else counters.put(key, counters.get(key)-1);
                }
                System.out.println(counters);
            }
            i++;
        }
        int finalSum = 0;
        for (Integer key:counters.keySet()) finalSum += counters.get(key);
        System.out.println("與精確值最多相差"+(i-finalSum)/(k+1));

    }
}

這個有點複雜，看一下模擬的效果

Scala

import scala.util.Random

object FrequentElement2 {
  def main(args: Array[String]): Unit = {
      val k=3
      var counters = collection.mutable.HashMap[Int,Int]()
      var i = 0
      while (i<10){
        val element = Random.nextInt(5)
        if (counters.size < k){//模擬k個計數器
          println("正在計數")
          counters.put(element, counters.getOrElse(element, 0)+1);
          println(counters);
        }else{//k個計數器都用上的時候，進行一輪減1
          println("開始一輪減1");
          println(counters);
          counters = counters.map{case (key,value)=>(key, value-1)}.filter(_._2>0)
          println(counters)
        }
        i+=1
      }
    println(counters)
    val finalSum = counters.values.sum
    println("與精確值最多相差"+(i-finalSum)/(k+1));
  }
}

Python

import random
k = 3
counters = {}
i = 0
while i < 10:
    element = random.randint(0, 5)
    if len(counters) < k:
        print("正在計數")
        counters[element] = counters.get(element, 0)+1
        print(counters)
    else:
        print("開始一輪減1")
        for key, value in counters.items():
            if value > 1:
                value -= 1
        counters = dict((key, value) for key, value in counters.items() if value != 0)
        print(counters)
    i+=1
finalSum = sum(counters.values())
print("與精確值最多相差%d"%((i-finalSum)/(k+1)))