算法第四版 1.3.34 RandomBag

最新推荐文章于 2022-02-15 23:57:54 发布

原创最新推荐文章于 2022-02-15 23:57:54 发布 · 758 阅读

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本文介绍了一个简单的随机数据袋实现，使用Java编程语言，并利用Fisher-Yates洗牌算法确保每次迭代返回的元素都是随机顺序。文章详细展示了如何通过自定义类`RandomBag`来存储并随机返回元素。

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要保证完全随机，可以用StdRandom.shuffle(Object[] o)

该函数源代码如下：

    /**
     * Rearranges the elements of the specified array in uniformly random order.
     *
     * @param  a the array to shuffle
     * @throws IllegalArgumentException if {@code a} is {@code null}
     */
    public static void shuffle(Object[] a) {
        validateNotNull(a);
        int n = a.length;
        for (int i = 0; i < n; i++) {
            int r = i + uniform(n-i);     // between i and n-1
            Object temp = a[i];
            a[i] = a[r];
            a[r] = temp;
        }
    }

该函数运用了Fisher–Yates shuffle 洗牌算法

算法思路如下：https://gaohaoyang.github.io/2016/10/16/shuffle-algorithm/#top

package Cap1;

import java.util.Iterator;

import edu.princeton.cs.introcs.StdOut;
import edu.princeton.cs.introcs.StdRandom;

public class RandomBag<Item> implements Iterable<Item> {

	/**
	 * @param args
	 */
	private Item[] a = (Item[]) new Object[1];
	private int N = 0;
	public boolean isEmpty(){
		return N==0;
	}
	public int size(){
		return N;
	}
	private void resize(int length){
		Item[] temp = (Item[]) new Object[length];
		for(int i=0;i<N;i++){
			temp[i] = a[i];
		}
		a = temp;
	}
	public void add(Item item){
		if(N == a.length) resize(2*a.length);
		a[N++] = item; 
	}
	
	@Override
	public Iterator<Item> iterator() {
		// TODO Auto-generated method stub
		return new RandomBagIterator();
	}
	private class RandomBagIterator implements Iterator<Item>{

		private int[] seq = new int[N];
		private int cur = 0;
		public RandomBagIterator(){
			for(int i=0;i<seq.length;i++)
				seq[i] = i;
			StdRandom.shuffle(seq);
		}
		@Override
		public boolean hasNext() {
			// TODO Auto-generated method stub
			return cur != seq.length;
		}

		@Override
		public Item next() {
			// TODO Auto-generated method stub
			return a[seq[cur++]];
		}

		@Override
		public void remove() {
			// TODO Auto-generated method stub
			
		}
		
	}
	public static void main(String[] args) {
		RandomBag<Integer> r = new RandomBag<Integer>();
		int[] cnt = new int[4];
		for(int i=0;i<4;i++)
			r.add(i);
		for(int i=0;i<100000;i++){
			Iterator<Integer> it = r.iterator();
			cnt[it.next()]++;
		}
		for(int i=0;i<cnt.length;i++)
			StdOut.println(i + " " + (double)cnt[i]/100000);
	}
	

	

}