android - 使用 nextAfter(双开始，双向)；在安卓系统中

Question

我需要帮助。我将计算一个测量变量，然后取前 100 个值并取它们的平均值。请记住，过去 6 周我一直在自学，对某些人来说显而易见的东西对我来说不一定显而易见。

本质上，说“double x”是变量，我有很多接近的值。我需要的是一种计算这些值的前 100 个值的总和(然后是平均值)的方法。

在我的研究中，我能看到最适合我需要的东西是'nextAfter(double start, double direction);在此之前，使用 'max' 来确定最大值，这是否是正确的起点:

double xm = max(x); 静态 double (xm, x < xm);

我的问题是如何获得前 100 个值(最大值和 99 个 nextAfter 的值)的总和 - 取平均值很容易 - 只需除以 100。

Answer 1

要计算从源中读取的最大 n 值的平均值，您需要至少存储这些值。由于在结束前的任何给定点，您都不知道总体上最大的 n 值中的一些是否会稍后出现，因此您需要跟踪看到的最大 n 值远。

一个简单的方法是将最大值存储在堆或优先级队列中，因为这样可以轻松添加新值并查找(和删除)存储的最小值。默认的 PriorityQueue 非常适合这个任务，因为它使用元素的自然排序，因此 poll ing 删除了最小的存储元素。如果想要计算 n 最小元素的平均值，则需要使用带有自定义 Comparator 的 PriorityQueue(或者在这个特殊的在这种情况下，简单地否定所有值并使用自然排序也可以。

实现所需的惰性方法(更少的代码)是简单地将每个传入值添加到队列中，如果队列的大小超过 n [那么它必须是 n+1 ] 从队列中移除最小的元素:

// vp is the value provider
while(vp.hasNext()) {
    // read the next value and add it to the queue
    pq.add(vp.nextValue());
    if (pq.size() > topSize) {
        pq.poll();
    }

稍微复杂一点的方法是先检查是否需要添加新的值，需要时才修改队列，

double newValue = vp.nextValue();
// Check if we have to put the new value in the queue
// that is the case when the queue is not yet full, or the smallest
// stored value is smaller than the new
if (pq.size() < topSize || pq.peek() < newValue) {
    // remove the smallest value from the queue only if it is full
    if (pq.size() == topSize()) {
        pq.poll();
    }
    pq.add(newValue);
}

这种方式可能更有效，因为向队列添加一个值和删除最小值都是 O(log size) 操作，而与最小存储值比较是 O( 1)。因此，如果有许多值小于之前看到的最大 n，则第二种方法可以节省一些工作。

如果性能很重要，请注意 PriorityQueue 不能存储原始类型，如 double，因此存储(和平均计算的检索)涉及装箱(包装一个double 对象中的 double 值)分别。拆箱(从 Double 对象中提取 double 值)，并因此间接从队列的底层数组到实际值。这些成本可以通过自己使用原始 double[] 实现基于堆的优先级队列来避免。 (但这应该很少是必要的，通常，装箱和间接的成本只占整个处理的一小部分。)

一个头脑简单的完整工作示例:

import java.util.PriorityQueue;

/**
 * Example class to collect the largest values from a stream and compute their
 * average.
 */
public class Average {
    // number of values we want to save
    private int topSize;
    // number of values read so far
    private long count = 0;
    // priority queue to save the largest topSize values
    private PriorityQueue<Double> pq;
    // source of read values, could be a file reader, a device reader, or whatever
    private ValueProvider vp;

    /**
     * Construct an <code>Average</code> to sample the largest <code>n</code>
     * values from the source.
     *
     * @param tops Number of values to save for averaging.
     * @param v Source of the values to sample.
     *
     * @throws IllegalArgumentException when the specified number of values is less than one.
     */
    public Average(int tops, ValueProvider v) throws IllegalArgumentException {
        if (tops < 1) {
            throw new IllegalArgumentException("Can't get average of fewer than one values.");
        }
        topSize = tops;
        vp = v;
        // Initialise queue to needed capacity; topSize + 1, since we first add
        // and then poll. Thus no resizing should ever be necessary.
        pq = new PriorityQueue<Double>(topSize+1);
    }

    /**
     * Compute the average of the values stored in the <code>PriorityQueue<Double></code>
     *
     * @param prio The queue to average.
     * @return the average of the values stored in the queue.
     */
    public static double average(PriorityQueue<Double> prio) throws IllegalArgumentException {
        if (prio == null || prio.size() == 0) {
            throw new IllegalArgumentException("Priority queue argument is null or empty.");
        }
        double sum = 0;
        for(Double d : prio) {
            sum += d;
        }
        return sum/prio.size();
    }

    /**
     * Reads values from the provider until exhausted, reporting the average
     * of the largest <code>topSize</code> values read so far from time to time
     * and when the source is exhausted.
     */
    public void collectAverage() {
        while(vp.hasNext()) {
            // read the next value and add it to the queue
            pq.add(vp.nextValue());
            ++count;
            // If the queue was already full, we now have
            // topSize + 1 values in it, so we remove the smallest.
            // That is, conveniently, what the default PriorityQueue<Double>
            // gives us. If we wanted for example the smallest, we'd need
            // to use a PriorityQueue with a custom Comparator (or negate
            // the values).
            if (pq.size() > topSize) {
                pq.poll();
            }
            // Occasionally report the running average of the largest topSize
            // values read so far. This may not be desired.
            if (count % (topSize*25) == 0 || count < 11) {
                System.out.printf("Average of top %d values after collecting %d is %f\n",
                                   pq.size(), count, average(pq));
            }
        }
        // Report final average. Returning the average would be a natural choice too.
        System.out.printf("Average of top %d values of %d total is %f\n",
                           pq.size(), count, average(pq));
    }

    public static void main(String[] args) {
        Average a = new Average(100, new SimpleProvider(123456));
        a.collectAverage();
    }
}

使用界面

/**
 * Interface for a source of <code>double</code>s.
 */
public interface ValueProvider {
    /**
     * Gets the next value from the source.
     *
     * @return The next value if there is one.
     * @throws RuntimeException if the source is exhausted.
     */
    public double nextValue() throws RuntimeException;

    /**
     * Checks whether the source has more values to deliver.
     *
     * @return whether there is at least one more value to be obtained from the source.
     */
    public boolean hasNext();
}

和实现类

/**
 * Simple provider of a stream of <code>double</code>s.
 */
public class SimpleProvider implements ValueProvider {
    // State determining which value to return next.
    private long state = 0;
    // Last allowed state.
    private final long end;

    /**
     * Construct a provider of <code>e</code> values.
     *
     * @param e the number of values to yield.
     */
    public SimpleProvider(long e) {
        end = e > 0 ? e : 0;
    }

    /**
     * Default constructor to provide 10000 values.
     */
    public SimpleProvider() {
        this(10000);
    }

    public double nextValue() {
        ++state;
        return Math.log(state)*Math.sin(state) + Math.cos(state/2.0);
    }

    public boolean hasNext() {
        return state < end;
    }
}