Question

我试图洗牌一副牌。

public void shuffle()
{
    int rand1;
    int rand2;
    Random randomGenerator = new Random();

    for (int i = 0; i < deck.size(); i++) {

        // pick a random index between 0 and size of the deck - 1
        rand1 = randomGenerator.nextInt(10);
         rand2 = randomGenerator.nextInt(10);

        // swap rand1 and rand2
        Card temp = deck.get (rand1);
        deck.set(rand1, deck.get (rand2)); 
        deck.set(rand2, temp); 
    }
}

然后它说：

java.lang.IndexOutOfBoundsException; Index:8; size:4(in java.util.ArrayList)

怎么了？

Answer 1

这一行

rand1 = randomGenerator.nextInt(10);

应该是

rand1 = randomGenerator.nextInt(deck.size());

，同样适用于rand2。

Answer 2

另一个灵魂（只是有人需要知道）

创建一个包含1到deck.size()的列表。然后使用Collection.shuffle对其进行随机播放。然后列表将以相等的可能性进行洗牌

Answer 3

如果这不是作业，请使用Collections.shuffle作为@StinePike建议。如果是，你应该知道你没有正确地洗牌。也就是说，并非所有结果都同样可能。这是你的算法产生的直方图（用@Jason的修正）改组3个数字：

[1, 2, 3]: ***********************
[2, 3, 1]: ******************
[3, 2, 1]: **********************
[3, 1, 2]: ******************
[2, 1, 3]: **********************
[1, 3, 2]: **********************

这是正确算法产生的：

[1, 2, 3]: *********************
[2, 3, 1]: *********************
[3, 2, 1]: *********************
[3, 1, 2]: *********************
[2, 1, 3]: *********************
[1, 3, 2]: *********************

Collections.shuffle（deck）是正确的，就像这样：

public static <T> void shuffle(List<T> deck){
    int rand;
    for (int i = 0; i < deck.size()-1; i++) {
        // We randomly choose rand between i (inclusive) and deck.size()-1
        rand = i + randomGenerator.nextInt(deck.size()-i);

        // swap i and rand
        T temp = deck.get(rand);
        deck.set(rand, deck.get(i)); 
        deck.set(i, temp);
    }
}

这是我用来生成直方图的代码。请注意，排列并不总是以相同的顺序出现，但相同的排列（[2,3,1]和[3,1,2]）一直不太可能。

public static void main(String[] args) {
    Map<List<Integer>,Long> histogram = new HashMap<>();
    int logReps = 20;
    int scaleStar = logReps-7;
    List<Integer> list = new ArrayList<Integer>(Arrays.asList(1,2,3));
    for (int i = 0; i<(1<<logReps); i++) {
        List<Integer> templist = new ArrayList<Integer>(list);
        badShuffle(templist);
        if( histogram.containsKey(templist)) {
            histogram.put(templist,histogram.get(templist)+1L);
        } else {
            histogram.put(templist,1L);
        }
    }
    for(Entry<List<Integer>,Long> entry : histogram.entrySet()) {
        System.out.print(entry.getKey()+": ");
        for (int i=0; i<(entry.getValue()>>>scaleStar); i++) {
            System.out.print("*");
        }
        System.out.println();
    }
}
static Random randomGenerator = new Random();
public static void badShuffle(List<Integer> deck){
    int rand1;
    int rand2;

    for (int i = 0; i < deck.size(); i++) {

        // pick a random index between 0 and size of the deck - 1
        rand1 = randomGenerator.nextInt(deck.size());
         rand2 = randomGenerator.nextInt(deck.size());

        // swap rand1 and rand2
        Integer temp = deck.get (rand1);
        deck.set(rand1, deck.get (rand2)); 
        deck.set(rand2, temp); 
    }
}
public static <T> void goodShuffle(List<T> deck){
    int rand;
    for (int i = 0; i < deck.size()-1; i++) {
        // We randomly choose rand between i (inclusive) and deck.size()-1
        rand = i + randomGenerator.nextInt(deck.size()-i);

        // swap i and rand
        T temp = deck.get(rand);
        deck.set(rand, deck.get(i)); 
        deck.set(i, temp);
    }
}
public static void libraryShuffle(List<Integer> deck){
    Collections.shuffle(deck);
}

IndexOutOfBoundsException试图改组牌

3 个答案: