提取和删除不完全与ArrayList一起工作的模式

Question

我正在为AP Computer Science做一项作业，其中包括生成一个listarray（在一定范围内），然后用html" target="_blank">方法处理它们，去除超过或小于程序前面确定的阈值的对象。这里是我编写的代码的一个例子，同时还有我的老师提供的前提条件。

/**
* @param orig is a List of Integer
* @param mid is an int > 2
* @return a new List of Integer that contains, in order, all the numbers in
* orig that are >= mid / 2
*/public static ArrayList<Integer> extractUpper(ArrayList<Integer> orig, int mid) {
    ArrayList<Integer> myArray = new ArrayList<Integer>();
    for(Integer a: orig) {
        if (a >= mid/2)
            myArray.add(a);
    }
    return myArray;
}

/**
* @param orig is a List of Integer
* @param mid is an int > 2
* @return none PostCondition: all numbers less than  mid / 2 have been 
*  removed from orig
*/
public static void deleteUpper(ArrayList<Integer> orig, int mid) {
    for(int j = 0; j < orig.size(); j++) {
        if (orig.get(j) >= (mid/2))
            orig.remove(j);
    }

}

对我来说，这看起来应该工作得很好，但是当我运行这个：

ic static void main(String[] args) {
    //a.
    int listLen = 10;
    int listMax = 20;
    System.out.println("listLen equals " + listLen + " and listMax equals " + listMax);
    System.out.println();

    //b.
    System.out.println("Generating a fixed-length ArrayList of length " + listLen + " with all values >= 0 and < " + listMax);
    ArrayList<Integer> Array1 = Main.buildFixedList(listLen, listMax);
    System.out.println(Array1);

    //c.        
    System.out.print("The numbers in this ArrayList >= " + listMax/2 + " are: ");
    ArrayList<Integer> Array2 = Main.extractUpper(Array1, listMax);
    System.out.println(Array2);

    //d.
    System.out.print("After deleting numbers > " + listMax/2 + " the modified list is: ");
    Main.deleteUpper(Array1, listMax);
    System.out.println(Array1);

    //e.
    System.out.print("After deletion, the numbers in the List >= " + listMax/2 + " are: ");
    ArrayList<Integer> Array3 = Main.extractUpper(Array1, listMax);
    System.out.println(Array3);

    //f.
    System.out.println();

我的输出似乎忽略了某些数字，有些比其他的更频繁。

listLen equals 10 and listMax equals 20

Generating a fixed-length ArrayList of length 10 with all values >= 0 and < 20
[14, 16, 12, 9, 8, 11, 14, 16, 1]
The numbers in this ArrayList >= 10 are: [14, 16, 12, 11, 14, 16]
After deleting numbers > 10 the modified list is: [16, 9, 8, 14, 1]
After deletion, the numbers in the List >= 10 are: [16, 14]

>=10和<10方法偶尔也会起作用，但我认为现在它更像是一个废话。在这个特定的示例中，>=10方法起作用，但<10则无效。我不知道我的代码出了什么问题。

编辑：谢谢你所有的回复，我很感谢你的帮助。我已经编辑了extractUpper和deleteUpper方法，并且获得了更高的成功率，但是代码似乎忽略了一些数字。代码如下：

 /**
* @param orig is a List of Integer
* @param mid is an int > 2
* @return a new List of Integer that contains, in order, all the numbers in
* orig that are >= mid / 2
*/public static ArrayList<Integer> extractUpper(ArrayList<Integer> orig, int mid) {
    ArrayList<Integer> myArray = new ArrayList<Integer>();
    for (int i = 0;  i < orig.size(); i++){
        if(orig.get(i) >= mid/2) {
            myArray.add(orig.get(i));
        }
    }
    return myArray;
}

/**
* @param orig is a List of Integer
* @param mid is an int > 2
* @return none PostCondition: all numbers less than  mid / 2 have been 
*  removed from orig
*/
public static void deleteUpper(ArrayList<Integer> orig, int mid) {
    for ( int i = orig.size()-1;  i >= 0; i--){
        if (i < orig.size()) {
            if(orig.get(i) >= mid/2) {
                orig.remove(i);
                i++;
            }
        }
        else
            i--;
    }

}

下面是直接来自程序的几个输出：

listLen equals 10 and listMax equals 20

Generating a fixed-length ArrayList of length 10 with all values >= 0 and < 20
[4, 15, 8, 11, 18, 16, 7, 3, 6]
The numbers in this ArrayList >= 10 are: [15, 11, 18, 16]
After deleting numbers > 10 the modified list is: [4, 8, 7, 3, 6]
After deletion, the numbers in the List >= 10 are: []

Generating a fixed-length ArrayList of length 10 with all values >= 0 and < 20
[6, 3, 9, 16, 4, 4, 17, 8, 4]
The numbers in this ArrayList >= 10 are: []
After deleting numbers > 10 the modified list is: [6, 3, 9, 4, 4, 8, 4]
After deletion, the numbers in the List >= 10 are: []

listLen equals 10 and listMax equals 20

Generating a fixed-length ArrayList of length 10 with all values >= 0 and < 20
[4, 5, 0, 4, 12, 12, 1, 12, 10]
The numbers in this ArrayList >= 10 are: [12, 12, 12, 10]
After deleting numbers > 10 the modified list is: [4, 5, 0, 4, 1, 12]
After deletion, the numbers in the List >= 10 are: [12]

Generating a fixed-length ArrayList of length 10 with all values >= 0 and < 20
[15, 16, 2, 8, 1, 7, 3, 0, 15]
The numbers in this ArrayList >= 10 are: [12]
After deleting numbers > 10 the modified list is: [2, 8, 1, 7, 3, 0]
After deletion, the numbers in the List >= 10 are: [12]

Answer 1

问题出在deleteuper方法中。

当您从列表中删除一个项目时，该列表中的索引会发生变化--如果您删除项目3，那么以前在索引4处可访问的项目现在变成了编号3。

在您的实现中，您总是增加索引指针，不管删除是否发生。这意味着，如果连续两个项目符合删除标准，则只删除第一个项目。

Iterator<Integer> i = orig.iterator();
while (i.hasNext()) {
    if (i.next() >= mid) {
        i.remove();
    }
}

for (int i=0;  i<orig.size(); ) {
    if (orig.get(i) >= mid) {
        orig.remove(i);
    }
    else {
        i++;
    }
}