如何使用数组/数组列表将值更改为星号

Question

我对 Java 很陌生，我正在尝试使用数组和数组列表编写一个程序，您可以在其中输入任意数量的值，并使用星号输出两个参数之间的值。 例如：

[5,14,23,43,54,15]
1-10: *
11-20: **
21-30:*
31-40: 
41-50:*
51-60: *

等等。这是我到目前为止所拥有的，但我遇到了错误和超出范围的异常。谁能说我是否走在正确的轨道上？任何帮助表示赞赏！

package arraylists;

import java.util.ArrayList;
import java.util.Scanner;

public class numberslists {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner reader = new Scanner(System.in);
        ArrayList numbers = new ArrayList();
        int [] number = new int[10];
        int x, count = 0;
        System.out.println("how many numbers would you like?");
        count = reader.nextInt();
        System.out.println("enter in those numbers please");
        for (x=0; x < count; x++){
            number[x] = reader.nextInt();
            numbers.add(number[x]);
        }
        System.out.println(numbers);
        int x10 = numbers.indexOf(number[x] < 10);
        numbers.remove(x10);
        System.out.println(numbers);
    }
}

Answer 1

简而言之，正如拉希鲁所说，你需要换行：int x10 = numbers.indexOf(number[x] < 10);

您的代码的主要问题是表达式number[x] < 10 返回一个布尔值（真或假）。因此numbers.indexOf(number[x] < 10) 将返回 1 或 -1。

最后，当代码到达numbers.remove(x10); 并且如果为-1（表示假），那么您将获得java.lang.ArrayIndexOutOfBoundsException，因为没有办法执行numbers.remove(-1);。请参阅documentation。

您的代码还有改进的余地。以下是您可以做什么的建议。但请在尝试修复自己的代码后查看此建议（这样您可以获得更好的学习体验）。

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class CountOcurrancesInArray {

    private static Scanner reader = new Scanner(System.in);
    private static List<Integer> numbers = new ArrayList<Integer>(); // Use generics when possible: <Integer>
    public static void main(String[] args) {
            int x, count = 0;
            System.out.println("how many numbers would you like?");
            count = reader.nextInt();
            System.out.println("enter in those numbers please");
            for (x=0; x < count; x++){
                // I don't see a need for this line. number[x] = reader.nextInt();
                numbers.add(reader.nextInt());
            }
            System.out.println(numbers);
            int[] comparingNumbers = requestComparingNubers();
            System.out.println("You entered these numbers: " + numbers);
            String matchingNumbers = checkForNumbersInTheList(comparingNumbers);
            System.out.println("Numbers between " + comparingNumbers[0] + "-" + comparingNumbers[1] + ":" + matchingNumbers);
    }

    /**
     * Counts how many entries are in the list between 'comparingNumbersInput'
     * @param comparingNumbersInput
     * @return number of entries as asterisks "*"
     */
    private static String checkForNumbersInTheList(int[] comparingNumbersInput) {
        String result = "";
        for(Integer i : numbers) {
            if (i >= comparingNumbersInput[0] && i <= comparingNumbersInput[1]) {
                result += "*";
            }
        }
        return result;
    }

    /**
     * Asks the user to enter 2 numbers to be compared against the all the numbers in the list.
     * @return returns a int[2] sorted ascendingly
     */
    private static int[] requestComparingNubers() {
        int [] result = new int[2];
        System.out.println("Counting how many numbers there are in between x and y.");
        System.out.println("What is the first number?");
        result[0]=reader.nextInt();
        System.out.println("What is the second number?");
        result[1]=reader.nextInt();
        // Sort comparingList
        if (result[0] > result[1]) {
            int temp = result[1];
            result[1] = result[0];
            result[0] = temp;
        }
        return result;
    }
}

Answer 2

从用户获取计数后声明数组。

int x, count = 0;
System.out.println("how many numbers would you like?");
count = reader.nextInt();
int [] number = new int[count];

还请查看导致错误的代码行。

Answer 3

对我来说，这作为 Map 更有意义，您可以在其中为输入数组中找到的每个范围存储一个计数器。现在这意味着您必须首先确定每个输入是否适合的范围，然后更新与该范围匹配的计数器。由于我们必须将范围计算为输出字符串，并且无论如何您都希望将计数器表示为星号字符串，将范围存储为Map.key 的字符串，将计数器存储为星号字符串作为@987654322 @ 效果很好。

这里有一些示例代码，其中数字是用户输入的原始值的ArrayList。

  //Declare a Map that stores the range as a String ( "01-10") as the key 
  //and a counter in astericks as the value
  Map<String,String> counters = new HashMap<>();

  //Loop over the array ov values
  for(Integer value: numbers){
    //For each value calculate the diviser by diving by 10
    Integer lowRange = value / 10;
    //To get the low range, multiply the diviser by 10 and add 1
    lowRange = (10 * lowRange) + 1;
    //The high range is 9 mor ethan the low range
    Integer highRange = lowRange + 9;
    //Finally calcualte what the range looks like as a String
    //Note that it handles "1" as a special case by prepending a "0" to make the value "01"
    String rangeString = ((lowRange < 10) ? "0" + lowRange : lowRange) + "-" + highRange;

    //Now check the map to see if the rangeString exists as a key, meaning 
    //we have previously found a value in the same range 
    String count = "";
    if(counters.containsKey(rangeString)){
      //If we found the same range, get the previous count
      count = counters.get(rangeString);
    }

    //Place the count back into the map keyed off of the range and add an asterick to the count String
    counters.put(rangeString, count + "*");
  }

  //Finally iterate over all keys in the map, printing the results of the counters for each
  for(String range: counters.keySet()){
     System.out.println(range + " " + counters.get(range));
  }

作为输出示例，如果用户输入值：

[5,14,23,43,54,15,41]

输出将是：

01-10 *
11-20 **
41-50 **
51-60 *
21-30 *

Answer 4

Java 数组是从零开始的索引。例如，如果您声明一个包含 10 个元素的数组，则这些元素的索引将从 0 到 9。

在下面的代码 sn-p 中，当 java 完成“for”循环时

  for (x=0; x < count; x++){
    number[x] = reader.nextInt();
    numbers.add(number[x]);
}

x 变量的值将等于您输入到数字数组中的元素数（x = count）。

所以，当你得到 x 位置的元素时，如下所示：

 int x10 = numbers.indexOf(number[x] < 10);

如果 x

 numbers.remove(x10);

如果 x >= 10，将在 number[x] 处发生 ArrayIndexOutOfBoundsException

Answer 5

又一个家庭作业问题

import java.util.ArrayList;
import java.util.Scanner;
import java.util.*;

public class numberlists {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner reader = new Scanner(System.in);
        LinkedList < Integer > numbers = new LinkedList < Integer > ();
        //int [] number = new int[10]; no need, and the input is variable size
        int x, count = 0;
        System.out.println("how many numbers would you like?");
        count = reader.nextInt();
        System.out.println("enter in those numbers please");
        Map < Integer, Integer > range_numbers = new HashMap < Integer, Integer > ();


        for (x = 0; x < count; x++) {
            //number[x] = reader.nextInt();   no need
            numbers.add(reader.nextInt());
            int rs = ((int) numbers.getLast() / 10) * 10 + 1; //range start for number i.e rs(15)=11
            if (!range_numbers.containsKey(rs)) { //check if has number in that range
                range_numbers.put(rs, 1);
            } else { //gets the prev count, add 1 and stores back for range
                range_numbers.put(rs, range_numbers.get(rs) + 1);
            }

        }

        System.out.println(numbers);
        Map < Integer, Integer > sortedpairs = new TreeMap < Integer, Integer > (range_numbers); // need to sort

        for (Map.Entry < Integer, Integer > pair: sortedpairs.entrySet()) {
            System.out.printf("\n%d-%d: %s", pair.getKey(), pair.getKey() + 9,
                new String(new char[pair.getValue()]).replace("\0", "*"));
            //little trick to repeat any string n times
        }

    }
}

享受吧。