Question

我是编程新手，以前从未使用过扫描仪。我遇到扫描仪无法识别某些字符的问题，或者至少这是我认为的问题所在。我必须编写的程序应该计算两组非负整数的交集，并集和集差。用户应输入两组，以逗号分隔，并用方括号括起来。例如：[1、2、3] + [4、3、10、0]。还要求我使用TreeSet，并使用适当的TreeSet方法对这两个集合执行请求的操作。目前，我收到的输出是：

输入非负整数列表，用逗号分隔并括在方括号中。
例如：[1、2、3] + [4、3、10、0]。

输入序列：[0,1,2,3] + [4,5,6] 输入错误：在设置开始处应为'['。

任何帮助将不胜感激。

import java.util.Scanner;
import java.util.TreeSet;  
public class setCalculator {
    static Scanner input = new Scanner(System.in);
    

    
    public static void main(String[] args) { 
        
        System.out.println("Enter a list of non-negative integers, separated by commas, and enclosed in square brackets.  ");
         
        System.out.println("For example: [1, 2, 3] + [4, 3, 10, 0]. ");
 
        
        while(true) {
            System.out.print("\nEnter Sequences: ");
            if(input.hasNext("\n")) {
                break;
            } try {
                compute();
            } catch (IllegalArgumentException e) {
                System.out.println("Error in input: " + e.getMessage());
            }
            input.next();
        }
        
    }
    
    public static void compute(){
        
         TreeSet<Integer> setA, setB;  // The two sets of integers.
         
         setA = readSet();
        if (! input.hasNext("\\+") && ! input.hasNext("\\-") && ! input.hasNext("\\*"))
            throw new IllegalArgumentException("Expected *, +, or  - after first set.");
        setB = readSet();
        if( input.hasNext("\n"))
            throw new IllegalArgumentException("Extra unexpected input.");
        if(input.hasNext("\\+"))
            setA.addAll(setB);     // Union.
         else if (input.hasNext("\\*"))
            setA.retainAll(setB);  // Intersection.
         else
            setA.removeAll(setB);  // Set difference.
         
         System.out.print("Value:  " + setA);
         
         /*
          * Start with an empty set.
Read the '[' that begins the set.
Repeat:
   Read the next number and add it to the set.
   If the next character is ']':
      break.
   Read the comma that separates one number from the next.
Read the ']'.
Return the set.
          */
    }
    
    private static TreeSet<Integer> readSet() {     
        TreeSet<Integer> set = new TreeSet<Integer>();
        if(! input.hasNext("\\[")) {
            throw new IllegalArgumentException("Expected '[' at start of set.");
            
        }
        if(input.hasNext("\\[")){
            input.nextLine();
            return set;
        }
        while (true) {
            // Read the next integer and add it to the set.
         
         if (! input.hasNextInt())
            throw new IllegalArgumentException("Expected an integer.");
         int n = input.nextInt(); // Read the integer.
         set.add(Integer.valueOf(n));  // (Could have just said set.add(n)!)
         if (input.hasNext("\\)"))
            break;  // ']' marks the end of the set.
         else if (input.hasNext("\\,"))
            input.next(); // Read a comma and continue.
         else
            throw new IllegalArgumentException("Expected ',' or ']'.");
      }

        input.next(); // Read the ']' that ended the set.

      return set;
}
}

Answer 1

问题可能出在您使用Scanner.hasNext()上。特别是在if(input.hasNext("\\[")){上。如果我没记错的话，这将检查完整的令牌，这显然不仅仅是“ [”。

总体而言，您的方法使问题变得更加复杂。我个人不会使用扫描仪完成整个任务。只需从扫描器中获取输入，然后对检索到的String进行输入验证就容易得多。

此简短代码段将从扫描仪获取输入，将其保存在字符串中，并从提供的字符串中解析Set。（但是肯定有更有效的解决方案）

编辑：

public static void main(String args[]) {
    Scanner scanner = new Scanner(System.in);
    System.out.println("Enter the sequence:");
    // read the whole input
    String input = scanner.nextLine();
    // find the operator
    String op = findOperator(input);
    // split the String on "+"
    String[] sets = input.split("\\" + op);
    if (sets.length != 2) {
        // raise exception for incorrect input
        return;
    }
    TreeSet<Integer> setA = parseSet(sets[0]);
    TreeSet<Integer> setB = parseSet(sets[1]);
    TreeSet<Integer> resultSet = computeResultSet(setA, setB, op);
    System.out.println(resultSet);
}

private static String findOperator(String input) {
    char[] operators = { '+', '-', '*' };
    for (char c : operators) {
        if (input.indexOf(c) != -1) {
            return "" + c;
        }
    }
    // operator not found -> wrong input -> exception handling
    return "";
}

private static TreeSet<Integer> parseSet(String input) {
    TreeSet<Integer> outputSet = new TreeSet<Integer>();
    // remove whitespaces
    input = input.trim();
    // check if the input has the correct format
    if (!input.startsWith("[") || !input.endsWith("]")) {
        // exception for incorrect input
        return outputSet;
    }
    // remove the braces
    input = input.substring(1, input.length() - 1);
    // add the integers to the set
    String[] digits = input.split(",");
    for (String singleDigit : digits) {
        outputSet.add(Integer.parseInt(singleDigit.trim()));
    }
    return outputSet;
}

private static TreeSet<Integer> computeResultSet(TreeSet<Integer> setA, TreeSet<Integer> setB, String op) {
    TreeSet<Integer> resultSet = new TreeSet<Integer>();
    if (op.equals("+")) {
        setA.addAll(setB);
    } else if (op.equals("-")) {
        setA.removeAll(setB);
    } else if (op.equals("*")) {
        setA.retainAll(setB);
    }
    resultSet = setA;
    return resultSet;
}

输入：

Enter the sequence:
[1,2,3] - [2,3]

输出：

[1]

扫描仪无法识别某些字符时出现问题

1 个答案: