Question

我们试图完成我们的课程项目。但是当我们尝试运行下面的java代码时，有4个错误我们无法弄清楚。这些是错误。

Bst.java:137: error: illegal start of expression
                                     for()
                                         ^
Bst.java:139: error: illegal start of expression
                                    }
                                    ^
Bst.java:140: error: 'else' without 'if'
                                    else{
                                    ^
Bst.java:164: error: reached end of file while parsing

这是我们在MIU的Java代码2016 Spring Data Structure BST示例代码（对于项目＃2）。我们可以添加/修改类，但应该维护类的基本结构。

import java.util.*;
import java.util.*;
class Node { 
    int data; // data
    Node lc; //left child pointer
    Node rc; //right child pointer   
    Node(int data){     // constructor
        this.data=data;
        lc=null;
        rc=null;
    }
}
class BST {
    Node root;
    BST(){
        root=null;
    }
    void insert(int newData){ // This is not the full version
        Node newNode=new Node(newData);
        if(root==null) root=newNode; // root was null
        else { // root was not null, so find the place to attach the new node
            Node ptr=root; // Node pointer
            while(true){
                if(newNode.data==ptr.data) { 
                    System.out.println("The node already exist.");
                    break;
                }
                else if(newNode.data<ptr.data) {
                    if(ptr.lc!=null) ptr=ptr.lc;
                    else {
                        ptr.lc=newNode;
                        System.out.println(newNode.data+" was successfully inserted.");  
                        break;
                    }
                } 
                else if(newNode.data>ptr.data) {
                    if(ptr.rc!=null) ptr=ptr.rc;
                    else {
                        ptr.rc=newNode;
                        System.out.println(newNode.data+" was successfully inserted."); 
                        break;
                    }
                } 
            }
        }             
    }
    Node delete(Node root,int key){     // delete the nodes 
            if (root == null) return null;
            if (root.data > key) {
                root.lc = delete(root.lc, key);
                }
                else if (root.data < key) {
            root.rc = delete(root.rc, key);
        }
        return null;
    }
    boolean search(Node root,int key){ // if the search is successful return the Node or return null

        if(root==null){
            return false;
        }
        if(root.data== key) {
            return true;
        }
            if(root.data!= key){
                return false;
            }
        return true;
    }
    int number_nodes(Node n){ // the counting left child
        if(n == null)
        return 0;
        if(n.lc ==null && n.rc==null) 
        return 1;
        else
            return 1+number_nodes(n.lc);
    }
    int rnumber_nodes(Node n){ // the counting right child
        if(n == null)
        return 0;
        if(n.lc ==null && n.rc==null) 
        return 1;
        else
        return 1+number_nodes(n.rc);
    }
    void inorder(Node n){ // recursive inorder travelsal
        if(n==null) return;
        System.out.print("[");
        inorder(n.lc);
        System.out.print(n.data);
        inorder(n.rc);
        System.out.print("]");
    }
    void preorder(Node n){ // recursive preorder travelsal
        if(n==null) return;
        System.out.print("[");
        System.out.print(n.data);
        preorder(n.lc);        
        preorder(n.rc);
        System.out.print("]");
    }
    void postorder(Node n){ // recursive postorder travelsal
        if(n==null) return;
        System.out.print("[");      
        postorder(n.lc);        
        postorder(n.rc);
        System.out.print(n.data);
        System.out.print("]");
    }
}
public class Bst { // change the name into your IDs
    public static void main(String[] args) {
        BST bst=new BST();
        Scanner sScan=new Scanner(System.in); // menu
        Scanner iScan=new Scanner(System.in); // data
        while(true){
            System.out.print("\n(q)uit,(i)nsert,(d)elete,(s)earch,i(n)order,(p)reorder,p(o)storder,(h)ow many:");
            String uChoice=sScan.next();
            if(uChoice.equalsIgnoreCase("i")){
                System.out.print("Enter a number to insert:");
                int uData=iScan.nextInt();
                bst.insert(uData);
            }
            else if(uChoice.equalsIgnoreCase("d")){  // deletion
                System.out.print("enter the delete number");
                Scanner s=new Scanner(System.in); // to use new scanner
                int delete_num=s.nextInt(); // 
                bst.delete(bst.root,delete_num);
            }
            else if(uChoice.equalsIgnoreCase("s")){  // search
                System.out.print("enter the search number");
                Scanner s=new Scanner(System.in);
                int search_num=s.nextInt(); 
                    if(bst.search(bst.root,search_num)){
                        for(){
                        System.out.println(" your number is found"); // to tell you your # is found or not found
                    }
                    }
                    else{
                        System.out.println(" your number is not found");
                    }
            }
            else if(uChoice.equalsIgnoreCase("n")){  // in order
                bst.inorder(bst.root);
            }
            else if(uChoice.equalsIgnoreCase("p")){  // pre order
                bst.preorder(bst.root);
            }
            else if(uChoice.equalsIgnoreCase("o")){  // post order
                bst.postorder(bst.root);
            }
            else if(uChoice.equalsIgnoreCase("h")){  // how many
                int x,y;
                x=bst.number_nodes(bst.root);
                y=bst.rnumber_nodes(bst.root);
                int total=x+y;
                System.out.print(total);
            }
            if(uChoice.equalsIgnoreCase("q")) break; // quit
        }
    }
}

Answer 1

您必须学习for循环的语法。 java中的无限循环写如下：

for(;;){ }

我建议你在这种情况下使用while循环。以下是两种情况的示例：

if(bst.search(bst.root,search_num)){
     for(;;){
           System.out.println(" your number is found");  
     }
}
else{
     System.out.println(" your number is not found");

}

使用while循环：

if(bst.search(bst.root,search_num)){
         while(true){
               System.out.println(" your number is found");  
         }
    }
    else{
         System.out.println(" your number is not found");
   }

事实上，在你的情况下，根本不需要循环或while循环所以代码如下：

if(bst.search(bst.root,search_num)){
   System.out.println(" your number is found");  
}
else{
    System.out.println(" your number is not found");
}

Answer 2

在java 无限循环 中写成：

for(;;){ }

而不是for( ){ }

所以你缺少分号的事实是使for循环声明无效/不完整，这就是错误的原因！

代码中的非法表达式（Java）

2 个答案: