数组实现Queue的删除逻辑错误排查请求

我一眼就发现了导致队列删除后异常为空的原因——你的Remove函数里存在一个赋值/相等判断混淆的经典低级错误！

错误根源 #

在Remove函数的这个判断逻辑里：

if (front = rear) {
    front = rear = -1;
}

你把相等比较运算符==误写成了赋值运算符=。当插入1、2、3后，front=0，rear=2：

• 执行front = rear时，会把rear的值（2）赋给front，此时front直接变成2
• 这个赋值表达式的结果是2（非零值），在C++中会被视为true，于是触发front = rear = -1的逻辑，直接把整个队列的首尾指针重置为空状态
• 这就是为什么删除第一个元素后，队列直接显示为空的核心原因

修复后的完整代码 #

只需要把Remove函数里的赋值操作改成相等判断即可，我也顺带优化了空队列时的提示信息：

#include<iostream>
#include<conio.h>
using namespace std;

int Remove(int[]);
int Insert(int[], int);
void Display(int[], int, int);

int front = -1, rear = -1;

void main() {
    const int size = 50;
    int Q[size];
    int item, res;
    char ch = 'y';

    while (ch == 'y' || ch == 'Y') {
        cout << "\n Enter item for insertion : ";
        cin >> item;
        res = Insert(Q, item);
        if (res == -1) {
            cout << "\n OVERFLOW!";
            _getch();
        }
        cout << "\n Now the Queue is : ";
        Display(Q, front, rear);
        cout << "\n Do you wish to insert more elements into the array(y/n)? : ";
        cin >> ch;
    }

    cout << "\n Now the deletion of elements from the Queue begins....";
    ch = 'y';
    while (ch == 'y' || ch == 'Y') {
        res = Remove(Q);
        if (res == -1) {
            cout << "\n UNDERFLOW!";
            _getch();
        } else {
            cout << "\n The element deleted from the Queue is : " << res << endl;
            cout << "\n Now, the Queue(Front--to--Rear) is : ";
            Display(Q, front, rear);
        }
        cout << "\n Do you wish to delete more elements from the Queue(y/n)? : ";
        cin >> ch;
    }
    _getch();
    return;
}

int Insert(int Q[], int ele) {
    const int size = 50;
    if (rear == size - 1) {
        return -1;
    } else {
        if (rear == -1) {
            front = rear = 0;
            Q[rear] = ele;
        } else {
            rear++;
            Q[rear] = ele;
        }
        return 0;
    }
}

// 修复后的Remove函数
int Remove(int Q[]) {
    int ret;
    if (front == -1) {
        return -1;
    } else {
        ret = Q[front];
        // 把赋值运算符=改为相等判断运算符==
        if (front == rear) {
            front = rear = -1;
        } else {
            front++;
        }
    }
    return ret;
}

void Display(int Q[], int front, int rear) {
    if (front == -1) {
        cout << "Empty Queue";
        return;
    }
    for (int i = front; i < rear; i++) {
        cout << Q[i] << " <- ";
    }
    cout << Q[rear] << endl;
}

验证效果 #

插入1、2、3后执行第一次删除操作：

• 会正确返回元素1，front变为1，rear保持为2
• Display函数会输出2 <- 3，完全符合队列FIFO的预期行为

内容的提问来源于stack exchange，提问作者Amap