当使用 goto 语句时，代码不会在循环内想要的地方输出

Question

我正在尝试制作一个简单的井字游戏。现在我正在努力使计算机无法放置在二维数组中的某些地方。计算机设置为随机放置，我正在使用循环和转到使其随机，直到找到合适的位置。

#include <iostream>
#include <string>
#include <ctime>
#include <Windows.h>

using namespace std;

string team;
string a = "  ";
const int rows = 5;
const int elements = 5;

string Board[rows][elements] = { a, "| ", a, "| ", a,
    "- ", "+ ", "- ", "+ ", "- ",
    a, "| ", a, "| ", a,
    "- ", "+ ", "- ", "+ ", "- ",
    a, "| ", a, "| ", a };

void showBoard()
{
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < elements; j++) {
            cout << Board[i][j];
        }
        cout << endl;
    }
}

int main()
{
    int nonFilled = 0;
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < elements; j++) {
            if (Board[i][j] == a && a == "  ")
                nonFilled++;
        }
    }

    int circleFilled = 0;
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < elements; j++) {
            if (Board[i][j] == a && a == "0 ")
                circleFilled++;
        }
    }
    int crossFilled = 0;
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < elements; j++) {
            if (Board[i][j] == a && a == "X ")
                crossFilled++;
        }
    }

    cout << "Welcome to tic-tac-toe game." << endl;
    cout << endl;

    showBoard();

    cout << "Please select team if circle or cross: (0/X)" << endl;
    cin >> team;

    if (team == "0") {
        int ifCircle = 1;
        cout << "You have selected circle." << endl;
        while (nonFilled > circleFilled + crossFilled) {
            srand(time(NULL));
            int x, y = 0;
            int xx, xy = 0;
            cout << "Select square: " << endl;
            cout << "Y cord (0-4): ";
            cin >> x;
            cout << endl;
            cout << "X cord (0-4): ";
            cin >> y;
            if (x == 0 || y == 0)
                return 0;

            /*if (Board[x - 1][y - 1] = "| ", "+ ", "- ")
            {
                cout << "You cannot place one there. " << endl;
                nonFilled = 100;
            }*/

            int b = 1;
            if (Board[x - 1][y - 1] == a)
                b = 1;
            else
                b = 2;

            switch (b) {
            case 1:
                Board[x - 1][y - 1] = "0 ";
                break;

            case 2:
                cout << "You cannot place one there. " << endl;
                continue;
            }

            /* if (Board[x - 1][y - 1] == a)
            {
                Board[x - 1][y - 1] = "0 ";
            }
            else
            {
                cout << "You cannot place one there." << endl;
                
            }*/

            cout << endl;

            showBoard();
            cout << "The opponent will now pick a square:" << endl;
            system("pause");

            xx = rand() % rows;
            xy = rand() % elements;

            int c = 1;
            if (Board[xx][xy] == a)
                c = 1;
            else if (Board[xx][xy] == "| ", "+ ", "- ")
                c = 2;

            switch (c) {

            case 1:
                Board[xx][xy] = "X ";
                break;

            case 2: {

            LOOP: // here is my label for the goto statement

                while (true) // this loop
                {

                    xx = rand() % rows;
                    xy = rand() % elements;

                    if (Board[xx][xy] == "| ", "+ ", "- ") {
                        goto LOOP; // goto statement
                    }
                    else {
                        Board[xx][xy] = "X ";
                    }
                }
            }
            }

            cout << endl;

            cout << "The opponent has picked:" << endl;
            showBoard();
        }
    }

    else if (team == "X") {
        int ifCircle = 0;
    }
    system("pause");
    return 0;
}

有问题的循环在最底部，我不确定是我放置标签的方式有问题还是我使用语句的方式有问题，还是代码的不同部分有问题.

我查看了大量关于 while 循环内 goto 语句的问题，但我找不到任何问题。

Answer 1

眼前的问题既不在于 LOOP: 标签的位置也不在于 goto 语句的使用（我不是参与关于该关键字是否应该 ever 在 C++ 程序中使用的争论。

问题出在以下行：

if (Board[xx][xy] == "| ", "+ ", "- ") {

（以及前面几行类似的 if else... 语句）。

这不是做你想做的事！事实上，它将 always return 一个 true 值，因为表达式的净结果很简单， if ("- ") - 这将永远是一个non-null（即 non-zero）字符串文字的地址。

您需要的（如果您希望将索引 Board[][] 字符串与三个文字中的任何一个相匹配）如下：

if (Board[xx][xy] == "| " || Board[xx][xy] == "+ " || Board[xx][xy] == "- ") {
    //...

您的代码使用 comma operator 计算每个 comma-separated 表达式（按 left-to-right 顺序），丢弃除最后一个 (right-most) 之外的每个表达式价值;总体结果就是 right-most 表达式的结果。

编辑：应用我上面建议的修复后，我注意到另一个问题：你的 while 循环，就目前而言，将 never 退出（它会 goto 到 LOOP: 或保持运行）。这是修复循环的一种（快速）方法，它也消除了对 goto 语句的需要：

            case 2:
            {
                bool done = false;
                while (!done)
                {
                    xx = rand() % rows;
                    xy = rand() % elements;

                    if (Board[xx][xy] == "| " || Board[xx][xy] == "+ " || Board[xx][xy] == "- ") {
                        continue;
                    }
                    else {
                        Board[xx][xy] = "X ";
                        done = true;
                    }
                }
            }

有更多 'elegant' 方法可以达到相同的结果，但希望您至少能够理解（并理解）我对您的代码所做的相当小的更改。请随时要求任何进一步的说明 and/or 解释。

当使用 goto 语句时，代码不会在循环内想要的地方输出

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