问题是 C++ 中的“错误：需要分号”

Question

using namespace std;
#define MAX_STACK_SIZE 50

class Stack {
public:
    int top = -1;
    Location data[MAX_STACK_SIZE];
    bool isEmpty() { return top == -1; }
    bool isFull() { return top == MAX_STACK_SIZE - 1; }
    void push(Location& e) {
        if (!isFull()) {
            data[++top] = e;
        }
    }
    Location& pop() {
        if (!isEmpty()) {
            return data[top--];
        }
    }
    Location& peek() {
        return data[top];
    }
};

struct Location {
    int row;
    int col;
    Location(int r,int c): row(r), col(c){}
};

int main()
{
    const int MAX_SIZE = 6;
    char map[MAX_SIZE][MAX_SIZE] = {
        {'1','1','1','1','1','1'},
        {'e','0','1','0','0','1'},
        {'1','0','0','0','1','1'},
        {'1','0','1','0','1','1'},
        {'1','0','1','0','0','x'},
        {'1','1','1','1','1','1'},
    };

    bool isValid(int r, int c) {
        if (r < 0 || c < 0 || r >= MAX_SIZE || c >= MAX_SIZE) return false;
        else  return map[r][c] = '0' || map[r][c] = 'x';
    }
}

I am trying to implement maze navigation as a stack. The isValid function says i need ';' Marking the potential fixes ' bool isValid(int r,int c); ' They suggest me use it like this. What is the problem? Please help me TT

Answer 1

It is not allowed in C++ to have regular functions inside other functions. You have to use closure/lambda function syntax.

Ie rewrite string bool isValid(int r, int c) { to auto isValid = [&](int r, int c) -> bool { , plus add ; at the end of such lambda function definition.

Then you can use isValid like regular function afterwards.

Full code of main() function with needed fixes down below:

Try it online!

int main()
{
    const int MAX_SIZE = 6;
    char map[MAX_SIZE][MAX_SIZE] = {
        {'1','1','1','1','1','1'},
        {'e','0','1','0','0','1'},
        {'1','0','0','0','1','1'},
        {'1','0','1','0','1','1'},
        {'1','0','1','0','0','x'},
        {'1','1','1','1','1','1'},
    };

    auto isValid = [&](int r, int c) -> bool {
        if (r < 0 || c < 0 || r >= MAX_SIZE || c >= MAX_SIZE) return false;
        else  return map[r][c] == '0' || map[r][c] == 'x';
    };

    bool try_is_valid = isValid(1, 2);

    return 0;
}

Answer 2

While Arty's answer works, it doesn't really address your confusion. The simple answer is: C++ doesn't allow nested function definitions. So you can't define function isValid inside function main . (Arty's answer shows you how to circumvent this restriction, but I would say it is rather too advanced for what you need.)

So you have to define the isValid function outside main . isValid won't be called from anywhere else, so it is good practice to declare it static so that it doesn't clash with other functions of the same name that may be defined in different source files. (And even if you only have one source file, it's still a good habit to get into.)

isValid has to be defined or declared before you call it. It is a matter of taste whether to define a static function ahead of the calling code, like this:

static bool isValid(int r, int c) {
    if (r < 0 || c < 0 || r >= MAX_SIZE || c >= MAX_SIZE) return false;
    else  return map[r][c] = '0' || map[r][c] = 'x';
}

int main() {
    if (isValid(...)) etc.
}

or to declare if first and define it later, like this:

static bool isValid(int r, int c);

int main() {
    if (isValid(...)) etc.
}

static bool isValid(int r, int c) {
    if (r < 0 || c < 0 || r >= MAX_SIZE || c >= MAX_SIZE) return false;
    else  return map[r][c] = '0' || map[r][c] = 'x';
}

I prefer the second method myself, but many people prefer the first.

The disadvantage with this method (and the advantage with Arty's method) is that isValid doesn't have access to variables declared in main . So it can't see MAX_SIZE or map . So in this case you need to declare these variables outside the main function too, making them static for the same reason.