while (op_top >= 0) { char op = op_stack[op_top]; op_top--; double b = num_stack[num_top]; num_top--; double a = num_stack[num_top]; num_top--; double result = 0; if (op == '+') { result = a + b; } else if (op == '-') { result = a - b; } else if (op == '*') { result = a * b; } else if (op == '/') { result = a / b; } else if (op == '%') { result = (int)a % (int)b; } else if (op == 's') { result = sqrt(a); } else if (op == '^') { result = pow(a, b); } num_top++; num_stack[num_top] = result; } return num_stack[0]; } 帮我在代码内部中对这个代码进行注释

下面是对代码进行注释的版本：

// 计算逆波兰表达式的函数
double calc(char *expr) {
    int len = strlen(expr);
    int num_top = -1; // 数字栈栈顶指针
    int op_top = -1; // 操作符栈栈顶指针
    double num_stack[len]; // 存放数字的栈
    char op_stack[len]; // 存放操作符的栈
    
    // 遍历表达式中的每个字符
    for (int i = 0; i < len; i++) {
        char ch = expr[i];
        if (ch >= '0' && ch <= '9') { // 如果是数字，则将其入栈
            double num = ch - '0'; // 将字符转换成数字
            i++;
            while (i < len && expr[i] >= '0' && expr[i] <= '9') { // 处理多位数字
                num = num * 10 + (expr[i] - '0');
                i++;
            }
            i--;
            num_top++;
            num_stack[num_top] = num;
        } else if (ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == '%' || ch == 's' || ch == '^') { // 如果是操作符，则将其入栈
            op_top++;
            op_stack[op_top] = ch;
        } else if (ch == ' ') { // 如果是空格，则跳过
            continue;
        } else { // 如果是非法字符，则返回错误
            return 0;
        }
        
        // 处理操作符
        while (op_top >= 0 && (op_stack[op_top] == 's' || op_stack[op_top] != '+' && op_stack[op_top] != '-' || ch == '*' || ch == '/')) {
            char op = op_stack[op_top];
            op_top--;
            double b = num_stack[num_top];
            num_top--;
            double a = num_stack[num_top];
            num_top--;
            double result = 0;
            if (op == '+') {
                result = a + b;
            } else if (op == '-') {
                result = a - b;
            } else if (op == '*') {
                result = a * b;
            } else if (op == '/') {
                result = a / b;
            } else if (op == '%') {
                result = (int)a % (int)b;
            } else if (op == 's') {
                result = sqrt(a);
            } else if (op == '^') {
                result = pow(a, b);
            }
            num_top++;
            num_stack[num_top] = result;
        }
        op_top++;
        op_stack[op_top] = ch;
    }
    
    // 处理剩余操作符
    while (op_top >= 0) {
        char op = op_stack[op_top];
        op_top--;
        double b = num_stack[num_top];
        num_top--;
        double a = num_stack[num_top];
        num_top--;
        double result = 0;
        if (op == '+') {
            result = a + b;
        } else if (op == '-') {
            result = a - b;
        } else if (op == '*') {
            result = a * b;
        } else if (op == '/') {
            result = a / b;
        } else if (op == '%') {
            result = (int)a % (int)b;
        } else if (op == 's') {
            result = sqrt(a);
        } else if (op == '^') {
            result = pow(a, b);
        }
        num_top++;
        num_stack[num_top] = result;
    }
    
    return num_stack[0];
}