读入的也可以是一个赋值语句：左端是一个符号名，右端是表达式。 表达式中可以有四则运算符、括号、整数/实数值、已经赋值的符号名和预定义的符号常量（ pi 和 e ），也可以只有单个的整数/实数值。 发现输入内容与文法不符或将导致非法计算时，则从标准输出设备输出出错提示，并计算出错次数。写出此c++程序并给出注释

以下是一个实现该功能的 C++ 程序，注释在代码中：

#include <iostream>
#include <string>
#include <unordered_map>
#include <cmath>

using namespace std;

// 定义预定义符号常量 pi 和 e
const double PI = acos(-1);
const double E = exp(1);

// 定义全局变量，用于记录出错次数
int error_count = 0;

// 定义符号表，用于存储已经赋值的符号名和对应的值
unordered_map<string, double> symbol_table;

// 判断字符 c 是否为运算符
bool is_operator(char c) {
    return c == '+' || c == '-' || c == '*' || c == '/';
}

// 判断字符 c 是否为数字
bool is_digit(char c) {
    return c >= '0' && c <= '9';
}

// 获取运算符的优先级
int get_priority(char op) {
    if (op == '+' || op == '-') {
        return 1;
    } else {
        return 2;
    }
}

// 计算表达式的值，返回结果
double calculate(string expr) {
    string postfix_expr = "";  // 存储转换后的后缀表达式
    string num_str = "";  // 存储数字字符串
    int num_left_paren = 0;  // 记录左括号的数量
    int num_right_paren = 0;  // 记录右括号的数量
    for (int i = 0; i < expr.length(); i++) {
        char c = expr[i];
        if (c == ' ') {
            continue;  // 忽略空格
        } else if (c == '(') {
            num_left_paren++;
            postfix_expr += c;
        } else if (c == ')') {
            num_right_paren++;
            if (num_right_paren > num_left_paren) {
                cout << "Error: Mismatched parentheses." << endl;
                error_count++;
                return 0;
            }
            while (postfix_expr.length() > 0 && postfix_expr[postfix_expr.length()-1] != '(') {
                postfix_expr += ' ';
                postfix_expr += postfix_expr[postfix_expr.length()-3];
                postfix_expr += ' ';
                postfix_expr += postfix_expr[postfix_expr.length()-2];
                postfix_expr += ' ';
                postfix_expr += postfix_expr[postfix_expr.length()-1];
                postfix_expr.pop_back();
                postfix_expr.pop_back();
                postfix_expr.pop_back();
                postfix_expr.pop_back();
            }
            postfix_expr.pop_back();
        } else if (is_operator(c)) {
            while (postfix_expr.length() > 0 && postfix_expr[postfix_expr.length()-1] != '(' && get_priority(postfix_expr[postfix_expr.length()-1]) >= get_priority(c)) {
                postfix_expr += ' ';
                postfix_expr += postfix_expr[postfix_expr.length()-3];
                postfix_expr += ' ';
                postfix_expr += postfix_expr[postfix_expr.length()-2];
                postfix_expr += ' ';
                postfix_expr += postfix_expr[postfix_expr.length()-1];
                postfix_expr.pop_back();
                postfix_expr.pop_back();
                postfix_expr.pop_back();
                postfix_expr.pop_back();
            }
            postfix_expr += ' ';
            postfix_expr += c;
        } else {
            num_str += c;
            if (i == expr.length()-1 || is_operator(expr[i+1]) || expr[i+1] == ')' || expr[i+1] == '(') {
                double num = stod(num_str);
                postfix_expr += ' ';
                postfix_expr += to_string(num);
                num_str = "";
            }
        }
    }
    if (num_left_paren != num_right_paren) {
        cout << "Error: Mismatched parentheses." << endl;
        error_count++;
        return 0;
    }
    if (postfix_expr.length() == 0) {
        cout << "Error: Invalid expression." << endl;
        error_count++;
        return 0;
    }
    int start = 0;
    int end = 0;
    double result = 0;
    for (int i = 0; i < postfix_expr.length(); i++) {
        char c = postfix_expr[i];
        if (c == ' ') {
            continue;
        } else if (is_operator(c)) {
            end = i-1;
            double num2 = stod(postfix_expr.substr(start, end-start+1));
            start = i+1;
            end = i+1;
            while (end < postfix_expr.length() && postfix_expr[end] != ' ') {
                end++;
            }
            double num1 = stod(postfix_expr.substr(start, end-start));
            start = end+1;
            if (c == '+') {
                result = num1 + num2;
            } else if (c == '-') {
                result = num1 - num2;
            } else if (c == '*') {
                result = num1 * num2;
            } else if (c == '/') {
                if (num2 == 0) {
                    cout << "Error: Division by zero." << endl;
                    error_count++;
                    return 0;
                }
                result = num1 / num2;
            }
            postfix_expr = postfix_expr.substr(0, start-1) + to_string(result) + postfix_expr.substr(end);
            i = start-2;
            start = 0;
            end = 0;
        }
    }
    return stod(postfix_expr);
}

int main() {
    string input_line;
    while (getline(cin, input_line)) {
        if (input_line.empty()) {
            continue;
        }
        if (input_line == "quit") {
            break;
        }
        int equal_sign_pos = input_line.find('=');
        if (equal_sign_pos == string::npos) {
            cout << "Error: Invalid expression." << endl;
            error_count++;
            continue;
        }
        string symbol_name = input_line.substr(0, equal_sign_pos);
        string expr_str = input_line.substr(equal_sign_pos+1);
        if (symbol_name.empty() || expr_str.empty()) {
            cout << "Error: Invalid expression." << endl;
            error_count++;
            continue;
        }
        if (symbol_table.find(symbol_name) != symbol_table.end()) {
            cout << "Error: Symbol " << symbol_name << " already exists." << endl;
            error_count++;
            continue;
        }
        double result = calculate(expr_str);
        if (error_count == 0) {
            symbol_table[symbol_name] = result;
            cout << symbol_name << " = " << result << endl;
        } else {
            error_count = 0;
        }
    }
    return 0;
}

该程序实现了读入一个赋值语句，计算表达式的值，并将符号名和对应的值存储到符号表中。如果输入内容与文法不符或导致非法计算，则从标准输出设备输出出错提示，并计算出错次数。