表达式求值和转换

#includeusing namespace std;#include#include#include#include#includeconst int maxn = 100020;stacknum;          //数字栈stackop;          //运算符栈void eval()                   //用于中缀表达式计算{    auto b = num.top();             //考虑运算顺序，先取出来的是第二个运算数    num.pop();    auto a = num.top();    num.pop();    auto c = op.top();    op.pop();    double x = 0;    if (c == '+')x = a + b;    else if (c == '-')x = a - b;    else if (c == '*')x = a * b;    else x = a / b;    num.push(x);}    bool eval_(char c)                  //用于后缀表达式{    auto b = num.top();             //考虑运算顺序，先取出来的是第二个运算数    num.pop();    auto a = num.top();    num.pop();    int x = 0;    if (c == '+')x = a + b;    else if (c == '-')x = a - b;    else if (c == '*')x = a * b;    else {        if (b == 0)        {            cout << "Error: " << a << "/0";            return false;        }        x = a / b;    }    num.push(x);    return true;}//中缀表达式求值函数void  evalmerge(string str){           unordered_mappr{ {'+',1},{'-',1},{'*',2},{'/',2} };     //定义一个哈希表，存储优先级    for (int i = 0; i < str.size(); i++)    {        auto c = str[i];        if (isdigit(c)||c=='-'&&(i==0||str[i-1]=='('))        {            int flag = 1, xiaoshuwei = 1;            double x = c - '0', j = i + 1;            if (c == '-')            {                x = 0;            }            while (j = pr[c])//符号栈没空，并且栈顶的运算符优先级大于当前运算符            {                eval(); //操作栈顶的运算符                                           }            op.push(c); //新的运算符压入栈        }    }    while (op.size())   //把最后剩下的运算符操作完    {        eval();    }    cout << num.top();}//后缀表达式求值函数，以#结尾，每个字符用空格隔开 负数粘在一起void evallast(string str){    for (int i = 0; i < str.size() - 1; i++)    {        auto c = str[i];        if (isdigit(c) || (c == '-' && i + 1 < str.size() - 1 && str[i + 1] != ' '))        {            int x = c - '0', j = i + 1;            if (c == '-')            {                x = 0;            }            while (j < str.size() - 1 && isdigit(str[j]))            {                x = x * 10 + str[j++] - '0';            //这里的j是要自增的            }            i = j - 1;                 //别忘记了更新i            if (c == '-')            {                x = -x;            }            num.push(x);        }//取出数字              else if (c != ' ')        {            if (num.size() < 2)            {                cout << "Expression Error: " << num.top();                return;            }            else            {                bool flag = eval_(c);                if (!flag)                {                    return;                }            }        }    }    if (num.size() == 1)        cout < 1)        {            cout << "Expression Error: " << num.top();        }    }   }//中缀转后缀string transe(string str)           {    string ans;                           //存储结果表达式    stackshu;    stackfu;    unordered_mappr{ {'+',1},{'-',1},{'*',2},{'/',2} };     //定义一个哈希表，存储优先级    for (int i = 0; i < str.size(); i++)    {        auto c = str[i];        if (isdigit(c) || c == '-' && (i == 0 || str[i - 1] == '('))        {            string temp;            temp += c;            int x = c - '0', j = i + 1;                       while (j = pr[c])//符号栈没空，并且栈顶的运算符优先级大于当前运算符            {                auto temp = fu.top();                fu.pop();                ans += temp;                ans += " ";            }            fu.push(c); //新的运算符压入栈        }    }    while (fu.size())   //把最后剩下的运算符操作完    {        auto temp = fu.top();        fu.pop();        ans += temp;        ans += " ";    }    ans += '#';    return ans;}int main(){    string s;    cin >> s;    evalmerge(s);    cout << endl;    while (!num.empty())    {        num.pop();    }    while (!op.empty())    {        op.pop();    }    string ss = transe(s);    cout << ss << endl;        return 0;}