code:

#include <iostream>
#include <string>
#include <map>
#include <cctype>
#include <sstream>

using namespace std;



enum token_value {
    NAME, NUMBER, END,
    PLUS='+', MINUS='-', MUL='*', DIV='/',
    PRINT=';', ASSIGN='=', LP='(', RP=')'
};

token_value curr_tok = PRINT;

double expr(bool get)
{
    double term(bool);
    double left = term(get);
    for (;;)
        switch (curr_tok){
        case PLUS:
            left += term(true);
            break;
        case MINUS:
            left -= term(true);
            break;
        default:
            return left;
        }
}

double term(bool get)
{
    double error(const string&);
    double prim(bool);
    double left = prim(get);
    for (;;)
        switch(curr_tok) {
        case MUL:
            left *= prim(true);
            break;
        case DIV:
            if (double d = prim(true)) {
                left /= d;
                break;
            }
            return error("divide by 0");
        default:
            return left;
        }}

double number_value;
string string_value;

double prim(bool get)
{
    map<string,double>table;
     double error(const string&);
    token_value get_token();
    double expr(bool);
    if(get) get_token();

    switch(curr_tok) {
    case NUMBER:
    {
        double v = number_value;
        get_token();
        return v;
    }
    case NAME:
    {
        double& v = table[string_value];
        if(get_token() == ASSIGN) v = expr(true);
        return v;
    }
    case MINUS:
        return -prim(true);
    case LP:
    {
        double e = expr(true);
        if(curr_tok != RP) return error(") expected");
        get_token();
        return e;
    }
    default:
        return error("primary expected");
    }
}

token_value get_token()
{
     double error(const string&);
    char ch;
    do {
        if(!cin.get(ch)) return curr_tok = END;
    } while(ch != '\n' && isspace(ch));
    switch (ch){
    case ';':
    case '\n':
        return curr_tok = PRINT;
    case '*':
    case '/':
    case '-':
    case '+':
    case '(':
    case ')':
    case '=':
        return curr_tok = token_value(ch);
    case '0': case '1': case '2': case '3': case '4':
    case '5': case '6': case '7': case '8': case '9':
    case '.':
        cin.putback(ch);
        cin >>  number_value;
        return curr_tok=NUMBER;
    default:
        if (isalpha(ch)) {
            string_value = ch;
            while (cin.get(ch) && isalnum(ch)) string_value.push_back(ch);
            cin.putback(ch);
            return curr_tok=NAME;
        }
    }
        error ("bad token");
        return curr_tok = PRINT;
}
        int no_of_errors;

double error(const string& s)
        {
            no_of_errors++;
            cerr << "error:" << s << '\n';
                return 1;
        }

istream* input;
int main(int argc, char* argv[])
{
    map<string,double>table;
    switch (argc) {
    case 1: 
        input = &cin;
        break;
    case 2:
        input = new istringstream(argv[1]);
        break;
default:
    error ("too many arguments");
    return 1;
        }
    table["pi"] = 3.1415926535897932385;
    table["e"] = 2.7182818284590452354;

    while (*input){
        get_token();
        if (curr_tok == END)break;
        if (curr_tok == PRINT)continue;
        cout << expr(false) << '\n';
    }
    if (input != &cin)delete input;
    return no_of_errors;
}

这是在书上抄下来的c++代码,你可以编译然后在终端运行一下.然而我想给它加个ui,那就用qt吧!下面是用设计师绘制的界面

深度截图_designer_20181104132728.png

问题来了.
1.原来的程序的输入用的是cin,但是qt的控件QTextEdit似乎是用text()函数获取输入的字符串,这里要如何转换呢.
2.原输出是直接cout << expr(flase) <<endl,qt的控件QTextBrowser是用textBrowser->setText(QString::number(result,'f',2));这样的,如何转换呢