c++ - Stroustrup 的 C++ 书籍练习

Question

我正在从 Stroustrup 的书中学习 C++:Programming: Principles and Practice using C++。我在第 6 章，我们正在编写一个计算器，现在:

I have to add a factorial clause which bids stronger than * and /. In the original program we had 3 levels, primary (floating point literals & parentheses), terms (* and /) and expressions (+ or -). What expression does is call term, to collect its left hand side and so on. In order to implement the factorial I've added a fact() between termand primary(), in order to bid it tighter. Before adding fact() the calculator was working perfectly fine. Then I added fact in order to take data from primary and term is now taking data from fact.

现在回答问题:

Q1:正如我已经说过的，在添加 fact 之前一切都很好。现在输出的唯一正确方程是 term。当我尝试做其他算术时，它只会打印出最后输入的数字。我哪里搞砸了其他操作。

Q2:(有点跑题)为什么当我尝试退出程序时(通过输入 'q'，我花了三四次输入 'q' 直到它退出。

代码如下:

#include "std_lib_facilities.h"


class Token {
public:
    char kind;        // what kind of token
    double value;     // for numbers: a value
    Token(char ch)    // make a Token from a char
        :kind(ch), value(0) { }
    Token(char ch, double val)     // make a Token from a char and a double
        :kind(ch), value(val) { }
};

//------------------------------------------------------------------------------

class Token_stream {
public:
    Token_stream();         // make a Token_stream that reads from cin
    Token get();            // get a Token
    void putback(Token t);  // put a Token back
private:
    bool full;      // is there a Token in the buffer
    Token buffer;   // here is where we keep a Token put back using putback();
};

// Constructor
Token_stream::Token_stream()
    :full(false),buffer(0)
{
}

Token_stream ts;

void Token_stream::putback(Token t)
{
    if (full)  error("putback() into a full buffer"); // checks if we're using putback() twice
    buffer = t;  // copy t to buffer
    full = true; // buffer is now full
}

Token Token_stream::get()
{
    if (full) {     // do we already have a Token ready?
                // remove Token from buffer
        full = false;
        return buffer;
    }

    char ch;
    cin >> ch; // note that >> skips whitespace

    switch(ch) {
    case ';':   // for "print"
    case 'q':   // for "quit"
    case '(': case ')': case '{': case '}': case '!': case'+': case '-': case'/': case '*':
        return Token(ch);  // let each character represent itself
    case '.':
    case '0': case '1': case '2': case '3': case '4':
    case '5': case '6': case '7': case '8': case '9':
    {
        cin.putback(ch);        // put digit back into the input stream
        double val;
        cin >> val;
        return Token('8',val);  // let '8' represent a "number"
        break;
    }
    default:
        error("Bad token");
        return 0;
    }
}



Token get_token()    // read a token from cin
{
    char ch;
    cin >> ch;    // note that >> skips whitespace (space, newline, tab, etc.)

    switch (ch) {

    case 'q':
    case ';':
    case '(': case ')': case '{': case '}': case '!': case '+': case '-': case '*': case '/':
        return Token(ch);        // let each character represent itself
    case '.':
    case '0': case '1': case '2': case '3': case '4':
    case '5': case '6': case '7': case '8': case '9':
        {
            cin.putback(ch);         // put digit back into the input stream
            double val;
            cin >> val;              // read a floating-point number
            return Token('8',val);   // let '8' represent "a number"
        }
    default:
        error("Bad token");
        return 0;
    }
}

//------------------------------------------------------------------------------

double expression();  // read and evaluate a Expression

//------------------------------------------------------------------------------

double term();        // read and evaluate a Term

//------------------------------------------------------------------------------

double primary()
{
    Token t = ts.get();
    switch (t.kind) {
    case '(':    // handle '(' expression ')'
        {
            double d = expression();
            t = ts.get();
            if (t.kind != ')') error("')' expected");
            return d;
        }
    case '{':    // handle '(' expression ')'
            {
                double d = expression();
                t = ts.get();
                if (t.kind != '}') error("'}' expected");
                return d;
            }
    case '8':            // we use '8' to represent a number
        return t.value;  // return the number's value
    default:
        return 0;
        error("primary expected");
    }
}

//------------------------------------------------------------------------------

int main()
try {
        double val = 0;
        while(cin)
        {
            Token t = ts.get();

            if(t.kind == 'q') break; //'q' for quit
            if(t.kind == ';')
                cout << "=" << val << "\n";
            else
                ts.putback(t);
            val = expression();
        }

    keep_window_open("q");
}
catch (exception& e) {
    cerr << e.what() << endl;
    keep_window_open ("q");
    return 1;
}
catch (...) {
    cerr << "exception \n";
    keep_window_open ("q");
    return 2;
}

//------------------------------------------------------------------------------

double expression()
{
    double left = term();      // read and evaluate a Term
    Token t = ts.get();        // get the next token

    while(true) {
        switch(t.kind) {
        case '+':
            left += term();    // evaluate Term and add
            t = ts.get();
            break;
        case '-':
            left -= term();    // evaluate Term and subtract
            t = ts.get();
            break;
        default:
            ts.putback(t);
            return left;       // finally: no more + or -: return the answer
        }
    }
}

//------------------------------------------------------------------------------

double factorial(double val)
{
    double res=1;
    for(int i=1; i<=val; i++)
        res *= i;
    return res;
}

double fact()
{
    double left = primary();
    Token t = ts.get();

    switch(t.kind)
    {
    case '!':
        {   double res = factorial(left);
            return res;
        }
    default:
        return left;
    }
}

//------------------------------------------------------------------------------

double term()
{
    double left = fact();
    Token t = ts.get();     // get the next token

    while(true) {
        switch (t.kind) {
        case '*':
            left *= fact();
            t = ts.get();
            break;
        case '/':
            {
                double d = fact();
                if (d == 0) error("divide by zero");
                left /= d;
                t = ts.get();
                break;
            }
        default:
            ts.putback(t);
            return left;
        }
    }
}

抱歉，如果我不够具体。这是我第一次发布这样的主题。

Answer 1

从你的第二个问题开始:

你需要按 q 几次，因为你把它作为一个参数:

keep_window_open("q");

因此，每次您退出 while 循环时，它都会等待您输入 q 以关闭控制台。

关于您的阶乘函数:

指定你的计算器语法:写出现有操作的优先顺序，即包含它们的函数的调用顺序。这将使您更容易合并新功能。

最好将 factorial 作为附加 case 放入 primary() 中，因为 括号 有比 term() 中的乘法、除法等优先级更高(调用更早)。

这是一种可能的实现方式:

double primary(){
    Token t = ts.get();
    switch (t.kind) {
    case '{':{
        double d = expression();
        t = ts.get();
        if (t.kind != '}') error("'}' expected");
        return d;
    }
    case '(':    // handles '(' expression ')'{    
        double d = expression();
        t = ts.get();
        if (t.kind != ')') error("')' expected");
        return d;
    }
    case '8': case '!':{
        // include a test whether the number is integer and > 0 
        if(is_factorial()){
            double d  = factorial(t.value);
            t = ts.get();
            return d;
        }
        else return t.value;
    }
    default:
        error("primary expected");
    }
}

哪里:

/*
   Non-member method: factorial.
   Use: double fact = factorial(double);
   This funnction provides factorial operator.
*/
double factorial(double num){
   if(num <= 1) return 1;
   return num*factorial(num-1);
}

和

/*
   Non-member method: is_factorial.
   Use: bool fact = is_factorial(void);
   This funnction returns true if a number
   is followed by factorial opertor. 
   Used as an indicator to call factorial function.
*/
bool is_factorial(){
    Token t = ts.get();
    if (t.kind == '!'){
        ts.putback(t);
        return true;
    }
    ts.putback(t);
    return false;
}