我从Stroustrup的书中学习C ++:编程:使用C ++的原理与实践。我正在第6章,我们正在编写一个计算器,现在:
我必须添加一个因子条款,其出价高于
*
和/
。在原始程序中,我们有3个级别primary
(浮点文字和括号),terms
(*和/)和expressions
(+或 - )。expression
做的是致电term
,收集左手边等等。为了实现阶乘,我在fact()
和term
之间添加了primary()
,以便将其投标更严格。 在添加fact()
之前,计算器工作正常。然后我添加了fact
以便从primary
获取数据,而term
现在正在从fact
获取数据。
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。现在唯一正确的输出方程是term
。当我尝试做其他算术时,它只打印我最后输入的数字。我在哪里弄乱了其他操作。
Q2 :(有点偏离主题)为什么当我尝试退出程序时(输入'q'
,我需要三到四次输入'q'
才能退出。
以下是代码:
#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;
}
}
}
很抱歉,如果我不够具体。 这是我第一次发布这样的帖子。
答案 0 :(得分:2)
从第二个问题开始:
您需要多次按q
几次作为参数:
keep_window_open("q");
因此,每次退出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:
指定您的计算器语法:编写现有操作的优先顺序,即调用顺序包含它们的函数。这将使您更容易合并新功能。
最好将factorial
作为附加case
添加到primary()
中,因为括号具有比 term()
中的乘法,除法等。
以下是一种可能的实施方式:
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");
}
}
其中:
/*
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;
}