简单计算器(完整版)

//
// This is example code from Chapter 6.7 "Trying the second version" of
// "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
//

#include "myhead.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 (get() is defined elsewhere)
void putback(Token t); // put a Token back
void ignore(char c);
private:
bool full; // is there a Token in the buffer?
Token buffer; // here is where we keep a Token put back using putback()
};

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

// The constructor just sets full to indicate that the buffer is empty:
Token_stream::Token_stream()//000 填空
:full(false), buffer(0) // no Token in buffer
{
}

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

// The putback() member function puts its argument back into the Token_stream's buffer:
void Token_stream::putback(Token t)//000
{
if (full) error("putback() into a full buffer");
buffer = t; // copy t to buffer
full = true; // buffer is now full
}

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

Token Token_stream::get()//000 填空
{
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 (space, newline, tab, etc.)

switch (ch)
{
case ';': // for "print"
case 'q': // for "quit"
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);
double val;
cin >> val; // read a floating-point number
return Token('8',val); // let '8' represent "a number"
}
default:
error("Bad token");
}
}

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

Token_stream ts; // provides get() and putback()

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

double expression(); // declaration so that primary()

can call expression()

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

// deal with numbers and parentheses
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 '8': // we use '8' to represent a number
return t.value; // return the number's value
case '-':
return -primary();
case '+':
return primary();
default:
error("primary expected");
}
}

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

// deal with *, /, and %
double term()
{
double left = primary();
Token t = ts.get(); // get the next token from token stream

while(true)
{
switch (t.kind)
{
case '*':
left *= primary();
t = ts.get();
break;
case '/':
{
double d = primary();
if (d == 0) error("divide by zero");
left /= d;
t = ts.get();
break;
}
case '%':
{
double d=primary();
int i1=int(left);
if(i1!=left)error("left-hand operand of % not int");
int i2=int(d);
if(i2!=d)error("right-hand operand of % not int");
if(i2==0)error(" %:divide by zero");
left=i1%i2;
t=ts.get();
break;
}
default:
ts.putback(t);
return left;
}
}
}

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

// deal with + and -
double expression()
{
double left = term(); // read and evaluate a Term
Token t = ts.get(); // get the next token from token stream

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:
return left; // finally: no more + or -: return the answer
}
}
}

//------------------------------------------------------------------------------
//------------------------------------------------------------------------------
void Token_stream::ignore(char ch)
{
if(full&& ch==buffer.kind)
{
full=false;
return;
}
full=false;
char c=0;
while(cin>>ch)
if(ch==c)
return;
}
//------------------------------------------------------------------------------
void clean_up_mess()
{
ts.ignore(';');
}
//------------------------------------------------------------------------------
void calculate()
{

while(cin)
try{
cout<<">";
Token t=ts.get();
while(t.kind==';')
t=ts.get();
if(t.kind=='q')
return;
ts.putback(t);
cout<<"="<}
catch(exception&e)
{
cerr<clean_up_mess;
}
}
//------------------------------------------------------------------------------

int main()
{
cout<<"请输入表达式，几个表达式之间以\';\'隔开，结束请输入\'q\'\n";
try
{
while (cin)
{
cout<<">";
Token t = ts.get();
while(t.kind==';')
t=ts.get();
if (t.kind == 'q')
{
return 0;
}
ts.putback(t);
cout<<"="<}
return 0;
}
catch (exception& e)
{
cerr << "error: " << e.what() << '\n';
return 1;
}
catch (...)
{
cerr << "Oops: unknown exception!\n";
return 2;
}
}
//----------------------------------------------------------