c设计一个简单计算器.doc
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#include iostream.h
#include string
using namespace std;
enum Token_value{
NAME, NUMBER, END,
PLUS = +, MINUS = -,MUL = *,DIV = /,
PRINT = ;,ASSIGN = =,LP = (,RP = )
};
Token_value curr_tok = PRINT;
// fuction list//////////////////////////////////////
double expr(bool get);
double term(bool get);
double prim(bool get);
Token_value get_token();
double error(const string s);
//////////////////////////////////////////////////////////
double expr(bool get) //plus and minus
{ 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) // multiply and divide
{ double left = prim(get);
for(;;)
{ switch(curr_tok){
case MUL:
left*=prim(true);
break;
case DIV:
if(double d = prim(true)){ // no zero!!
left/= d;
break;
}
return error(divide by zero!!\n);
default :
return left;
}
}
}
double number_value;
double string_value;
double prim(bool get)
{ if(get) get_token();
switch(curr_tok){
case NUMBER:
{ double v = number_value;
get_token();
return v;
}
case NAME:
{ double v;
//double v = table[string_value];
//this table reserved the name mapped with variable
//now we dont use it!
if(get_token()==ASSIGN)
v = expr(true);
return v;
}
case MINUS: // negative
{ return -prim(true);
}
case LP:
{ double e = expr(true);
if(curr_tok!=RP)return error()expected);
get_token(); // absorb )
return e;
}
default:
return error(primary expected); // no primary
}
}
Token_value get_token()
{ char ch =0;
cinch;
switch(ch){
case 0:
return curr_tok=END; //return and assignment
case ;:
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);
cinnumber_value;
return curr_tok = NUMBER;
default:
if(isalpha(ch))
{ cin.putback(ch);
cinstring_value;
retu
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