#include "std_lib_facilities"
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(); Token get(); void putback(Token t); private: bool full; Token buffer; } ; Token_stream::Token_stream() :full(false),buffer(0) { } void Token_stream::putback(Token t)
{
if(full) error ("putback() into a full buffer");
buffer=t;
full=true;
}
Token Token_stream::get() { if(full){ full=false; return buffer; } char ch; cin >> ch; switch (ch) {
case 'q':
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");
}
}
Token_stream ts;
double expression(); double primary() // read and evaluate a 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 default: error("primary expected"); } }
double term()
{
double left = primary();
Token t = ts.get(); // get the next token
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; } default: ts.putback(t); return left; }
}
} 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 } }
}
//------------------------------------------------------------------------------
int main() {
double val=0;
while (cin){ Token t=ts.get(); if (t.kind == 'q') break; // 'q' for "quit" if(t.kind==';') cout<<"="<<val<<endl; else ts.putback(t); val=expression();
}
}
//-------------------------------------------------------------------Please anyone tell me how this program works step by step ex - when I put "5" where it checked first like which path it followed and then for next input "+ " and then from next 10 .
I struck with this , when I was studying book written by Bjarne Stroustrup
" Principles and Practice using C++"" in chapter 6 .