Macwelt 1/Macwelt DVD 1.toast/Web-Publishing/HTML-Editoren/Alpha ƒ/Mode Examples/C++-Example.c

C/C++ Source or Header | 2000-10-30 | 5.4 KB | 216 lines

// C++-Example.c
//
// Included in the Alpha distribution as an example of the C++ mode
//
// source of original document:
//
// http://www.research.att.com/~bs
//
// copywrite (c) 1985 Bjarne Stroustrup
// <[e-mail]>


// The desk calculator
//
// reads from std::cin
// uses namespaces, but no exceptions
//
// The C++ Programming Language, pp 163-171, sec 8.2, Namespaces


// uses += rather than push_back() for string
// to work around standard library bug

// No guarantees offered. Constructive comments to [e-mail]

/*
        program:
                END                        // END is end-of-input
                expr_list END

        expr_list:
                expression PRINT           // PRINT is ';'
                expression PRINT expr_list

        expression:
                expression + term
                expression - term
                term

        term:
                term / primary
                term * primary
                primary

        primary:
                NUMBER
                NAME
                NAME = expression
                - primary
                ( expression )
*/

#include<iostream>
#include<map>
#include<string>
#include<cctype>

// note: no "using namespace std;"

namespace Error {

        int no_of_errors;

        double error(const char* s)
        {
                        cerr << "error: " << s << '\n';
                        no_of_errors++;
                        return 1;
        }
}

namespace Lexer {

        enum Token_value {
                NAME,   NUMBER,         END,
                PLUS='+',       MINUS='-',      MUL='*',        DIV='/',
                PRINT=';',      ASSIGN='=',     LP='(',         RP=')'
        };

        Token_value curr_tok;
        double number_value;
        string string_value;

        Token_value get_token();
}


Lexer::Token_value Lexer::get_token()
{
        char ch = 0;
        cin>>ch;

        switch (ch) {
        case 0:
                return END;
        case ';':
                return curr_tok=PRINT;
        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);
                cin >> number_value;
                return curr_tok=NUMBER;
        default:                        // NAME, NAME =, or error
                if (isalpha(ch)) {      // warning isalpha is a macro
                                        // in some implementations
                        std::cin.putback(ch);
                        std::cin>>string_value;
                        return curr_tok=NAME;
                }
                Error::error("bad token");
                return curr_tok=PRINT;
        }
}

map<string,double> table;       // global; see sec9.???

namespace Parser {
        double prim(bool get);          // handle primaries
        double term(bool get);          // multiply and divide
        double expr(bool get);          // add and subtract

        using Lexer::get_token;         // commonly used names
        using Lexer::curr_tok;
        using Error::error;
}

double Parser::prim(bool get)           // handle primaries
{
        if (get) get_token();

        switch (curr_tok) {
        case Lexer::NUMBER:     // floating point constant
                get_token();
                return Lexer::number_value;
        case Lexer::NAME:
        {       double& v = table[Lexer::string_value];
                if (get_token() == Lexer::ASSIGN) v = expr(1);
                return v;
        }
        case Lexer::MINUS:              // unary minus
                return -prim(1);
        case Lexer::LP:
        {       double e = expr(1);
                if (curr_tok != Lexer::RP) return error(") expected");
                get_token();                      // eat ')'
                return e;
        }
        case Lexer::END:
                return 1;
        default:
                return error("primary expected");
        }
}

double Parser::term(bool get)           // multiply and divide
{
        double left = prim(get);

        for (;;)
        switch (curr_tok) {
        case Lexer::MUL:
                left *= prim(true);
                break;
        case Lexer::DIV:
                if (double d = prim(true)) {
                left /= d;
                        break;
                }
                return error("divide by 0");
        default:
                return left;
        }
}

double Parser::expr(bool get)           // add and subtract
{
        double left = term(get);

        for(;;)                 // ``forever''
        switch (curr_tok) {
        case Lexer::PLUS:
                left += term(true);
                break;
        case Lexer::MINUS:
                left -= term(true);
                break;
        default:
                return left;
                }
}

int main(int argc, char* argv[])
{
        // insert pre-defined names:
        table["pi"] = 3.1415926535897932385;
        table["e"]  = 2.7182818284590452354;

        while (std::cin) {
                Lexer::get_token();
                if (Lexer::curr_tok == Lexer::END) break;
                if (Lexer::curr_tok == Lexer::PRINT) continue;
                std::cout << Parser::expr(false) << '\n';
        }

        return Error::no_of_errors;
}