https://rentry.org/PPP2_p209

// Code derived from Stroustrup's PPP2 book
// § 6.7 Trying the second version
//  -and beginning on p 209

#include <iostream>
#include <stdexcept>
#include <string>

using namespace std;

void error(const char* s) { throw runtime_error(s); }

//------------------------------------------------------------------------------
class Token {
 public:
  Token(char ch) : kind{ch} {}

  Token(char ch, double val) : kind{ch}, value{val} {}

  char   kind  = '0';
  double value = 0.0;
};

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

 private:
  bool  full;    // is there a Token in the buffer?
  Token buffer;  // here is where we keep a Token put back using putback()
};

//------------------------------------------------------------------------------
Token_stream::Token_stream() : full{false}, buffer{0}  // no Token in buffer
{
}

//------------------------------------------------------------------------------
// read a token from cin
Token Token_stream::get()
{
  if (full) {  // check if we already have a Token ready
    full = false;
    return buffer;
  }

  char ch;
  cin >> ch;  // note that >> skips whitespace (space, newline, tab, etc.)

  // clang-format off
  switch (ch) {
    case ';':
    case 'q':
    case '(': case ')': case '+': case '-': case '*': case '/':
      return Token{ch};
      break;
    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"
      break;
    }
    default:
      error("Bad token");
      return Token{'K'};  // invalid, shouldn't reach here
  }
  // clang-format on
}

//------------------------------------------------------------------------------
// The putback() member function puts its argument back into the Token_stream's
// buffer:
void Token_stream::putback(Token t)
{
  if (full)
    error("putback() into a full buffer");

  buffer = t;     // copy t to buffer
  full   = true;  // buffer is now full
}

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

double expression();

Token_stream ts;

//------------------------------------------------------------------------------
// 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;
      break;
    }
    case '8':          // we use '8' to represent a number
      return t.value;  // return the number's value
      break;
    default:
      error("primary expected");
      // shouldn't reach here
      return 0.0;
  }
}

//------------------------------------------------------------------------------
// deal with *, /
double term()
{
  double left = primary();

  Token t = ts.get();

  while (true) {
    switch (t.kind) {
      case '*': left *= primary(); break;
      case '/': {
        double d = primary();
        if (d == 0)
          error("divide by zero");

        left /= d;
        break;
      }
      default: ts.putback(t); return left;
    }

    t = ts.get();
  }
}

//------------------------------------------------------------------------------
// deal with + and -
double expression()
{
  double left = term();

  Token t = ts.get();

  while (true) {
    switch (t.kind) {
      case '+': left += term(); break;
      case '-': left -= term(); break;
      default: ts.putback(t); return left;
    }

    t = ts.get();
  }
}

//------------------------------------------------------------------------------
int main()
try {
  double val = 0;

  while (cin) {
    Token t = ts.get();

    if (t.kind == 'q')  // 'q' for quit
      break;

    if (t.kind == ';')  // ';' for "print now"
      cout << "= " << val << '\n';
    else
      ts.putback(t);

    val = expression();
  }

} catch (exception& e) {
  cerr << e.what() << '\n';
  return 1;

} catch (...) {
  cerr << "exception \n";
  return 2;
}

build & run:

g++ -std=c++20 -O2 -Wall -pedantic ./ch_06/main_p209.cpp && ./a.out

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Pub: 01 Apr 2023 12:59 UTC
Edit: 02 May 2023 21:19 UTC
Views: 483